Abstract database behind generic interface and implement alternative drivers (#322)

- [x] Design interface
- [x] Implement Sled backend
  - [x] Re-implement the SledCountedTree hack ~~on Sled backend~~ on all backends (i.e. over the abstraction)
- [x] Convert Garage code to use generic interface
- [x] Proof-read converted Garage code
- [ ] Test everything well
- [x] Implement sqlite backend
- [x] Implement LMDB backend
- [ ] (Implement Persy backend?)
- [ ] (Implement other backends? (like RocksDB, ...))
- [x] Implement backend choice in config file and garage server module
- [x] Add CLI for converting between DB formats
- Exploit the new interface to put more things in transactions
  - [x] `.updated()` trigger on Garage tables

Fix #284

**Bugs**

- [x] When exporting sqlite, trees iterate empty??
- [x] LMDB doesn't work

**Known issues for various back-ends**

- Sled:
  - Eats all my RAM and also all my disk space
  - `.len()` has to traverse the whole table
  - Is actually quite slow on some operations
  - And is actually pretty bad code...
- Sqlite:
  - Requires a lock to be taken on all operations. The lock is also taken when iterating on a table with `.iter()`, and the lock isn't released until the iterator is dropped. This means that we must be VERY carefull to not do anything else inside a `.iter()` loop or else we will have a deadlock! Most such cases have been eliminated from the Garage codebase, but there might still be some that remain. If your Garage-over-Sqlite seems to hang/freeze, this is the reason.
  - (adapter uses a bunch of unsafe code)
- Heed (LMDB):
  - Not suited for 32-bit machines as it has to map the whole DB in memory.
  - (adpater uses a tiny bit of unsafe code)

**My recommendation:** avoid 32-bit machines and use LMDB as much as possible.

**Converting databases** is actually quite easy. For example from Sled to LMDB:

```bash
cd src/db
cargo run --features cli --bin convert -- -i path/to/garage/meta/db -a sled -o path/to/garage/meta/db.lmdb -b lmdb
```

Then, just add this to your `config.toml`:

```toml
db_engine = "lmdb"
```

Co-authored-by: Alex Auvolat <alex@adnab.me>
Reviewed-on: https://git.deuxfleurs.fr/Deuxfleurs/garage/pulls/322
Co-authored-by: Alex <alex@adnab.me>
Co-committed-by: Alex <alex@adnab.me>

39 files changed, 2362 insertions, 413 deletions

diff --git a/src/api/admin/cluster.rs b/src/api/admin/cluster.rs
index 6d01317d..4b7716a3 100644
--- a/src/api/admin/cluster.rs
+++ b/src/api/admin/cluster.rs
@@ -19,6 +19,7 @@ pub async fn handle_get_cluster_status(garage: &Arc<Garage>) -> Result<Response<
 	let res = GetClusterStatusResponse {
 		node: hex::encode(garage.system.id),
 		garage_version: garage.system.garage_version(),
+		db_engine: garage.db.engine(),
 		known_nodes: garage
 			.system
 			.get_known_nodes()
@@ -98,6 +99,7 @@ fn get_cluster_layout(garage: &Arc<Garage>) -> GetClusterLayoutResponse {
 struct GetClusterStatusResponse {
 	node: String,
 	garage_version: &'static str,
+	db_engine: String,
 	known_nodes: HashMap<String, KnownNodeResp>,
 	layout: GetClusterLayoutResponse,
 }
diff --git a/src/block/Cargo.toml b/src/block/Cargo.toml
index 9cba69ee..80346aca 100644
--- a/src/block/Cargo.toml
+++ b/src/block/Cargo.toml
@@ -14,6 +14,7 @@ path = "lib.rs"
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
 [dependencies]
+garage_db = { version = "0.8.0", path = "../db" }
 garage_rpc = { version = "0.7.0", path = "../rpc" }
 garage_util = { version = "0.7.0", path = "../util" }
 garage_table = { version = "0.7.0", path = "../table" }
@@ -27,8 +28,6 @@ tracing = "0.1.30"
 rand = "0.8"
 zstd = { version = "0.9", default-features = false }
 
-sled = "0.34"
-
 rmp-serde = "0.15"
 serde = { version = "1.0", default-features = false, features = ["derive", "rc"] }
 serde_bytes = "0.11"
diff --git a/src/block/manager.rs b/src/block/manager.rs
index 9b2d9cad..32ba0431 100644
--- a/src/block/manager.rs
+++ b/src/block/manager.rs
@@ -1,3 +1,5 @@
+use core::ops::Bound;
+
 use std::convert::TryInto;
 use std::path::{Path, PathBuf};
 use std::sync::Arc;
@@ -17,10 +19,12 @@ use opentelemetry::{
 	Context, KeyValue,
 };
 
+use garage_db as db;
+use garage_db::counted_tree_hack::CountedTree;
+
 use garage_util::data::*;
 use garage_util::error::*;
 use garage_util::metrics::RecordDuration;
-use garage_util::sled_counter::SledCountedTree;
 use garage_util::time::*;
 use garage_util::tranquilizer::Tranquilizer;
 
@@ -91,9 +95,9 @@ pub struct BlockManager {
 
 	rc: BlockRc,
 
-	resync_queue: SledCountedTree,
+	resync_queue: CountedTree,
 	resync_notify: Notify,
-	resync_errors: SledCountedTree,
+	resync_errors: CountedTree,
 
 	system: Arc<System>,
 	endpoint: Arc<Endpoint<BlockRpc, Self>>,
@@ -108,7 +112,7 @@ struct BlockManagerLocked();
 
 impl BlockManager {
 	pub fn new(
-		db: &sled::Db,
+		db: &db::Db,
 		data_dir: PathBuf,
 		compression_level: Option<i32>,
 		background_tranquility: u32,
@@ -123,12 +127,14 @@ impl BlockManager {
 		let resync_queue = db
 			.open_tree("block_local_resync_queue")
 			.expect("Unable to open block_local_resync_queue tree");
-		let resync_queue = SledCountedTree::new(resync_queue);
+		let resync_queue =
+			CountedTree::new(resync_queue).expect("Could not count block_local_resync_queue");
 
 		let resync_errors = db
 			.open_tree("block_local_resync_errors")
 			.expect("Unable to open block_local_resync_errors tree");
-		let resync_errors = SledCountedTree::new(resync_errors);
+		let resync_errors =
+			CountedTree::new(resync_errors).expect("Could not count block_local_resync_errors");
 
 		let endpoint = system
 			.netapp
@@ -219,11 +225,44 @@ impl BlockManager {
 	/// to fix any mismatch between the two.
 	pub async fn repair_data_store(&self, must_exit: &watch::Receiver<bool>) -> Result<(), Error> {
 		// 1. Repair blocks from RC table.
-		for (i, entry) in self.rc.rc.iter().enumerate() {
-			let (hash, _) = entry?;
-			let hash = Hash::try_from(&hash[..]).unwrap();
-			self.put_to_resync(&hash, Duration::from_secs(0))?;
-			if i & 0xFF == 0 && *must_exit.borrow() {
+		let mut next_start: Option<Hash> = None;
+		loop {
+			// We have to do this complicated two-step process where we first read a bunch
+			// of hashes from the RC table, and then insert them in the to-resync queue,
+			// because of SQLite. Basically, as long as we have an iterator on a DB table,
+			// we can't do anything else on the DB. The naive approach (which we had previously)
+			// of just iterating on the RC table and inserting items one to one in the resync
+			// queue can't work here, it would just provoke a deadlock in the SQLite adapter code.
+			// This is mostly because the Rust bindings for SQLite assume a worst-case scenario
+			// where SQLite is not compiled in thread-safe mode, so we have to wrap everything
+			// in a mutex (see db/sqlite_adapter.rs and discussion in PR #322).
+			let mut batch_of_hashes = vec![];
+			let start_bound = match next_start.as_ref() {
+				None => Bound::Unbounded,
+				Some(x) => Bound::Excluded(x.as_slice()),
+			};
+			for entry in self
+				.rc
+				.rc
+				.range::<&[u8], _>((start_bound, Bound::Unbounded))?
+			{
+				let (hash, _) = entry?;
+				let hash = Hash::try_from(&hash[..]).unwrap();
+				batch_of_hashes.push(hash);
+				if batch_of_hashes.len() >= 1000 {
+					break;
+				}
+			}
+			if batch_of_hashes.is_empty() {
+				break;
+			}
+
+			for hash in batch_of_hashes.into_iter() {
+				self.put_to_resync(&hash, Duration::from_secs(0))?;
+				next_start = Some(hash)
+			}
+
+			if *must_exit.borrow() {
 				return Ok(());
 			}
 		}
@@ -264,46 +303,69 @@ impl BlockManager {
 	}
 
 	/// Get lenght of resync queue
-	pub fn resync_queue_len(&self) -> usize {
-		self.resync_queue.len()
+	pub fn resync_queue_len(&self) -> Result<usize, Error> {
+		// This currently can't return an error because the CountedTree hack
+		// doesn't error on .len(), but this will change when we remove the hack
+		// (hopefully someday!)
+		Ok(self.resync_queue.len())
 	}
 
 	/// Get number of blocks that have an error
-	pub fn resync_errors_len(&self) -> usize {
-		self.resync_errors.len()
+	pub fn resync_errors_len(&self) -> Result<usize, Error> {
+		// (see resync_queue_len comment)
+		Ok(self.resync_errors.len())
 	}
 
 	/// Get number of items in the refcount table
-	pub fn rc_len(&self) -> usize {
-		self.rc.rc.len()
+	pub fn rc_len(&self) -> Result<usize, Error> {
+		Ok(self.rc.rc.len()?)
 	}
 
 	//// ----- Managing the reference counter ----
 
 	/// Increment the number of time a block is used, putting it to resynchronization if it is
 	/// required, but not known
-	pub fn block_incref(&self, hash: &Hash) -> Result<(), Error> {
-		if self.rc.block_incref(hash)? {
+	pub fn block_incref(
+		self: &Arc<Self>,
+		tx: &mut db::Transaction,
+		hash: Hash,
+	) -> db::TxOpResult<()> {
+		if self.rc.block_incref(tx, &hash)? {
 			// When the reference counter is incremented, there is
 			// normally a node that is responsible for sending us the
 			// data of the block. However that operation may fail,
 			// so in all cases we add the block here to the todo list
 			// to check later that it arrived correctly, and if not
 			// we will fecth it from someone.
-			self.put_to_resync(hash, 2 * BLOCK_RW_TIMEOUT)?;
+			let this = self.clone();
+			tokio::spawn(async move {
+				if let Err(e) = this.put_to_resync(&hash, 2 * BLOCK_RW_TIMEOUT) {
+					error!("Block {:?} could not be put in resync queue: {}.", hash, e);
+				}
+			});
 		}
 		Ok(())
 	}
 
 	/// Decrement the number of time a block is used
-	pub fn block_decref(&self, hash: &Hash) -> Result<(), Error> {
-		if self.rc.block_decref(hash)? {
+	pub fn block_decref(
+		self: &Arc<Self>,
+		tx: &mut db::Transaction,
+		hash: Hash,
+	) -> db::TxOpResult<()> {
+		if self.rc.block_decref(tx, &hash)? {
 			// When the RC is decremented, it might drop to zero,
 			// indicating that we don't need the block.
 			// There is a delay before we garbage collect it;
 			// make sure that it is handled in the resync loop
 			// after that delay has passed.
-			self.put_to_resync(hash, BLOCK_GC_DELAY + Duration::from_secs(10))?;
+			let this = self.clone();
+			tokio::spawn(async move {
+				if let Err(e) = this.put_to_resync(&hash, BLOCK_GC_DELAY + Duration::from_secs(10))
+				{
+					error!("Block {:?} could not be put in resync queue: {}.", hash, e);
+				}
+			});
 		}
 		Ok(())
 	}
@@ -503,12 +565,12 @@ impl BlockManager {
 		});
 	}
 
-	fn put_to_resync(&self, hash: &Hash, delay: Duration) -> Result<(), sled::Error> {
+	fn put_to_resync(&self, hash: &Hash, delay: Duration) -> db::Result<()> {
 		let when = now_msec() + delay.as_millis() as u64;
 		self.put_to_resync_at(hash, when)
 	}
 
-	fn put_to_resync_at(&self, hash: &Hash, when: u64) -> Result<(), sled::Error> {
+	fn put_to_resync_at(&self, hash: &Hash, when: u64) -> db::Result<()> {
 		trace!("Put resync_queue: {} {:?}", when, hash);
 		let mut key = u64::to_be_bytes(when).to_vec();
 		key.extend(hash.as_ref());
@@ -547,13 +609,8 @@ impl BlockManager {
 	// - Ok(true) -> a block was processed (successfully or not)
 	// - Ok(false) -> no block was processed, but we are ready for the next iteration
 	// - Err(_) -> a Sled error occurred when reading/writing from resync_queue/resync_errors
-	async fn resync_iter(
-		&self,
-		must_exit: &mut watch::Receiver<bool>,
-	) -> Result<bool, sled::Error> {
-		if let Some(first_pair_res) = self.resync_queue.iter().next() {
-			let (time_bytes, hash_bytes) = first_pair_res?;
-
+	async fn resync_iter(&self, must_exit: &mut watch::Receiver<bool>) -> Result<bool, db::Error> {
+		if let Some((time_bytes, hash_bytes)) = self.resync_queue.first()? {
 			let time_msec = u64::from_be_bytes(time_bytes[0..8].try_into().unwrap());
 			let now = now_msec();
 
@@ -561,7 +618,7 @@ impl BlockManager {
 				let hash = Hash::try_from(&hash_bytes[..]).unwrap();
 
 				if let Some(ec) = self.resync_errors.get(hash.as_slice())? {
-					let ec = ErrorCounter::decode(ec);
+					let ec = ErrorCounter::decode(&ec);
 					if now < ec.next_try() {
 						// if next retry after an error is not yet,
 						// don't do resync and return early, but still
@@ -602,7 +659,7 @@ impl BlockManager {
 					warn!("Error when resyncing {:?}: {}", hash, e);
 
 					let err_counter = match self.resync_errors.get(hash.as_slice())? {
-						Some(ec) => ErrorCounter::decode(ec).add1(now + 1),
+						Some(ec) => ErrorCounter::decode(&ec).add1(now + 1),
 						None => ErrorCounter::new(now + 1),
 					};
 
@@ -966,7 +1023,7 @@ impl ErrorCounter {
 		}
 	}
 
-	fn decode(data: sled::IVec) -> Self {
+	fn decode(data: &[u8]) -> Self {
 		Self {
 			errors: u64::from_be_bytes(data[0..8].try_into().unwrap()),
 			last_try: u64::from_be_bytes(data[8..16].try_into().unwrap()),
diff --git a/src/block/metrics.rs b/src/block/metrics.rs
index f0f541a3..477add66 100644
--- a/src/block/metrics.rs
+++ b/src/block/metrics.rs
@@ -1,6 +1,6 @@
 use opentelemetry::{global, metrics::*};
 
-use garage_util::sled_counter::SledCountedTree;
+use garage_db::counted_tree_hack::CountedTree;
 
 /// TableMetrics reference all counter used for metrics
 pub struct BlockManagerMetrics {
@@ -23,7 +23,7 @@ pub struct BlockManagerMetrics {
 }
 
 impl BlockManagerMetrics {
-	pub fn new(resync_queue: SledCountedTree, resync_errors: SledCountedTree) -> Self {
+	pub fn new(resync_queue: CountedTree, resync_errors: CountedTree) -> Self {
 		let meter = global::meter("garage_model/block");
 		Self {
 			_resync_queue_len: meter
diff --git a/src/block/rc.rs b/src/block/rc.rs
index ec3ea44e..ce6defad 100644
--- a/src/block/rc.rs
+++ b/src/block/rc.rs
@@ -1,5 +1,7 @@
 use std::convert::TryInto;
 
+use garage_db as db;
+
 use garage_util::data::*;
 use garage_util::error::*;
 use garage_util::time::*;
@@ -7,31 +9,41 @@ use garage_util::time::*;
 use crate::manager::BLOCK_GC_DELAY;
 
 pub struct BlockRc {
-	pub(crate) rc: sled::Tree,
+	pub(crate) rc: db::Tree,
 }
 
 impl BlockRc {
-	pub(crate) fn new(rc: sled::Tree) -> Self {
+	pub(crate) fn new(rc: db::Tree) -> Self {
 		Self { rc }
 	}
 
 	/// Increment the reference counter associated to a hash.
 	/// Returns true if the RC goes from zero to nonzero.
-	pub(crate) fn block_incref(&self, hash: &Hash) -> Result<bool, Error> {
-		let old_rc = self
-			.rc
-			.fetch_and_update(&hash, |old| RcEntry::parse_opt(old).increment().serialize())?;
-		let old_rc = RcEntry::parse_opt(old_rc);
+	pub(crate) fn block_incref(
+		&self,
+		tx: &mut db::Transaction,
+		hash: &Hash,
+	) -> db::TxOpResult<bool> {
+		let old_rc = RcEntry::parse_opt(tx.get(&self.rc, &hash)?);
+		match old_rc.increment().serialize() {
+			Some(x) => tx.insert(&self.rc, &hash, x)?,
+			None => unreachable!(),
+		};
 		Ok(old_rc.is_zero())
 	}
 
 	/// Decrement the reference counter associated to a hash.
 	/// Returns true if the RC is now zero.
-	pub(crate) fn block_decref(&self, hash: &Hash) -> Result<bool, Error> {
-		let new_rc = self
-			.rc
-			.update_and_fetch(&hash, |old| RcEntry::parse_opt(old).decrement().serialize())?;
-		let new_rc = RcEntry::parse_opt(new_rc);
+	pub(crate) fn block_decref(
+		&self,
+		tx: &mut db::Transaction,
+		hash: &Hash,
+	) -> db::TxOpResult<bool> {
+		let new_rc = RcEntry::parse_opt(tx.get(&self.rc, &hash)?).decrement();
+		match new_rc.serialize() {
+			Some(x) => tx.insert(&self.rc, &hash, x)?,
+			None => tx.remove(&self.rc, &hash)?,
+		};
 		Ok(matches!(new_rc, RcEntry::Deletable { .. }))
 	}
 
@@ -44,12 +56,15 @@ impl BlockRc {
 	/// deletion time has passed
 	pub(crate) fn clear_deleted_block_rc(&self, hash: &Hash) -> Result<(), Error> {
 		let now = now_msec();
-		self.rc.update_and_fetch(&hash, |rcval| {
-			let updated = match RcEntry::parse_opt(rcval) {
-				RcEntry::Deletable { at_time } if now > at_time => RcEntry::Absent,
-				v => v,
+		self.rc.db().transaction(|mut tx| {
+			let rcval = RcEntry::parse_opt(tx.get(&self.rc, &hash)?);
+			match rcval {
+				RcEntry::Deletable { at_time } if now > at_time => {
+					tx.remove(&self.rc, &hash)?;
+				}
+				_ => (),
 			};
-			updated.serialize()
+			tx.commit(())
 		})?;
 		Ok(())
 	}
diff --git a/src/db/Cargo.toml b/src/db/Cargo.toml
new file mode 100644
index 00000000..6d8f64be
--- /dev/null
+++ b/src/db/Cargo.toml
@@ -0,0 +1,36 @@
+[package]
+name = "garage_db"
+version = "0.8.0"
+authors = ["Alex Auvolat <alex@adnab.me>"]
+edition = "2018"
+license = "AGPL-3.0"
+description = "Abstraction over multiple key/value storage engines that supports transactions"
+repository = "https://git.deuxfleurs.fr/Deuxfleurs/garage"
+readme = "../../README.md"
+
+[lib]
+path = "lib.rs"
+
+[[bin]]
+name = "convert"
+path = "bin/convert.rs"
+required-features = ["cli"]
+
+[dependencies]
+err-derive = "0.3"
+hexdump = "0.1"
+log = "0.4"
+
+heed = "0.11"
+rusqlite = { version = "0.27", features = ["bundled"] }
+sled = "0.34"
+
+# cli deps
+clap = { version = "3.1.18", optional = true, features = ["derive", "env"] }
+pretty_env_logger = { version = "0.4", optional = true }
+
+[dev-dependencies]
+mktemp = "0.4"
+
+[features]
+cli = ["clap", "pretty_env_logger"]
diff --git a/src/db/bin/convert.rs b/src/db/bin/convert.rs
new file mode 100644
index 00000000..9e45e61f
--- /dev/null
+++ b/src/db/bin/convert.rs
@@ -0,0 +1,76 @@
+use std::path::PathBuf;
+
+use garage_db::*;
+
+use clap::Parser;
+
+/// K2V command line interface
+#[derive(Parser, Debug)]
+#[clap(author, version, about, long_about = None)]
+struct Args {
+	/// Input DB path
+	#[clap(short = 'i')]
+	input_path: PathBuf,
+	/// Input DB engine
+	#[clap(short = 'a')]
+	input_engine: String,
+
+	/// Output DB path
+	#[clap(short = 'o')]
+	output_path: PathBuf,
+	/// Output DB engine
+	#[clap(short = 'b')]
+	output_engine: String,
+}
+
+fn main() {
+	let args = Args::parse();
+	pretty_env_logger::init();
+
+	match do_conversion(args) {
+		Ok(()) => println!("Success!"),
+		Err(e) => eprintln!("Error: {}", e),
+	}
+}
+
+fn do_conversion(args: Args) -> Result<()> {
+	let input = open_db(args.input_path, args.input_engine)?;
+	let output = open_db(args.output_path, args.output_engine)?;
+	output.import(&input)?;
+	Ok(())
+}
+
+fn open_db(path: PathBuf, engine: String) -> Result<Db> {
+	match engine.as_str() {
+		"sled" => {
+			let db = sled_adapter::sled::Config::default().path(&path).open()?;
+			Ok(sled_adapter::SledDb::init(db))
+		}
+		"sqlite" | "sqlite3" | "rusqlite" => {
+			let db = sqlite_adapter::rusqlite::Connection::open(&path)?;
+			Ok(sqlite_adapter::SqliteDb::init(db))
+		}
+		"lmdb" | "heed" => {
+			std::fs::create_dir_all(&path).map_err(|e| {
+				Error(format!("Unable to create LMDB data directory: {}", e).into())
+			})?;
+
+			let map_size = if u32::MAX as usize == usize::MAX {
+				eprintln!(
+					"LMDB is not recommended on 32-bit systems, database size will be limited"
+				);
+				1usize << 30 // 1GB for 32-bit systems
+			} else {
+				1usize << 40 // 1TB for 64-bit systems
+			};
+
+			let db = lmdb_adapter::heed::EnvOpenOptions::new()
+				.max_dbs(100)
+				.map_size(map_size)
+				.open(&path)
+				.unwrap();
+			Ok(lmdb_adapter::LmdbDb::init(db))
+		}
+		e => Err(Error(format!("Invalid DB engine: {}", e).into())),
+	}
+}
diff --git a/src/db/counted_tree_hack.rs b/src/db/counted_tree_hack.rs
new file mode 100644
index 00000000..bbe943a2
--- /dev/null
+++ b/src/db/counted_tree_hack.rs
@@ -0,0 +1,127 @@
+//! This hack allows a db tree to keep in RAM a counter of the number of entries
+//! it contains, which is used to call .len() on it.  This is usefull only for
+//! the sled backend where .len() otherwise would have to traverse the whole
+//! tree to count items.  For sqlite and lmdb, this is mostly useless (but
+//! hopefully not harmfull!). Note that a CountedTree cannot be part of a
+//! transaction.
+
+use std::sync::{
+	atomic::{AtomicUsize, Ordering},
+	Arc,
+};
+
+use crate::{Result, Tree, TxError, Value, ValueIter};
+
+#[derive(Clone)]
+pub struct CountedTree(Arc<CountedTreeInternal>);
+
+struct CountedTreeInternal {
+	tree: Tree,
+	len: AtomicUsize,
+}
+
+impl CountedTree {
+	pub fn new(tree: Tree) -> Result<Self> {
+		let len = tree.len()?;
+		Ok(Self(Arc::new(CountedTreeInternal {
+			tree,
+			len: AtomicUsize::new(len),
+		})))
+	}
+
+	pub fn len(&self) -> usize {
+		self.0.len.load(Ordering::SeqCst)
+	}
+
+	pub fn is_empty(&self) -> bool {
+		self.len() == 0
+	}
+
+	pub fn get<K: AsRef<[u8]>>(&self, key: K) -> Result<Option<Value>> {
+		self.0.tree.get(key)
+	}
+
+	pub fn first(&self) -> Result<Option<(Value, Value)>> {
+		self.0.tree.first()
+	}
+
+	pub fn iter(&self) -> Result<ValueIter<'_>> {
+		self.0.tree.iter()
+	}
+
+	// ---- writing functions ----
+
+	pub fn insert<K, V>(&self, key: K, value: V) -> Result<Option<Value>>
+	where
+		K: AsRef<[u8]>,
+		V: AsRef<[u8]>,
+	{
+		let old_val = self.0.tree.insert(key, value)?;
+		if old_val.is_none() {
+			self.0.len.fetch_add(1, Ordering::SeqCst);
+		}
+		Ok(old_val)
+	}
+
+	pub fn remove<K: AsRef<[u8]>>(&self, key: K) -> Result<Option<Value>> {
+		let old_val = self.0.tree.remove(key)?;
+		if old_val.is_some() {
+			self.0.len.fetch_sub(1, Ordering::SeqCst);
+		}
+		Ok(old_val)
+	}
+
+	pub fn compare_and_swap<K, OV, NV>(
+		&self,
+		key: K,
+		expected_old: Option<OV>,
+		new: Option<NV>,
+	) -> Result<bool>
+	where
+		K: AsRef<[u8]>,
+		OV: AsRef<[u8]>,
+		NV: AsRef<[u8]>,
+	{
+		let old_some = expected_old.is_some();
+		let new_some = new.is_some();
+
+		let tx_res = self.0.tree.db().transaction(|mut tx| {
+			let old_val = tx.get(&self.0.tree, &key)?;
+			let is_same = match (&old_val, &expected_old) {
+				(None, None) => true,
+				(Some(x), Some(y)) if x == y.as_ref() => true,
+				_ => false,
+			};
+			if is_same {
+				match &new {
+					Some(v) => {
+						tx.insert(&self.0.tree, &key, v)?;
+					}
+					None => {
+						tx.remove(&self.0.tree, &key)?;
+					}
+				}
+				tx.commit(())
+			} else {
+				tx.abort(())
+			}
+		});
+
+		match tx_res {
+			Ok(()) => {
+				match (old_some, new_some) {
+					(false, true) => {
+						self.0.len.fetch_add(1, Ordering::SeqCst);
+					}
+					(true, false) => {
+						self.0.len.fetch_sub(1, Ordering::SeqCst);
+					}
+					_ => (),
+				}
+				Ok(true)
+			}
+			Err(TxError::Abort(())) => Ok(false),
+			Err(TxError::Db(e)) => Err(e),
+		}
+	}
+}
diff --git a/src/db/lib.rs b/src/db/lib.rs
new file mode 100644
index 00000000..e9d3ea18
--- /dev/null
+++ b/src/db/lib.rs
@@ -0,0 +1,400 @@
+pub mod lmdb_adapter;
+pub mod sled_adapter;
+pub mod sqlite_adapter;
+
+pub mod counted_tree_hack;
+
+#[cfg(test)]
+pub mod test;
+
+use core::ops::{Bound, RangeBounds};
+
+use std::borrow::Cow;
+use std::cell::Cell;
+use std::sync::Arc;
+
+use err_derive::Error;
+
+#[derive(Clone)]
+pub struct Db(pub(crate) Arc<dyn IDb>);
+
+pub struct Transaction<'a>(&'a mut dyn ITx);
+
+#[derive(Clone)]
+pub struct Tree(Arc<dyn IDb>, usize);
+
+pub type Value = Vec<u8>;
+pub type ValueIter<'a> = Box<dyn std::iter::Iterator<Item = Result<(Value, Value)>> + 'a>;
+pub type TxValueIter<'a> = Box<dyn std::iter::Iterator<Item = TxOpResult<(Value, Value)>> + 'a>;
+
+// ----
+
+#[derive(Debug, Error)]
+#[error(display = "{}", _0)]
+pub struct Error(pub Cow<'static, str>);
+
+pub type Result<T> = std::result::Result<T, Error>;
+
+#[derive(Debug, Error)]
+#[error(display = "{}", _0)]
+pub struct TxOpError(pub(crate) Error);
+pub type TxOpResult<T> = std::result::Result<T, TxOpError>;
+
+pub enum TxError<E> {
+	Abort(E),
+	Db(Error),
+}
+pub type TxResult<R, E> = std::result::Result<R, TxError<E>>;
+
+impl<E> From<TxOpError> for TxError<E> {
+	fn from(e: TxOpError) -> TxError<E> {
+		TxError::Db(e.0)
+	}
+}
+
+pub fn unabort<R, E>(res: TxResult<R, E>) -> TxOpResult<std::result::Result<R, E>> {
+	match res {
+		Ok(v) => Ok(Ok(v)),
+		Err(TxError::Abort(e)) => Ok(Err(e)),
+		Err(TxError::Db(e)) => Err(TxOpError(e)),
+	}
+}
+
+// ----
+
+impl Db {
+	pub fn engine(&self) -> String {
+		self.0.engine()
+	}
+
+	pub fn open_tree<S: AsRef<str>>(&self, name: S) -> Result<Tree> {
+		let tree_id = self.0.open_tree(name.as_ref())?;
+		Ok(Tree(self.0.clone(), tree_id))
+	}
+
+	pub fn list_trees(&self) -> Result<Vec<String>> {
+		self.0.list_trees()
+	}
+
+	pub fn transaction<R, E, F>(&self, fun: F) -> TxResult<R, E>
+	where
+		F: Fn(Transaction<'_>) -> TxResult<R, E>,
+	{
+		let f = TxFn {
+			function: fun,
+			result: Cell::new(None),
+		};
+		let tx_res = self.0.transaction(&f);
+		let ret = f
+			.result
+			.into_inner()
+			.expect("Transaction did not store result");
+
+		match tx_res {
+			Ok(()) => {
+				assert!(matches!(ret, Ok(_)));
+				ret
+			}
+			Err(TxError::Abort(())) => {
+				assert!(matches!(ret, Err(TxError::Abort(_))));
+				ret
+			}
+			Err(TxError::Db(e2)) => match ret {
+				// Ok was stored -> the error occured when finalizing
+				// transaction
+				Ok(_) => Err(TxError::Db(e2)),
+				// An error was already stored: that's the one we want to
+				// return
+				Err(TxError::Db(e)) => Err(TxError::Db(e)),
+				_ => unreachable!(),
+			},
+		}
+	}
+
+	pub fn import(&self, other: &Db) -> Result<()> {
+		let existing_trees = self.list_trees()?;
+		if !existing_trees.is_empty() {
+			return Err(Error(
+				format!(
+					"destination database already contains data: {:?}",
+					existing_trees
+				)
+				.into(),
+			));
+		}
+
+		let tree_names = other.list_trees()?;
+		for name in tree_names {
+			let tree = self.open_tree(&name)?;
+			if tree.len()? > 0 {
+				return Err(Error(format!("tree {} already contains data", name).into()));
+			}
+
+			let ex_tree = other.open_tree(&name)?;
+
+			let tx_res = self.transaction(|mut tx| {
+				let mut i = 0;
+				for item in ex_tree.iter().map_err(TxError::Abort)? {
+					let (k, v) = item.map_err(TxError::Abort)?;
+					tx.insert(&tree, k, v)?;
+					i += 1;
+					if i % 1000 == 0 {
+						println!("{}: imported {}", name, i);
+					}
+				}
+				tx.commit(i)
+			});
+			let total = match tx_res {
+				Err(TxError::Db(e)) => return Err(e),
+				Err(TxError::Abort(e)) => return Err(e),
+				Ok(x) => x,
+			};
+
+			println!("{}: finished importing, {} items", name, total);
+		}
+		Ok(())
+	}
+}
+
+#[allow(clippy::len_without_is_empty)]
+impl Tree {
+	#[inline]
+	pub fn db(&self) -> Db {
+		Db(self.0.clone())
+	}
+
+	#[inline]
+	pub fn get<T: AsRef<[u8]>>(&self, key: T) -> Result<Option<Value>> {
+		self.0.get(self.1, key.as_ref())
+	}
+	#[inline]
+	pub fn len(&self) -> Result<usize> {
+		self.0.len(self.1)
+	}
+
+	#[inline]
+	pub fn first(&self) -> Result<Option<(Value, Value)>> {
+		self.iter()?.next().transpose()
+	}
+	#[inline]
+	pub fn get_gt<T: AsRef<[u8]>>(&self, from: T) -> Result<Option<(Value, Value)>> {
+		self.range((Bound::Excluded(from), Bound::Unbounded))?
+			.next()
+			.transpose()
+	}
+
+	/// Returns the old value if there was one
+	#[inline]
+	pub fn insert<T: AsRef<[u8]>, U: AsRef<[u8]>>(
+		&self,
+		key: T,
+		value: U,
+	) -> Result<Option<Value>> {
+		self.0.insert(self.1, key.as_ref(), value.as_ref())
+	}
+	/// Returns the old value if there was one
+	#[inline]
+	pub fn remove<T: AsRef<[u8]>>(&self, key: T) -> Result<Option<Value>> {
+		self.0.remove(self.1, key.as_ref())
+	}
+
+	#[inline]
+	pub fn iter(&self) -> Result<ValueIter<'_>> {
+		self.0.iter(self.1)
+	}
+	#[inline]
+	pub fn iter_rev(&self) -> Result<ValueIter<'_>> {
+		self.0.iter_rev(self.1)
+	}
+
+	#[inline]
+	pub fn range<K, R>(&self, range: R) -> Result<ValueIter<'_>>
+	where
+		K: AsRef<[u8]>,
+		R: RangeBounds<K>,
+	{
+		let sb = range.start_bound();
+		let eb = range.end_bound();
+		self.0.range(self.1, get_bound(sb), get_bound(eb))
+	}
+	#[inline]
+	pub fn range_rev<K, R>(&self, range: R) -> Result<ValueIter<'_>>
+	where
+		K: AsRef<[u8]>,
+		R: RangeBounds<K>,
+	{
+		let sb = range.start_bound();
+		let eb = range.end_bound();
+		self.0.range_rev(self.1, get_bound(sb), get_bound(eb))
+	}
+}
+
+#[allow(clippy::len_without_is_empty)]
+impl<'a> Transaction<'a> {
+	#[inline]
+	pub fn get<T: AsRef<[u8]>>(&self, tree: &Tree, key: T) -> TxOpResult<Option<Value>> {
+		self.0.get(tree.1, key.as_ref())
+	}
+	#[inline]
+	pub fn len(&self, tree: &Tree) -> TxOpResult<usize> {
+		self.0.len(tree.1)
+	}
+
+	/// Returns the old value if there was one
+	#[inline]
+	pub fn insert<T: AsRef<[u8]>, U: AsRef<[u8]>>(
+		&mut self,
+		tree: &Tree,
+		key: T,
+		value: U,
+	) -> TxOpResult<Option<Value>> {
+		self.0.insert(tree.1, key.as_ref(), value.as_ref())
+	}
+	/// Returns the old value if there was one
+	#[inline]
+	pub fn remove<T: AsRef<[u8]>>(&mut self, tree: &Tree, key: T) -> TxOpResult<Option<Value>> {
+		self.0.remove(tree.1, key.as_ref())
+	}
+
+	#[inline]
+	pub fn iter(&self, tree: &Tree) -> TxOpResult<TxValueIter<'_>> {
+		self.0.iter(tree.1)
+	}
+	#[inline]
+	pub fn iter_rev(&self, tree: &Tree) -> TxOpResult<TxValueIter<'_>> {
+		self.0.iter_rev(tree.1)
+	}
+
+	#[inline]
+	pub fn range<K, R>(&self, tree: &Tree, range: R) -> TxOpResult<TxValueIter<'_>>
+	where
+		K: AsRef<[u8]>,
+		R: RangeBounds<K>,
+	{
+		let sb = range.start_bound();
+		let eb = range.end_bound();
+		self.0.range(tree.1, get_bound(sb), get_bound(eb))
+	}
+	#[inline]
+	pub fn range_rev<K, R>(&self, tree: &Tree, range: R) -> TxOpResult<TxValueIter<'_>>
+	where
+		K: AsRef<[u8]>,
+		R: RangeBounds<K>,
+	{
+		let sb = range.start_bound();
+		let eb = range.end_bound();
+		self.0.range_rev(tree.1, get_bound(sb), get_bound(eb))
+	}
+
+	// ----
+
+	#[inline]
+	pub fn abort<R, E>(self, e: E) -> TxResult<R, E> {
+		Err(TxError::Abort(e))
+	}
+
+	#[inline]
+	pub fn commit<R, E>(self, r: R) -> TxResult<R, E> {
+		Ok(r)
+	}
+}
+
+// ---- Internal interfaces
+
+pub(crate) trait IDb: Send + Sync {
+	fn engine(&self) -> String;
+	fn open_tree(&self, name: &str) -> Result<usize>;
+	fn list_trees(&self) -> Result<Vec<String>>;
+
+	fn get(&self, tree: usize, key: &[u8]) -> Result<Option<Value>>;
+	fn len(&self, tree: usize) -> Result<usize>;
+
+	fn insert(&self, tree: usize, key: &[u8], value: &[u8]) -> Result<Option<Value>>;
+	fn remove(&self, tree: usize, key: &[u8]) -> Result<Option<Value>>;
+
+	fn iter(&self, tree: usize) -> Result<ValueIter<'_>>;
+	fn iter_rev(&self, tree: usize) -> Result<ValueIter<'_>>;
+
+	fn range<'r>(
+		&self,
+		tree: usize,
+		low: Bound<&'r [u8]>,
+		high: Bound<&'r [u8]>,
+	) -> Result<ValueIter<'_>>;
+	fn range_rev<'r>(
+		&self,
+		tree: usize,
+		low: Bound<&'r [u8]>,
+		high: Bound<&'r [u8]>,
+	) -> Result<ValueIter<'_>>;
+
+	fn transaction(&self, f: &dyn ITxFn) -> TxResult<(), ()>;
+}
+
+pub(crate) trait ITx {
+	fn get(&self, tree: usize, key: &[u8]) -> TxOpResult<Option<Value>>;
+	fn len(&self, tree: usize) -> TxOpResult<usize>;
+
+	fn insert(&mut self, tree: usize, key: &[u8], value: &[u8]) -> TxOpResult<Option<Value>>;
+	fn remove(&mut self, tree: usize, key: &[u8]) -> TxOpResult<Option<Value>>;
+
+	fn iter(&self, tree: usize) -> TxOpResult<TxValueIter<'_>>;
+	fn iter_rev(&self, tree: usize) -> TxOpResult<TxValueIter<'_>>;
+
+	fn range<'r>(
+		&self,
+		tree: usize,
+		low: Bound<&'r [u8]>,
+		high: Bound<&'r [u8]>,
+	) -> TxOpResult<TxValueIter<'_>>;
+	fn range_rev<'r>(
+		&self,
+		tree: usize,
+		low: Bound<&'r [u8]>,
+		high: Bound<&'r [u8]>,
+	) -> TxOpResult<TxValueIter<'_>>;
+}
+
+pub(crate) trait ITxFn {
+	fn try_on(&self, tx: &mut dyn ITx) -> TxFnResult;
+}
+
+pub(crate) enum TxFnResult {
+	Ok,
+	Abort,
+	DbErr,
+}
+
+struct TxFn<F, R, E>
+where
+	F: Fn(Transaction<'_>) -> TxResult<R, E>,
+{
+	function: F,
+	result: Cell<Option<TxResult<R, E>>>,
+}
+
+impl<F, R, E> ITxFn for TxFn<F, R, E>
+where
+	F: Fn(Transaction<'_>) -> TxResult<R, E>,
+{
+	fn try_on(&self, tx: &mut dyn ITx) -> TxFnResult {
+		let res = (self.function)(Transaction(tx));
+		let res2 = match &res {
+			Ok(_) => TxFnResult::Ok,
+			Err(TxError::Abort(_)) => TxFnResult::Abort,
+			Err(TxError::Db(_)) => TxFnResult::DbErr,
+		};
+		self.result.set(Some(res));
+		res2
+	}
+}
+
+// ----
+
+fn get_bound<K: AsRef<[u8]>>(b: Bound<&K>) -> Bound<&[u8]> {
+	match b {
+		Bound::Included(v) => Bound::Included(v.as_ref()),
+		Bound::Excluded(v) => Bound::Excluded(v.as_ref()),
+		Bound::Unbounded => Bound::Unbounded,
+	}
+}
diff --git a/src/db/lmdb_adapter.rs b/src/db/lmdb_adapter.rs
new file mode 100644
index 00000000..74622919
--- /dev/null
+++ b/src/db/lmdb_adapter.rs
@@ -0,0 +1,329 @@
+use core::ops::Bound;
+use core::ptr::NonNull;
+
+use std::collections::HashMap;
+use std::convert::TryInto;
+use std::sync::{Arc, RwLock};
+
+use heed::types::ByteSlice;
+use heed::{BytesDecode, Env, RoTxn, RwTxn, UntypedDatabase as Database};
+
+use crate::{
+	Db, Error, IDb, ITx, ITxFn, Result, TxError, TxFnResult, TxOpError, TxOpResult, TxResult,
+	TxValueIter, Value, ValueIter,
+};
+
+pub use heed;
+
+// -- err
+
+impl From<heed::Error> for Error {
+	fn from(e: heed::Error) -> Error {
+		Error(format!("LMDB: {}", e).into())
+	}
+}
+
+impl From<heed::Error> for TxOpError {
+	fn from(e: heed::Error) -> TxOpError {
+		TxOpError(e.into())
+	}
+}
+
+// -- db
+
+pub struct LmdbDb {
+	db: heed::Env,
+	trees: RwLock<(Vec<Database>, HashMap<String, usize>)>,
+}
+
+impl LmdbDb {
+	pub fn init(db: Env) -> Db {
+		let s = Self {
+			db,
+			trees: RwLock::new((Vec::new(), HashMap::new())),
+		};
+		Db(Arc::new(s))
+	}
+
+	fn get_tree(&self, i: usize) -> Result<Database> {
+		self.trees
+			.read()
+			.unwrap()
+			.0
+			.get(i)
+			.cloned()
+			.ok_or_else(|| Error("invalid tree id".into()))
+	}
+}
+
+impl IDb for LmdbDb {
+	fn engine(&self) -> String {
+		"LMDB (using Heed crate)".into()
+	}
+
+	fn open_tree(&self, name: &str) -> Result<usize> {
+		let mut trees = self.trees.write().unwrap();
+		if let Some(i) = trees.1.get(name) {
+			Ok(*i)
+		} else {
+			let tree = self.db.create_database(Some(name))?;
+			let i = trees.0.len();
+			trees.0.push(tree);
+			trees.1.insert(name.to_string(), i);
+			Ok(i)
+		}
+	}
+
+	fn list_trees(&self) -> Result<Vec<String>> {
+		let tree0 = match self.db.open_database::<heed::types::Str, ByteSlice>(None)? {
+			Some(x) => x,
+			None => return Ok(vec![]),
+		};
+
+		let mut ret = vec![];
+		let tx = self.db.read_txn()?;
+		for item in tree0.iter(&tx)? {
+			let (tree_name, _) = item?;
+			ret.push(tree_name.to_string());
+		}
+		drop(tx);
+
+		let mut ret2 = vec![];
+		for tree_name in ret {
+			if self
+				.db
+				.open_database::<ByteSlice, ByteSlice>(Some(&tree_name))?
+				.is_some()
+			{
+				ret2.push(tree_name);
+			}
+		}
+
+		Ok(ret2)
+	}
+
+	// ----
+
+	fn get(&self, tree: usize, key: &[u8]) -> Result<Option<Value>> {
+		let tree = self.get_tree(tree)?;
+
+		let tx = self.db.read_txn()?;
+		let val = tree.get(&tx, key)?;
+		match val {
+			None => Ok(None),
+			Some(v) => Ok(Some(v.to_vec())),
+		}
+	}
+
+	fn len(&self, tree: usize) -> Result<usize> {
+		let tree = self.get_tree(tree)?;
+		let tx = self.db.read_txn()?;
+		Ok(tree.len(&tx)?.try_into().unwrap())
+	}
+
+	fn insert(&self, tree: usize, key: &[u8], value: &[u8]) -> Result<Option<Value>> {
+		let tree = self.get_tree(tree)?;
+		let mut tx = self.db.write_txn()?;
+		let old_val = tree.get(&tx, key)?.map(Vec::from);
+		tree.put(&mut tx, key, value)?;
+		tx.commit()?;
+		Ok(old_val)
+	}
+
+	fn remove(&self, tree: usize, key: &[u8]) -> Result<Option<Value>> {
+		let tree = self.get_tree(tree)?;
+		let mut tx = self.db.write_txn()?;
+		let old_val = tree.get(&tx, key)?.map(Vec::from);
+		tree.delete(&mut tx, key)?;
+		tx.commit()?;
+		Ok(old_val)
+	}
+
+	fn iter(&self, tree: usize) -> Result<ValueIter<'_>> {
+		let tree = self.get_tree(tree)?;
+		let tx = self.db.read_txn()?;
+		TxAndIterator::make(tx, |tx| Ok(tree.iter(tx)?))
+	}
+
+	fn iter_rev(&self, tree: usize) -> Result<ValueIter<'_>> {
+		let tree = self.get_tree(tree)?;
+		let tx = self.db.read_txn()?;
+		TxAndIterator::make(tx, |tx| Ok(tree.rev_iter(tx)?))
+	}
+
+	fn range<'r>(
+		&self,
+		tree: usize,
+		low: Bound<&'r [u8]>,
+		high: Bound<&'r [u8]>,
+	) -> Result<ValueIter<'_>> {
+		let tree = self.get_tree(tree)?;
+		let tx = self.db.read_txn()?;
+		TxAndIterator::make(tx, |tx| Ok(tree.range(tx, &(low, high))?))
+	}
+	fn range_rev<'r>(
+		&self,
+		tree: usize,
+		low: Bound<&'r [u8]>,
+		high: Bound<&'r [u8]>,
+	) -> Result<ValueIter<'_>> {
+		let tree = self.get_tree(tree)?;
+		let tx = self.db.read_txn()?;
+		TxAndIterator::make(tx, |tx| Ok(tree.rev_range(tx, &(low, high))?))
+	}
+
+	// ----
+
+	fn transaction(&self, f: &dyn ITxFn) -> TxResult<(), ()> {
+		let trees = self.trees.read().unwrap();
+		let mut tx = LmdbTx {
+			trees: &trees.0[..],
+			tx: self
+				.db
+				.write_txn()
+				.map_err(Error::from)
+				.map_err(TxError::Db)?,
+		};
+
+		let res = f.try_on(&mut tx);
+		match res {
+			TxFnResult::Ok => {
+				tx.tx.commit().map_err(Error::from).map_err(TxError::Db)?;
+				Ok(())
+			}
+			TxFnResult::Abort => {
+				tx.tx.abort().map_err(Error::from).map_err(TxError::Db)?;
+				Err(TxError::Abort(()))
+			}
+			TxFnResult::DbErr => {
+				tx.tx.abort().map_err(Error::from).map_err(TxError::Db)?;
+				Err(TxError::Db(Error(
+					"(this message will be discarded)".into(),
+				)))
+			}
+		}
+	}
+}
+
+// ----
+
+struct LmdbTx<'a> {
+	trees: &'a [Database],
+	tx: RwTxn<'a, 'a>,
+}
+
+impl<'a> LmdbTx<'a> {
+	fn get_tree(&self, i: usize) -> TxOpResult<&Database> {
+		self.trees.get(i).ok_or_else(|| {
+			TxOpError(Error(
+				"invalid tree id (it might have been openned after the transaction started)".into(),
+			))
+		})
+	}
+}
+
+impl<'a> ITx for LmdbTx<'a> {
+	fn get(&self, tree: usize, key: &[u8]) -> TxOpResult<Option<Value>> {
+		let tree = self.get_tree(tree)?;
+		match tree.get(&self.tx, key)? {
+			Some(v) => Ok(Some(v.to_vec())),
+			None => Ok(None),
+		}
+	}
+	fn len(&self, _tree: usize) -> TxOpResult<usize> {
+		unimplemented!(".len() in transaction not supported with LMDB backend")
+	}
+
+	fn insert(&mut self, tree: usize, key: &[u8], value: &[u8]) -> TxOpResult<Option<Value>> {
+		let tree = *self.get_tree(tree)?;
+		let old_val = tree.get(&self.tx, key)?.map(Vec::from);
+		tree.put(&mut self.tx, key, value)?;
+		Ok(old_val)
+	}
+	fn remove(&mut self, tree: usize, key: &[u8]) -> TxOpResult<Option<Value>> {
+		let tree = *self.get_tree(tree)?;
+		let old_val = tree.get(&self.tx, key)?.map(Vec::from);
+		tree.delete(&mut self.tx, key)?;
+		Ok(old_val)
+	}
+
+	fn iter(&self, _tree: usize) -> TxOpResult<TxValueIter<'_>> {
+		unimplemented!("Iterators in transactions not supported with LMDB backend");
+	}
+	fn iter_rev(&self, _tree: usize) -> TxOpResult<TxValueIter<'_>> {
+		unimplemented!("Iterators in transactions not supported with LMDB backend");
+	}
+
+	fn range<'r>(
+		&self,
+		_tree: usize,
+		_low: Bound<&'r [u8]>,
+		_high: Bound<&'r [u8]>,
+	) -> TxOpResult<TxValueIter<'_>> {
+		unimplemented!("Iterators in transactions not supported with LMDB backend");
+	}
+	fn range_rev<'r>(
+		&self,
+		_tree: usize,
+		_low: Bound<&'r [u8]>,
+		_high: Bound<&'r [u8]>,
+	) -> TxOpResult<TxValueIter<'_>> {
+		unimplemented!("Iterators in transactions not supported with LMDB backend");
+	}
+}
+
+// ----
+
+type IteratorItem<'a> = heed::Result<(
+	<ByteSlice as BytesDecode<'a>>::DItem,
+	<ByteSlice as BytesDecode<'a>>::DItem,
+)>;
+
+struct TxAndIterator<'a, I>
+where
+	I: Iterator<Item = IteratorItem<'a>> + 'a,
+{
+	tx: RoTxn<'a>,
+	iter: Option<I>,
+}
+
+impl<'a, I> TxAndIterator<'a, I>
+where
+	I: Iterator<Item = IteratorItem<'a>> + 'a,
+{
+	fn make<F>(tx: RoTxn<'a>, iterfun: F) -> Result<ValueIter<'a>>
+	where
+		F: FnOnce(&'a RoTxn<'a>) -> Result<I>,
+	{
+		let mut res = TxAndIterator { tx, iter: None };
+
+		let tx = unsafe { NonNull::from(&res.tx).as_ref() };
+		res.iter = Some(iterfun(tx)?);
+
+		Ok(Box::new(res))
+	}
+}
+
+impl<'a, I> Drop for TxAndIterator<'a, I>
+where
+	I: Iterator<Item = IteratorItem<'a>> + 'a,
+{
+	fn drop(&mut self) {
+		drop(self.iter.take());
+	}
+}
+
+impl<'a, I> Iterator for TxAndIterator<'a, I>
+where
+	I: Iterator<Item = IteratorItem<'a>> + 'a,
+{
+	type Item = Result<(Value, Value)>;
+
+	fn next(&mut self) -> Option<Self::Item> {
+		match self.iter.as_mut().unwrap().next() {
+			None => None,
+			Some(Err(e)) => Some(Err(e.into())),
+			Some(Ok((k, v))) => Some(Ok((k.to_vec(), v.to_vec()))),
+		}
+	}
+}
diff --git a/src/db/sled_adapter.rs b/src/db/sled_adapter.rs
new file mode 100644
index 00000000..982f8d82
--- /dev/null
+++ b/src/db/sled_adapter.rs
@@ -0,0 +1,260 @@
+use core::ops::Bound;
+
+use std::cell::Cell;
+use std::collections::HashMap;
+use std::sync::{Arc, RwLock};
+
+use sled::transaction::{
+	ConflictableTransactionError, TransactionError, Transactional, TransactionalTree,
+	UnabortableTransactionError,
+};
+
+use crate::{
+	Db, Error, IDb, ITx, ITxFn, Result, TxError, TxFnResult, TxOpError, TxOpResult, TxResult,
+	TxValueIter, Value, ValueIter,
+};
+
+pub use sled;
+
+// -- err
+
+impl From<sled::Error> for Error {
+	fn from(e: sled::Error) -> Error {
+		Error(format!("Sled: {}", e).into())
+	}
+}
+
+impl From<sled::Error> for TxOpError {
+	fn from(e: sled::Error) -> TxOpError {
+		TxOpError(e.into())
+	}
+}
+
+// -- db
+
+pub struct SledDb {
+	db: sled::Db,
+	trees: RwLock<(Vec<sled::Tree>, HashMap<String, usize>)>,
+}
+
+impl SledDb {
+	pub fn init(db: sled::Db) -> Db {
+		let s = Self {
+			db,
+			trees: RwLock::new((Vec::new(), HashMap::new())),
+		};
+		Db(Arc::new(s))
+	}
+
+	fn get_tree(&self, i: usize) -> Result<sled::Tree> {
+		self.trees
+			.read()
+			.unwrap()
+			.0
+			.get(i)
+			.cloned()
+			.ok_or_else(|| Error("invalid tree id".into()))
+	}
+}
+
+impl IDb for SledDb {
+	fn engine(&self) -> String {
+		"Sled".into()
+	}
+
+	fn open_tree(&self, name: &str) -> Result<usize> {
+		let mut trees = self.trees.write().unwrap();
+		if let Some(i) = trees.1.get(name) {
+			Ok(*i)
+		} else {
+			let tree = self.db.open_tree(name)?;
+			let i = trees.0.len();
+			trees.0.push(tree);
+			trees.1.insert(name.to_string(), i);
+			Ok(i)
+		}
+	}
+
+	fn list_trees(&self) -> Result<Vec<String>> {
+		let mut trees = vec![];
+		for name in self.db.tree_names() {
+			let name = std::str::from_utf8(&name)
+				.map_err(|e| Error(format!("{}", e).into()))?
+				.to_string();
+			if name != "__sled__default" {
+				trees.push(name);
+			}
+		}
+		Ok(trees)
+	}
+
+	// ----
+
+	fn get(&self, tree: usize, key: &[u8]) -> Result<Option<Value>> {
+		let tree = self.get_tree(tree)?;
+		let val = tree.get(key)?;
+		Ok(val.map(|x| x.to_vec()))
+	}
+
+	fn len(&self, tree: usize) -> Result<usize> {
+		let tree = self.get_tree(tree)?;
+		Ok(tree.len())
+	}
+
+	fn insert(&self, tree: usize, key: &[u8], value: &[u8]) -> Result<Option<Value>> {
+		let tree = self.get_tree(tree)?;
+		let old_val = tree.insert(key, value)?;
+		Ok(old_val.map(|x| x.to_vec()))
+	}
+
+	fn remove(&self, tree: usize, key: &[u8]) -> Result<Option<Value>> {
+		let tree = self.get_tree(tree)?;
+		let old_val = tree.remove(key)?;
+		Ok(old_val.map(|x| x.to_vec()))
+	}
+
+	fn iter(&self, tree: usize) -> Result<ValueIter<'_>> {
+		let tree = self.get_tree(tree)?;
+		Ok(Box::new(tree.iter().map(|v| {
+			v.map(|(x, y)| (x.to_vec(), y.to_vec())).map_err(Into::into)
+		})))
+	}
+
+	fn iter_rev(&self, tree: usize) -> Result<ValueIter<'_>> {
+		let tree = self.get_tree(tree)?;
+		Ok(Box::new(tree.iter().rev().map(|v| {
+			v.map(|(x, y)| (x.to_vec(), y.to_vec())).map_err(Into::into)
+		})))
+	}
+
+	fn range<'r>(
+		&self,
+		tree: usize,
+		low: Bound<&'r [u8]>,
+		high: Bound<&'r [u8]>,
+	) -> Result<ValueIter<'_>> {
+		let tree = self.get_tree(tree)?;
+		Ok(Box::new(tree.range::<&'r [u8], _>((low, high)).map(|v| {
+			v.map(|(x, y)| (x.to_vec(), y.to_vec())).map_err(Into::into)
+		})))
+	}
+	fn range_rev<'r>(
+		&self,
+		tree: usize,
+		low: Bound<&'r [u8]>,
+		high: Bound<&'r [u8]>,
+	) -> Result<ValueIter<'_>> {
+		let tree = self.get_tree(tree)?;
+		Ok(Box::new(tree.range::<&'r [u8], _>((low, high)).rev().map(
+			|v| v.map(|(x, y)| (x.to_vec(), y.to_vec())).map_err(Into::into),
+		)))
+	}
+
+	// ----
+
+	fn transaction(&self, f: &dyn ITxFn) -> TxResult<(), ()> {
+		let trees = self.trees.read().unwrap();
+		let res = trees.0.transaction(|txtrees| {
+			let mut tx = SledTx {
+				trees: txtrees,
+				err: Cell::new(None),
+			};
+			match f.try_on(&mut tx) {
+				TxFnResult::Ok => {
+					assert!(tx.err.into_inner().is_none());
+					Ok(())
+				}
+				TxFnResult::Abort => {
+					assert!(tx.err.into_inner().is_none());
+					Err(ConflictableTransactionError::Abort(()))
+				}
+				TxFnResult::DbErr => {
+					let e = tx.err.into_inner().expect("No DB error");
+					Err(e.into())
+				}
+			}
+		});
+		match res {
+			Ok(()) => Ok(()),
+			Err(TransactionError::Abort(())) => Err(TxError::Abort(())),
+			Err(TransactionError::Storage(s)) => Err(TxError::Db(s.into())),
+		}
+	}
+}
+
+// ----
+
+struct SledTx<'a> {
+	trees: &'a [TransactionalTree],
+	err: Cell<Option<UnabortableTransactionError>>,
+}
+
+impl<'a> SledTx<'a> {
+	fn get_tree(&self, i: usize) -> TxOpResult<&TransactionalTree> {
+		self.trees.get(i).ok_or_else(|| {
+			TxOpError(Error(
+				"invalid tree id (it might have been openned after the transaction started)".into(),
+			))
+		})
+	}
+
+	fn save_error<R>(
+		&self,
+		v: std::result::Result<R, UnabortableTransactionError>,
+	) -> TxOpResult<R> {
+		match v {
+			Ok(x) => Ok(x),
+			Err(e) => {
+				let txt = format!("{}", e);
+				self.err.set(Some(e));
+				Err(TxOpError(Error(txt.into())))
+			}
+		}
+	}
+}
+
+impl<'a> ITx for SledTx<'a> {
+	fn get(&self, tree: usize, key: &[u8]) -> TxOpResult<Option<Value>> {
+		let tree = self.get_tree(tree)?;
+		let tmp = self.save_error(tree.get(key))?;
+		Ok(tmp.map(|x| x.to_vec()))
+	}
+	fn len(&self, _tree: usize) -> TxOpResult<usize> {
+		unimplemented!(".len() in transaction not supported with Sled backend")
+	}
+
+	fn insert(&mut self, tree: usize, key: &[u8], value: &[u8]) -> TxOpResult<Option<Value>> {
+		let tree = self.get_tree(tree)?;
+		let old_val = self.save_error(tree.insert(key, value))?;
+		Ok(old_val.map(|x| x.to_vec()))
+	}
+	fn remove(&mut self, tree: usize, key: &[u8]) -> TxOpResult<Option<Value>> {
+		let tree = self.get_tree(tree)?;
+		let old_val = self.save_error(tree.remove(key))?;
+		Ok(old_val.map(|x| x.to_vec()))
+	}
+
+	fn iter(&self, _tree: usize) -> TxOpResult<TxValueIter<'_>> {
+		unimplemented!("Iterators in transactions not supported with Sled backend");
+	}
+	fn iter_rev(&self, _tree: usize) -> TxOpResult<TxValueIter<'_>> {
+		unimplemented!("Iterators in transactions not supported with Sled backend");
+	}
+
+	fn range<'r>(
+		&self,
+		_tree: usize,
+		_low: Bound<&'r [u8]>,
+		_high: Bound<&'r [u8]>,
+	) -> TxOpResult<TxValueIter<'_>> {
+		unimplemented!("Iterators in transactions not supported with Sled backend");
+	}
+	fn range_rev<'r>(
+		&self,
+		_tree: usize,
+		_low: Bound<&'r [u8]>,
+		_high: Bound<&'r [u8]>,
+	) -> TxOpResult<TxValueIter<'_>> {
+		unimplemented!("Iterators in transactions not supported with Sled backend");
+	}
+}
diff --git a/src/db/sqlite_adapter.rs b/src/db/sqlite_adapter.rs
new file mode 100644
index 00000000..14bf35ff
--- /dev/null
+++ b/src/db/sqlite_adapter.rs
@@ -0,0 +1,500 @@
+use core::ops::Bound;
+
+use std::borrow::BorrowMut;
+use std::marker::PhantomPinned;
+use std::pin::Pin;
+use std::ptr::NonNull;
+use std::sync::{Arc, Mutex, MutexGuard};
+
+use log::trace;
+
+use rusqlite::{params, Connection, Rows, Statement, Transaction};
+
+use crate::{
+	Db, Error, IDb, ITx, ITxFn, Result, TxError, TxFnResult, TxOpError, TxOpResult, TxResult,
+	TxValueIter, Value, ValueIter,
+};
+
+pub use rusqlite;
+
+// --- err
+
+impl From<rusqlite::Error> for Error {
+	fn from(e: rusqlite::Error) -> Error {
+		Error(format!("Sqlite: {}", e).into())
+	}
+}
+
+impl From<rusqlite::Error> for TxOpError {
+	fn from(e: rusqlite::Error) -> TxOpError {
+		TxOpError(e.into())
+	}
+}
+
+// -- db
+
+pub struct SqliteDb(Mutex<SqliteDbInner>);
+
+struct SqliteDbInner {
+	db: Connection,
+	trees: Vec<String>,
+}
+
+impl SqliteDb {
+	pub fn init(db: rusqlite::Connection) -> Db {
+		let s = Self(Mutex::new(SqliteDbInner {
+			db,
+			trees: Vec::new(),
+		}));
+		Db(Arc::new(s))
+	}
+}
+
+impl SqliteDbInner {
+	fn get_tree(&self, i: usize) -> Result<&'_ str> {
+		self.trees
+			.get(i)
+			.map(String::as_str)
+			.ok_or_else(|| Error("invalid tree id".into()))
+	}
+
+	fn internal_get(&self, tree: &str, key: &[u8]) -> Result<Option<Value>> {
+		let mut stmt = self
+			.db
+			.prepare(&format!("SELECT v FROM {} WHERE k = ?1", tree))?;
+		let mut res_iter = stmt.query([key])?;
+		match res_iter.next()? {
+			None => Ok(None),
+			Some(v) => Ok(Some(v.get::<_, Vec<u8>>(0)?)),
+		}
+	}
+}
+
+impl IDb for SqliteDb {
+	fn engine(&self) -> String {
+		format!("sqlite3 v{} (using rusqlite crate)", rusqlite::version())
+	}
+
+	fn open_tree(&self, name: &str) -> Result<usize> {
+		let name = format!("tree_{}", name.replace(':', "_COLON_"));
+		let mut this = self.0.lock().unwrap();
+
+		if let Some(i) = this.trees.iter().position(|x| x == &name) {
+			Ok(i)
+		} else {
+			trace!("create table {}", name);
+			this.db.execute(
+				&format!(
+					"CREATE TABLE IF NOT EXISTS {} (
+						k BLOB PRIMARY KEY,
+						v BLOB
+					)",
+					name
+				),
+				[],
+			)?;
+			trace!("table created: {}, unlocking", name);
+
+			let i = this.trees.len();
+			this.trees.push(name.to_string());
+			Ok(i)
+		}
+	}
+
+	fn list_trees(&self) -> Result<Vec<String>> {
+		let mut trees = vec![];
+
+		trace!("list_trees: lock db");
+		let this = self.0.lock().unwrap();
+		trace!("list_trees: lock acquired");
+
+		let mut stmt = this.db.prepare(
+			"SELECT name FROM sqlite_schema WHERE type = 'table' AND name LIKE 'tree_%'",
+		)?;
+		let mut rows = stmt.query([])?;
+		while let Some(row) = rows.next()? {
+			let name = row.get::<_, String>(0)?;
+			let name = name.replace("_COLON_", ":");
+			let name = name.strip_prefix("tree_").unwrap().to_string();
+			trees.push(name);
+		}
+		Ok(trees)
+	}
+
+	// ----
+
+	fn get(&self, tree: usize, key: &[u8]) -> Result<Option<Value>> {
+		trace!("get {}: lock db", tree);
+		let this = self.0.lock().unwrap();
+		trace!("get {}: lock acquired", tree);
+
+		let tree = this.get_tree(tree)?;
+		this.internal_get(tree, key)
+	}
+
+	fn len(&self, tree: usize) -> Result<usize> {
+		trace!("len {}: lock db", tree);
+		let this = self.0.lock().unwrap();
+		trace!("len {}: lock acquired", tree);
+
+		let tree = this.get_tree(tree)?;
+		let mut stmt = this.db.prepare(&format!("SELECT COUNT(*) FROM {}", tree))?;
+		let mut res_iter = stmt.query([])?;
+		match res_iter.next()? {
+			None => Ok(0),
+			Some(v) => Ok(v.get::<_, usize>(0)?),
+		}
+	}
+
+	fn insert(&self, tree: usize, key: &[u8], value: &[u8]) -> Result<Option<Value>> {
+		trace!("insert {}: lock db", tree);
+		let this = self.0.lock().unwrap();
+		trace!("insert {}: lock acquired", tree);
+
+		let tree = this.get_tree(tree)?;
+		let old_val = this.internal_get(tree, key)?;
+
+		let sql = match &old_val {
+			Some(_) => format!("UPDATE {} SET v = ?2 WHERE k = ?1", tree),
+			None => format!("INSERT INTO {} (k, v) VALUES (?1, ?2)", tree),
+		};
+		let n = this.db.execute(&sql, params![key, value])?;
+		assert_eq!(n, 1);
+
+		Ok(old_val)
+	}
+
+	fn remove(&self, tree: usize, key: &[u8]) -> Result<Option<Value>> {
+		trace!("remove {}: lock db", tree);
+		let this = self.0.lock().unwrap();
+		trace!("remove {}: lock acquired", tree);
+
+		let tree = this.get_tree(tree)?;
+		let old_val = this.internal_get(tree, key)?;
+
+		if old_val.is_some() {
+			let n = this
+				.db
+				.execute(&format!("DELETE FROM {} WHERE k = ?1", tree), params![key])?;
+			assert_eq!(n, 1);
+		}
+
+		Ok(old_val)
+	}
+
+	fn iter(&self, tree: usize) -> Result<ValueIter<'_>> {
+		trace!("iter {}: lock db", tree);
+		let this = self.0.lock().unwrap();
+		trace!("iter {}: lock acquired", tree);
+
+		let tree = this.get_tree(tree)?;
+		let sql = format!("SELECT k, v FROM {} ORDER BY k ASC", tree);
+		DbValueIterator::make(this, &sql, [])
+	}
+
+	fn iter_rev(&self, tree: usize) -> Result<ValueIter<'_>> {
+		trace!("iter_rev {}: lock db", tree);
+		let this = self.0.lock().unwrap();
+		trace!("iter_rev {}: lock acquired", tree);
+
+		let tree = this.get_tree(tree)?;
+		let sql = format!("SELECT k, v FROM {} ORDER BY k DESC", tree);
+		DbValueIterator::make(this, &sql, [])
+	}
+
+	fn range<'r>(
+		&self,
+		tree: usize,
+		low: Bound<&'r [u8]>,
+		high: Bound<&'r [u8]>,
+	) -> Result<ValueIter<'_>> {
+		trace!("range {}: lock db", tree);
+		let this = self.0.lock().unwrap();
+		trace!("range {}: lock acquired", tree);
+
+		let tree = this.get_tree(tree)?;
+
+		let (bounds_sql, params) = bounds_sql(low, high);
+		let sql = format!("SELECT k, v FROM {} {} ORDER BY k ASC", tree, bounds_sql);
+
+		let params = params
+			.iter()
+			.map(|x| x as &dyn rusqlite::ToSql)
+			.collect::<Vec<_>>();
+
+		DbValueIterator::make::<&[&dyn rusqlite::ToSql]>(this, &sql, params.as_ref())
+	}
+	fn range_rev<'r>(
+		&self,
+		tree: usize,
+		low: Bound<&'r [u8]>,
+		high: Bound<&'r [u8]>,
+	) -> Result<ValueIter<'_>> {
+		trace!("range_rev {}: lock db", tree);
+		let this = self.0.lock().unwrap();
+		trace!("range_rev {}: lock acquired", tree);
+
+		let tree = this.get_tree(tree)?;
+
+		let (bounds_sql, params) = bounds_sql(low, high);
+		let sql = format!("SELECT k, v FROM {} {} ORDER BY k DESC", tree, bounds_sql);
+
+		let params = params
+			.iter()
+			.map(|x| x as &dyn rusqlite::ToSql)
+			.collect::<Vec<_>>();
+
+		DbValueIterator::make::<&[&dyn rusqlite::ToSql]>(this, &sql, params.as_ref())
+	}
+
+	// ----
+
+	fn transaction(&self, f: &dyn ITxFn) -> TxResult<(), ()> {
+		trace!("transaction: lock db");
+		let mut this = self.0.lock().unwrap();
+		trace!("transaction: lock acquired");
+
+		let this_mut_ref: &mut SqliteDbInner = this.borrow_mut();
+
+		let mut tx = SqliteTx {
+			tx: this_mut_ref
+				.db
+				.transaction()
+				.map_err(Error::from)
+				.map_err(TxError::Db)?,
+			trees: &this_mut_ref.trees,
+		};
+		let res = match f.try_on(&mut tx) {
+			TxFnResult::Ok => {
+				tx.tx.commit().map_err(Error::from).map_err(TxError::Db)?;
+				Ok(())
+			}
+			TxFnResult::Abort => {
+				tx.tx.rollback().map_err(Error::from).map_err(TxError::Db)?;
+				Err(TxError::Abort(()))
+			}
+			TxFnResult::DbErr => {
+				tx.tx.rollback().map_err(Error::from).map_err(TxError::Db)?;
+				Err(TxError::Db(Error(
+					"(this message will be discarded)".into(),
+				)))
+			}
+		};
+
+		trace!("transaction done");
+		res
+	}
+}
+
+// ----
+
+struct SqliteTx<'a> {
+	tx: Transaction<'a>,
+	trees: &'a [String],
+}
+
+impl<'a> SqliteTx<'a> {
+	fn get_tree(&self, i: usize) -> TxOpResult<&'_ str> {
+		self.trees.get(i).map(String::as_ref).ok_or_else(|| {
+			TxOpError(Error(
+				"invalid tree id (it might have been openned after the transaction started)".into(),
+			))
+		})
+	}
+
+	fn internal_get(&self, tree: &str, key: &[u8]) -> TxOpResult<Option<Value>> {
+		let mut stmt = self
+			.tx
+			.prepare(&format!("SELECT v FROM {} WHERE k = ?1", tree))?;
+		let mut res_iter = stmt.query([key])?;
+		match res_iter.next()? {
+			None => Ok(None),
+			Some(v) => Ok(Some(v.get::<_, Vec<u8>>(0)?)),
+		}
+	}
+}
+
+impl<'a> ITx for SqliteTx<'a> {
+	fn get(&self, tree: usize, key: &[u8]) -> TxOpResult<Option<Value>> {
+		let tree = self.get_tree(tree)?;
+		self.internal_get(tree, key)
+	}
+	fn len(&self, tree: usize) -> TxOpResult<usize> {
+		let tree = self.get_tree(tree)?;
+		let mut stmt = self.tx.prepare(&format!("SELECT COUNT(*) FROM {}", tree))?;
+		let mut res_iter = stmt.query([])?;
+		match res_iter.next()? {
+			None => Ok(0),
+			Some(v) => Ok(v.get::<_, usize>(0)?),
+		}
+	}
+
+	fn insert(&mut self, tree: usize, key: &[u8], value: &[u8]) -> TxOpResult<Option<Value>> {
+		let tree = self.get_tree(tree)?;
+		let old_val = self.internal_get(tree, key)?;
+
+		let sql = match &old_val {
+			Some(_) => format!("UPDATE {} SET v = ?2 WHERE k = ?1", tree),
+			None => format!("INSERT INTO {} (k, v) VALUES (?1, ?2)", tree),
+		};
+		let n = self.tx.execute(&sql, params![key, value])?;
+		assert_eq!(n, 1);
+
+		Ok(old_val)
+	}
+	fn remove(&mut self, tree: usize, key: &[u8]) -> TxOpResult<Option<Value>> {
+		let tree = self.get_tree(tree)?;
+		let old_val = self.internal_get(tree, key)?;
+
+		if old_val.is_some() {
+			let n = self
+				.tx
+				.execute(&format!("DELETE FROM {} WHERE k = ?1", tree), params![key])?;
+			assert_eq!(n, 1);
+		}
+
+		Ok(old_val)
+	}
+
+	fn iter(&self, _tree: usize) -> TxOpResult<TxValueIter<'_>> {
+		unimplemented!();
+	}
+	fn iter_rev(&self, _tree: usize) -> TxOpResult<TxValueIter<'_>> {
+		unimplemented!();
+	}
+
+	fn range<'r>(
+		&self,
+		_tree: usize,
+		_low: Bound<&'r [u8]>,
+		_high: Bound<&'r [u8]>,
+	) -> TxOpResult<TxValueIter<'_>> {
+		unimplemented!();
+	}
+	fn range_rev<'r>(
+		&self,
+		_tree: usize,
+		_low: Bound<&'r [u8]>,
+		_high: Bound<&'r [u8]>,
+	) -> TxOpResult<TxValueIter<'_>> {
+		unimplemented!();
+	}
+}
+
+// ----
+
+struct DbValueIterator<'a> {
+	db: MutexGuard<'a, SqliteDbInner>,
+	stmt: Option<Statement<'a>>,
+	iter: Option<Rows<'a>>,
+	_pin: PhantomPinned,
+}
+
+impl<'a> DbValueIterator<'a> {
+	fn make<P: rusqlite::Params>(
+		db: MutexGuard<'a, SqliteDbInner>,
+		sql: &str,
+		args: P,
+	) -> Result<ValueIter<'a>> {
+		let res = DbValueIterator {
+			db,
+			stmt: None,
+			iter: None,
+			_pin: PhantomPinned,
+		};
+		let mut boxed = Box::pin(res);
+		trace!("make iterator with sql: {}", sql);
+
+		unsafe {
+			let db = NonNull::from(&boxed.db);
+			let stmt = db.as_ref().db.prepare(sql)?;
+
+			let mut_ref: Pin<&mut DbValueIterator<'a>> = Pin::as_mut(&mut boxed);
+			Pin::get_unchecked_mut(mut_ref).stmt = Some(stmt);
+
+			let mut stmt = NonNull::from(&boxed.stmt);
+			let iter = stmt.as_mut().as_mut().unwrap().query(args)?;
+
+			let mut_ref: Pin<&mut DbValueIterator<'a>> = Pin::as_mut(&mut boxed);
+			Pin::get_unchecked_mut(mut_ref).iter = Some(iter);
+		}
+
+		Ok(Box::new(DbValueIteratorPin(boxed)))
+	}
+}
+
+impl<'a> Drop for DbValueIterator<'a> {
+	fn drop(&mut self) {
+		trace!("drop iter");
+		drop(self.iter.take());
+		drop(self.stmt.take());
+	}
+}
+
+struct DbValueIteratorPin<'a>(Pin<Box<DbValueIterator<'a>>>);
+
+impl<'a> Iterator for DbValueIteratorPin<'a> {
+	type Item = Result<(Value, Value)>;
+
+	fn next(&mut self) -> Option<Self::Item> {
+		let next = unsafe {
+			let mut_ref: Pin<&mut DbValueIterator<'a>> = Pin::as_mut(&mut self.0);
+			Pin::get_unchecked_mut(mut_ref).iter.as_mut()?.next()
+		};
+		let row = match next {
+			Err(e) => return Some(Err(e.into())),
+			Ok(None) => return None,
+			Ok(Some(r)) => r,
+		};
+		let k = match row.get::<_, Vec<u8>>(0) {
+			Err(e) => return Some(Err(e.into())),
+			Ok(x) => x,
+		};
+		let v = match row.get::<_, Vec<u8>>(1) {
+			Err(e) => return Some(Err(e.into())),
+			Ok(y) => y,
+		};
+		Some(Ok((k, v)))
+	}
+}
+
+// ----
+
+fn bounds_sql<'r>(low: Bound<&'r [u8]>, high: Bound<&'r [u8]>) -> (String, Vec<Vec<u8>>) {
+	let mut sql = String::new();
+	let mut params: Vec<Vec<u8>> = vec![];
+
+	match low {
+		Bound::Included(b) => {
+			sql.push_str(" WHERE k >= ?1");
+			params.push(b.to_vec());
+		}
+		Bound::Excluded(b) => {
+			sql.push_str(" WHERE k > ?1");
+			params.push(b.to_vec());
+		}
+		Bound::Unbounded => (),
+	};
+
+	match high {
+		Bound::Included(b) => {
+			if !params.is_empty() {
+				sql.push_str(" AND k <= ?2");
+			} else {
+				sql.push_str(" WHERE k <= ?1");
+			}
+			params.push(b.to_vec());
+		}
+		Bound::Excluded(b) => {
+			if !params.is_empty() {
+				sql.push_str(" AND k < ?2");
+			} else {
+				sql.push_str(" WHERE k < ?1");
+			}
+			params.push(b.to_vec());
+		}
+		Bound::Unbounded => (),
+	}
+
+	(sql, params)
+}
diff --git a/src/db/test.rs b/src/db/test.rs
new file mode 100644
index 00000000..cfcee643
--- /dev/null
+++ b/src/db/test.rs
@@ -0,0 +1,106 @@
+use crate::*;
+
+use crate::lmdb_adapter::LmdbDb;
+use crate::sled_adapter::SledDb;
+use crate::sqlite_adapter::SqliteDb;
+
+fn test_suite(db: Db) {
+	let tree = db.open_tree("tree").unwrap();
+
+	let ka: &[u8] = &b"test"[..];
+	let kb: &[u8] = &b"zwello"[..];
+	let kint: &[u8] = &b"tz"[..];
+	let va: &[u8] = &b"plop"[..];
+	let vb: &[u8] = &b"plip"[..];
+	let vc: &[u8] = &b"plup"[..];
+
+	assert!(tree.insert(ka, va).unwrap().is_none());
+	assert_eq!(tree.get(ka).unwrap().unwrap(), va);
+
+	let res = db.transaction::<_, (), _>(|mut tx| {
+		assert_eq!(tx.get(&tree, ka).unwrap().unwrap(), va);
+
+		assert_eq!(tx.insert(&tree, ka, vb).unwrap().unwrap(), va);
+
+		assert_eq!(tx.get(&tree, ka).unwrap().unwrap(), vb);
+
+		tx.commit(12)
+	});
+	assert!(matches!(res, Ok(12)));
+	assert_eq!(tree.get(ka).unwrap().unwrap(), vb);
+
+	let res = db.transaction::<(), _, _>(|mut tx| {
+		assert_eq!(tx.get(&tree, ka).unwrap().unwrap(), vb);
+
+		assert_eq!(tx.insert(&tree, ka, vc).unwrap().unwrap(), vb);
+
+		assert_eq!(tx.get(&tree, ka).unwrap().unwrap(), vc);
+
+		tx.abort(42)
+	});
+	assert!(matches!(res, Err(TxError::Abort(42))));
+	assert_eq!(tree.get(ka).unwrap().unwrap(), vb);
+
+	let mut iter = tree.iter().unwrap();
+	let next = iter.next().unwrap().unwrap();
+	assert_eq!((next.0.as_ref(), next.1.as_ref()), (ka, vb));
+	assert!(iter.next().is_none());
+	drop(iter);
+
+	assert!(tree.insert(kb, vc).unwrap().is_none());
+	assert_eq!(tree.get(kb).unwrap().unwrap(), vc);
+
+	let mut iter = tree.iter().unwrap();
+	let next = iter.next().unwrap().unwrap();
+	assert_eq!((next.0.as_ref(), next.1.as_ref()), (ka, vb));
+	let next = iter.next().unwrap().unwrap();
+	assert_eq!((next.0.as_ref(), next.1.as_ref()), (kb, vc));
+	assert!(iter.next().is_none());
+	drop(iter);
+
+	let mut iter = tree.range(kint..).unwrap();
+	let next = iter.next().unwrap().unwrap();
+	assert_eq!((next.0.as_ref(), next.1.as_ref()), (kb, vc));
+	assert!(iter.next().is_none());
+	drop(iter);
+
+	let mut iter = tree.range_rev(..kint).unwrap();
+	let next = iter.next().unwrap().unwrap();
+	assert_eq!((next.0.as_ref(), next.1.as_ref()), (ka, vb));
+	assert!(iter.next().is_none());
+	drop(iter);
+
+	let mut iter = tree.iter_rev().unwrap();
+	let next = iter.next().unwrap().unwrap();
+	assert_eq!((next.0.as_ref(), next.1.as_ref()), (kb, vc));
+	let next = iter.next().unwrap().unwrap();
+	assert_eq!((next.0.as_ref(), next.1.as_ref()), (ka, vb));
+	assert!(iter.next().is_none());
+	drop(iter);
+}
+
+#[test]
+fn test_lmdb_db() {
+	let path = mktemp::Temp::new_dir().unwrap();
+	let db = heed::EnvOpenOptions::new()
+		.max_dbs(100)
+		.open(&path)
+		.unwrap();
+	let db = LmdbDb::init(db);
+	test_suite(db);
+	drop(path);
+}
+
+#[test]
+fn test_sled_db() {
+	let path = mktemp::Temp::new_dir().unwrap();
+	let db = SledDb::init(sled::open(path.to_path_buf()).unwrap());
+	test_suite(db);
+	drop(path);
+}
+
+#[test]
+fn test_sqlite_db() {
+	let db = SqliteDb::init(rusqlite::Connection::open_in_memory().unwrap());
+	test_suite(db);
+}
diff --git a/src/garage/Cargo.toml b/src/garage/Cargo.toml
index 902f67f8..eb643160 100644
--- a/src/garage/Cargo.toml
+++ b/src/garage/Cargo.toml
@@ -21,6 +21,7 @@ path = "tests/lib.rs"
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
 [dependencies]
+garage_db = { version = "0.8.0", path = "../db" }
 garage_api = { version = "0.7.0", path = "../api" }
 garage_model = { version = "0.7.0", path = "../model" }
 garage_rpc = { version = "0.7.0", path = "../rpc" }
@@ -36,8 +37,6 @@ rand = "0.8"
 async-trait = "0.1.7"
 sodiumoxide = { version = "0.2.5-0", package = "kuska-sodiumoxide" }
 
-sled = "0.34"
-
 rmp-serde = "0.15"
 serde = { version = "1.0", default-features = false, features = ["derive", "rc"] }
 serde_bytes = "0.11"
diff --git a/src/garage/admin.rs b/src/garage/admin.rs
index bc1f494a..c662aa00 100644
--- a/src/garage/admin.rs
+++ b/src/garage/admin.rs
@@ -660,11 +660,11 @@ impl AdminRpcHandler {
 			}
 			Ok(AdminRpc::Ok(ret))
 		} else {
-			Ok(AdminRpc::Ok(self.gather_stats_local(opt)))
+			Ok(AdminRpc::Ok(self.gather_stats_local(opt)?))
 		}
 	}
 
-	fn gather_stats_local(&self, opt: StatsOpt) -> String {
+	fn gather_stats_local(&self, opt: StatsOpt) -> Result<String, Error> {
 		let mut ret = String::new();
 		writeln!(
 			&mut ret,
@@ -672,6 +672,7 @@ impl AdminRpcHandler {
 			self.garage.system.garage_version(),
 		)
 		.unwrap();
+		writeln!(&mut ret, "\nDatabase engine: {}", self.garage.db.engine()).unwrap();
 
 		// Gather ring statistics
 		let ring = self.garage.system.ring.borrow().clone();
@@ -689,59 +690,71 @@ impl AdminRpcHandler {
 			writeln!(&mut ret, "  {:?} {}", n, c).unwrap();
 		}
 
-		self.gather_table_stats(&mut ret, &self.garage.bucket_table, &opt);
-		self.gather_table_stats(&mut ret, &self.garage.key_table, &opt);
-		self.gather_table_stats(&mut ret, &self.garage.object_table, &opt);
-		self.gather_table_stats(&mut ret, &self.garage.version_table, &opt);
-		self.gather_table_stats(&mut ret, &self.garage.block_ref_table, &opt);
+		self.gather_table_stats(&mut ret, &self.garage.bucket_table, &opt)?;
+		self.gather_table_stats(&mut ret, &self.garage.key_table, &opt)?;
+		self.gather_table_stats(&mut ret, &self.garage.object_table, &opt)?;
+		self.gather_table_stats(&mut ret, &self.garage.version_table, &opt)?;
+		self.gather_table_stats(&mut ret, &self.garage.block_ref_table, &opt)?;
 
 		writeln!(&mut ret, "\nBlock manager stats:").unwrap();
 		if opt.detailed {
 			writeln!(
 				&mut ret,
 				"  number of RC entries (~= number of blocks): {}",
-				self.garage.block_manager.rc_len()
+				self.garage.block_manager.rc_len()?
 			)
 			.unwrap();
 		}
 		writeln!(
 			&mut ret,
 			"  resync queue length: {}",
-			self.garage.block_manager.resync_queue_len()
+			self.garage.block_manager.resync_queue_len()?
 		)
 		.unwrap();
 		writeln!(
 			&mut ret,
 			"  blocks with resync errors: {}",
-			self.garage.block_manager.resync_errors_len()
+			self.garage.block_manager.resync_errors_len()?
 		)
 		.unwrap();
 
-		ret
+		Ok(ret)
 	}
 
-	fn gather_table_stats<F, R>(&self, to: &mut String, t: &Arc<Table<F, R>>, opt: &StatsOpt)
+	fn gather_table_stats<F, R>(
+		&self,
+		to: &mut String,
+		t: &Arc<Table<F, R>>,
+		opt: &StatsOpt,
+	) -> Result<(), Error>
 	where
 		F: TableSchema + 'static,
 		R: TableReplication + 'static,
 	{
 		writeln!(to, "\nTable stats for {}", F::TABLE_NAME).unwrap();
 		if opt.detailed {
-			writeln!(to, "  number of items: {}", t.data.store.len()).unwrap();
+			writeln!(
+				to,
+				"  number of items: {}",
+				t.data.store.len().map_err(GarageError::from)?
+			)
+			.unwrap();
 			writeln!(
 				to,
 				"  Merkle tree size: {}",
-				t.merkle_updater.merkle_tree_len()
+				t.merkle_updater.merkle_tree_len()?
 			)
 			.unwrap();
 		}
 		writeln!(
 			to,
 			"  Merkle updater todo queue length: {}",
-			t.merkle_updater.todo_len()
+			t.merkle_updater.todo_len()?
 		)
 		.unwrap();
-		writeln!(to, "  GC todo queue length: {}", t.data.gc_todo_len()).unwrap();
+		writeln!(to, "  GC todo queue length: {}", t.data.gc_todo_len()?).unwrap();
+
+		Ok(())
 	}
 }
 
diff --git a/src/garage/repair.rs b/src/garage/repair.rs
index 830eac71..17e14b8b 100644
--- a/src/garage/repair.rs
+++ b/src/garage/repair.rs
@@ -64,13 +64,23 @@ impl Repair {
 
 	async fn repair_versions(&self, must_exit: &watch::Receiver<bool>) -> Result<(), Error> {
 		let mut pos = vec![];
+		let mut i = 0;
 
-		while let Some((item_key, item_bytes)) =
-			self.garage.version_table.data.store.get_gt(&pos)?
-		{
-			pos = item_key.to_vec();
+		while !*must_exit.borrow() {
+			let item_bytes = match self.garage.version_table.data.store.get_gt(pos)? {
+				Some((k, v)) => {
+					pos = k;
+					v
+				}
+				None => break,
+			};
+
+			i += 1;
+			if i % 1000 == 0 {
+				info!("repair_versions: {}", i);
+			}
 
-			let version = rmp_serde::decode::from_read_ref::<_, Version>(item_bytes.as_ref())?;
+			let version = rmp_serde::decode::from_read_ref::<_, Version>(&item_bytes)?;
 			if version.deleted.get() {
 				continue;
 			}
@@ -98,23 +108,30 @@ impl Repair {
 					))
 					.await?;
 			}
-
-			if *must_exit.borrow() {
-				break;
-			}
 		}
+		info!("repair_versions: finished, done {}", i);
 		Ok(())
 	}
 
 	async fn repair_block_ref(&self, must_exit: &watch::Receiver<bool>) -> Result<(), Error> {
 		let mut pos = vec![];
+		let mut i = 0;
 
-		while let Some((item_key, item_bytes)) =
-			self.garage.block_ref_table.data.store.get_gt(&pos)?
-		{
-			pos = item_key.to_vec();
+		while !*must_exit.borrow() {
+			let item_bytes = match self.garage.block_ref_table.data.store.get_gt(pos)? {
+				Some((k, v)) => {
+					pos = k;
+					v
+				}
+				None => break,
+			};
+
+			i += 1;
+			if i % 1000 == 0 {
+				info!("repair_block_ref: {}", i);
+			}
 
-			let block_ref = rmp_serde::decode::from_read_ref::<_, BlockRef>(item_bytes.as_ref())?;
+			let block_ref = rmp_serde::decode::from_read_ref::<_, BlockRef>(&item_bytes)?;
 			if block_ref.deleted.get() {
 				continue;
 			}
@@ -139,11 +156,8 @@ impl Repair {
 					})
 					.await?;
 			}
-
-			if *must_exit.borrow() {
-				break;
-			}
 		}
+		info!("repair_block_ref: finished, done {}", i);
 		Ok(())
 	}
 }
diff --git a/src/garage/server.rs b/src/garage/server.rs
index b58ad286..697d3358 100644
--- a/src/garage/server.rs
+++ b/src/garage/server.rs
@@ -2,6 +2,8 @@ use std::path::PathBuf;
 
 use tokio::sync::watch;
 
+use garage_db as db;
+
 use garage_util::background::*;
 use garage_util::config::*;
 use garage_util::error::Error;
@@ -31,13 +33,51 @@ pub async fn run_server(config_file: PathBuf) -> Result<(), Error> {
 
 	info!("Opening database...");
 	let mut db_path = config.metadata_dir.clone();
-	db_path.push("db");
-	let db = sled::Config::default()
-		.path(&db_path)
-		.cache_capacity(config.sled_cache_capacity)
-		.flush_every_ms(Some(config.sled_flush_every_ms))
-		.open()
-		.expect("Unable to open sled DB");
+	std::fs::create_dir_all(&db_path).expect("Unable to create Garage meta data directory");
+	let db = match config.db_engine.as_str() {
+		"sled" => {
+			db_path.push("db");
+			info!("Opening Sled database at: {}", db_path.display());
+			let db = db::sled_adapter::sled::Config::default()
+				.path(&db_path)
+				.cache_capacity(config.sled_cache_capacity)
+				.flush_every_ms(Some(config.sled_flush_every_ms))
+				.open()
+				.expect("Unable to open sled DB");
+			db::sled_adapter::SledDb::init(db)
+		}
+		"sqlite" | "sqlite3" | "rusqlite" => {
+			db_path.push("db.sqlite");
+			info!("Opening Sqlite database at: {}", db_path.display());
+			let db = db::sqlite_adapter::rusqlite::Connection::open(db_path)
+				.expect("Unable to open sqlite DB");
+			db::sqlite_adapter::SqliteDb::init(db)
+		}
+		"lmdb" | "heed" => {
+			db_path.push("db.lmdb");
+			info!("Opening LMDB database at: {}", db_path.display());
+			std::fs::create_dir_all(&db_path).expect("Unable to create LMDB data directory");
+			let map_size = if u32::MAX as usize == usize::MAX {
+				warn!("LMDB is not recommended on 32-bit systems, database size will be limited");
+				1usize << 30 // 1GB for 32-bit systems
+			} else {
+				1usize << 40 // 1TB for 64-bit systems
+			};
+
+			let db = db::lmdb_adapter::heed::EnvOpenOptions::new()
+				.max_dbs(100)
+				.map_size(map_size)
+				.open(&db_path)
+				.expect("Unable to open LMDB DB");
+			db::lmdb_adapter::LmdbDb::init(db)
+		}
+		e => {
+			return Err(Error::Message(format!(
+				"Unsupported DB engine: {} (options: sled, sqlite, lmdb)",
+				e
+			)));
+		}
+	};
 
 	info!("Initializing background runner...");
 	let watch_cancel = netapp::util::watch_ctrl_c();
diff --git a/src/garage/tests/bucket.rs b/src/garage/tests/bucket.rs
index ff5cc8da..b32af068 100644
--- a/src/garage/tests/bucket.rs
+++ b/src/garage/tests/bucket.rs
@@ -29,8 +29,7 @@ async fn test_bucket_all() {
 			.unwrap()
 			.iter()
 			.filter(|x| x.name.as_ref().is_some())
-			.find(|x| x.name.as_ref().unwrap() == "hello")
-			.is_some());
+			.any(|x| x.name.as_ref().unwrap() == "hello"));
 	}
 	{
 		// Get its location
@@ -75,13 +74,12 @@ async fn test_bucket_all() {
 	{
 		// Check bucket is deleted with List buckets
 		let r = ctx.client.list_buckets().send().await.unwrap();
-		assert!(r
+		assert!(!r
 			.buckets
 			.as_ref()
 			.unwrap()
 			.iter()
 			.filter(|x| x.name.as_ref().is_some())
-			.find(|x| x.name.as_ref().unwrap() == "hello")
-			.is_none());
+			.any(|x| x.name.as_ref().unwrap() == "hello"));
 	}
 }
diff --git a/src/model/Cargo.toml b/src/model/Cargo.toml
index 133fe44e..d908dc01 100644
--- a/src/model/Cargo.toml
+++ b/src/model/Cargo.toml
@@ -14,6 +14,7 @@ path = "lib.rs"
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
 [dependencies]
+garage_db = { version = "0.8.0", path = "../db" }
 garage_rpc = { version = "0.7.0", path = "../rpc" }
 garage_table = { version = "0.7.0", path = "../table" }
 garage_block = { version = "0.7.0", path = "../block" }
@@ -30,8 +31,6 @@ tracing = "0.1.30"
 rand = "0.8"
 zstd = { version = "0.9", default-features = false }
 
-sled = "0.34"
-
 rmp-serde = "0.15"
 serde = { version = "1.0", default-features = false, features = ["derive", "rc"] }
 serde_bytes = "0.11"
diff --git a/src/model/garage.rs b/src/model/garage.rs
index 2f99bd68..280f3dc7 100644
--- a/src/model/garage.rs
+++ b/src/model/garage.rs
@@ -2,6 +2,8 @@ use std::sync::Arc;
 
 use netapp::NetworkKey;
 
+use garage_db as db;
+
 use garage_util::background::*;
 use garage_util::config::*;
 
@@ -33,7 +35,7 @@ pub struct Garage {
 	pub config: Config,
 
 	/// The local database
-	pub db: sled::Db,
+	pub db: db::Db,
 	/// A background job runner
 	pub background: Arc<BackgroundRunner>,
 	/// The membership manager
@@ -71,7 +73,7 @@ pub struct GarageK2V {
 
 impl Garage {
 	/// Create and run garage
-	pub fn new(config: Config, db: sled::Db, background: Arc<BackgroundRunner>) -> Arc<Self> {
+	pub fn new(config: Config, db: db::Db, background: Arc<BackgroundRunner>) -> Arc<Self> {
 		let network_key = NetworkKey::from_slice(
 			&hex::decode(&config.rpc_secret).expect("Invalid RPC secret key")[..],
 		)
@@ -199,7 +201,7 @@ impl Garage {
 
 #[cfg(feature = "k2v")]
 impl GarageK2V {
-	fn new(system: Arc<System>, db: &sled::Db, meta_rep_param: TableShardedReplication) -> Self {
+	fn new(system: Arc<System>, db: &db::Db, meta_rep_param: TableShardedReplication) -> Self {
 		info!("Initialize K2V counter table...");
 		let counter_table = IndexCounter::new(system.clone(), meta_rep_param.clone(), db);
 		info!("Initialize K2V subscription manager...");
diff --git a/src/model/index_counter.rs b/src/model/index_counter.rs
index 123154d4..2602d5d9 100644
--- a/src/model/index_counter.rs
+++ b/src/model/index_counter.rs
@@ -6,6 +6,8 @@ use std::time::Duration;
 use serde::{Deserialize, Serialize};
 use tokio::sync::{mpsc, watch};
 
+use garage_db as db;
+
 use garage_rpc::ring::Ring;
 use garage_rpc::system::System;
 use garage_util::data::*;
@@ -114,10 +116,6 @@ impl<T: CounterSchema> TableSchema for CounterTable<T> {
 	type E = CounterEntry<T>;
 	type Filter = (DeletedFilter, Vec<Uuid>);
 
-	fn updated(&self, _old: Option<&Self::E>, _new: Option<&Self::E>) {
-		// nothing for now
-	}
-
 	fn matches_filter(entry: &Self::E, filter: &Self::Filter) -> bool {
 		if filter.0 == DeletedFilter::Any {
 			return true;
@@ -135,7 +133,7 @@ impl<T: CounterSchema> TableSchema for CounterTable<T> {
 
 pub struct IndexCounter<T: CounterSchema> {
 	this_node: Uuid,
-	local_counter: sled::Tree,
+	local_counter: db::Tree,
 	propagate_tx: mpsc::UnboundedSender<(T::P, T::S, LocalCounterEntry)>,
 	pub table: Arc<Table<CounterTable<T>, TableShardedReplication>>,
 }
@@ -144,7 +142,7 @@ impl<T: CounterSchema> IndexCounter<T> {
 	pub fn new(
 		system: Arc<System>,
 		replication: TableShardedReplication,
-		db: &sled::Db,
+		db: &db::Db,
 	) -> Arc<Self> {
 		let background = system.background.clone();
 
@@ -174,36 +172,36 @@ impl<T: CounterSchema> IndexCounter<T> {
 		this
 	}
 
-	pub fn count(&self, pk: &T::P, sk: &T::S, counts: &[(&str, i64)]) -> Result<(), Error> {
+	pub fn count(
+		&self,
+		tx: &mut db::Transaction,
+		pk: &T::P,
+		sk: &T::S,
+		counts: &[(&str, i64)],
+	) -> db::TxResult<(), Error> {
 		let tree_key = self.table.data.tree_key(pk, sk);
 
-		let new_entry = self.local_counter.transaction(|tx| {
-			let mut entry = match tx.get(&tree_key[..])? {
-				Some(old_bytes) => {
-					rmp_serde::decode::from_read_ref::<_, LocalCounterEntry>(&old_bytes)
-						.map_err(Error::RmpDecode)
-						.map_err(sled::transaction::ConflictableTransactionError::Abort)?
-				}
-				None => LocalCounterEntry {
-					values: BTreeMap::new(),
-				},
-			};
-
-			for (s, inc) in counts.iter() {
-				let mut ent = entry.values.entry(s.to_string()).or_insert((0, 0));
-				ent.0 += 1;
-				ent.1 += *inc;
-			}
-
-			let new_entry_bytes = rmp_to_vec_all_named(&entry)
-				.map_err(Error::RmpEncode)
-				.map_err(sled::transaction::ConflictableTransactionError::Abort)?;
-			tx.insert(&tree_key[..], new_entry_bytes)?;
+		let mut entry = match tx.get(&self.local_counter, &tree_key[..])? {
+			Some(old_bytes) => rmp_serde::decode::from_read_ref::<_, LocalCounterEntry>(&old_bytes)
+				.map_err(Error::RmpDecode)
+				.map_err(db::TxError::Abort)?,
+			None => LocalCounterEntry {
+				values: BTreeMap::new(),
+			},
+		};
+
+		for (s, inc) in counts.iter() {
+			let mut ent = entry.values.entry(s.to_string()).or_insert((0, 0));
+			ent.0 += 1;
+			ent.1 += *inc;
+		}
 
-			Ok(entry)
-		})?;
+		let new_entry_bytes = rmp_to_vec_all_named(&entry)
+			.map_err(Error::RmpEncode)
+			.map_err(db::TxError::Abort)?;
+		tx.insert(&self.local_counter, &tree_key[..], new_entry_bytes)?;
 
-		if let Err(e) = self.propagate_tx.send((pk.clone(), sk.clone(), new_entry)) {
+		if let Err(e) = self.propagate_tx.send((pk.clone(), sk.clone(), entry)) {
 			error!(
 				"Could not propagate updated counter values, failed to send to channel: {}",
 				e
diff --git a/src/model/k2v/item_table.rs b/src/model/k2v/item_table.rs
index 8b7cc08a..991fe66d 100644
--- a/src/model/k2v/item_table.rs
+++ b/src/model/k2v/item_table.rs
@@ -2,6 +2,7 @@ use serde::{Deserialize, Serialize};
 use std::collections::BTreeMap;
 use std::sync::Arc;
 
+use garage_db as db;
 use garage_util::data::*;
 
 use garage_table::crdt::*;
@@ -221,7 +222,12 @@ impl TableSchema for K2VItemTable {
 	type E = K2VItem;
 	type Filter = ItemFilter;
 
-	fn updated(&self, old: Option<&Self::E>, new: Option<&Self::E>) {
+	fn updated(
+		&self,
+		tx: &mut db::Transaction,
+		old: Option<&Self::E>,
+		new: Option<&Self::E>,
+	) -> db::TxOpResult<()> {
 		// 1. Count
 		let (old_entries, old_conflicts, old_values, old_bytes) = match old {
 			None => (0, 0, 0, 0),
@@ -239,7 +245,8 @@ impl TableSchema for K2VItemTable {
 			.map(|e| &e.partition.partition_key)
 			.unwrap_or_else(|| &new.unwrap().partition.partition_key);
 
-		if let Err(e) = self.counter_table.count(
+		let counter_res = self.counter_table.count(
+			tx,
 			&count_pk,
 			count_sk,
 			&[
@@ -248,14 +255,23 @@ impl TableSchema for K2VItemTable {
 				(VALUES, new_values - old_values),
 				(BYTES, new_bytes - old_bytes),
 			],
-		) {
-			error!("Could not update K2V counter for bucket {:?} partition {}; counts will now be inconsistent. {}", count_pk, count_sk, e);
+		);
+		if let Err(e) = db::unabort(counter_res)? {
+			// This result can be returned by `counter_table.count()` for instance
+			// if messagepack serialization or deserialization fails at some step.
+			// Warn admin but ignore this error for now, that's all we can do.
+			error!(
+				"Unable to update K2V item counter for bucket {:?} partition {}: {}. Index values will be wrong!",
+				count_pk, count_sk, e
+			);
 		}
 
 		// 2. Notify
 		if let Some(new_ent) = new {
 			self.subscriptions.notify(new_ent);
 		}
+
+		Ok(())
 	}
 
 	#[allow(clippy::nonminimal_bool)]
diff --git a/src/model/migrate.rs b/src/model/migrate.rs
index 7e61957a..25acb4b0 100644
--- a/src/model/migrate.rs
+++ b/src/model/migrate.rs
@@ -25,11 +25,15 @@ impl Migrate {
 			.open_tree("bucket:table")
 			.map_err(GarageError::from)?;
 
-		for res in tree.iter() {
+		let mut old_buckets = vec![];
+		for res in tree.iter().map_err(GarageError::from)? {
 			let (_k, v) = res.map_err(GarageError::from)?;
 			let bucket = rmp_serde::decode::from_read_ref::<_, old_bucket::Bucket>(&v[..])
 				.map_err(GarageError::from)?;
+			old_buckets.push(bucket);
+		}
 
+		for bucket in old_buckets {
 			if let old_bucket::BucketState::Present(p) = bucket.state.get() {
 				self.migrate_buckets050_do_bucket(&bucket, p).await?;
 			}
diff --git a/src/model/s3/block_ref_table.rs b/src/model/s3/block_ref_table.rs
index 9b3991bf..9589b4aa 100644
--- a/src/model/s3/block_ref_table.rs
+++ b/src/model/s3/block_ref_table.rs
@@ -1,6 +1,8 @@
 use serde::{Deserialize, Serialize};
 use std::sync::Arc;
 
+use garage_db as db;
+
 use garage_util::data::*;
 
 use garage_table::crdt::Crdt;
@@ -51,21 +53,22 @@ impl TableSchema for BlockRefTable {
 	type E = BlockRef;
 	type Filter = DeletedFilter;
 
-	fn updated(&self, old: Option<&Self::E>, new: Option<&Self::E>) {
-		#[allow(clippy::or_fun_call)]
-		let block = &old.or(new).unwrap().block;
+	fn updated(
+		&self,
+		tx: &mut db::Transaction,
+		old: Option<&Self::E>,
+		new: Option<&Self::E>,
+	) -> db::TxOpResult<()> {
+		let block = old.or(new).unwrap().block;
 		let was_before = old.map(|x| !x.deleted.get()).unwrap_or(false);
 		let is_after = new.map(|x| !x.deleted.get()).unwrap_or(false);
 		if is_after && !was_before {
-			if let Err(e) = self.block_manager.block_incref(block) {
-				warn!("block_incref failed for block {:?}: {}", block, e);
-			}
+			self.block_manager.block_incref(tx, block)?;
 		}
 		if was_before && !is_after {
-			if let Err(e) = self.block_manager.block_decref(block) {
-				warn!("block_decref failed for block {:?}: {}", block, e);
-			}
+			self.block_manager.block_decref(tx, block)?;
 		}
+		Ok(())
 	}
 
 	fn matches_filter(entry: &Self::E, filter: &Self::Filter) -> bool {
diff --git a/src/model/s3/object_table.rs b/src/model/s3/object_table.rs
index 3d9a89f7..62f5d8d9 100644
--- a/src/model/s3/object_table.rs
+++ b/src/model/s3/object_table.rs
@@ -2,6 +2,8 @@ use serde::{Deserialize, Serialize};
 use std::collections::BTreeMap;
 use std::sync::Arc;
 
+use garage_db as db;
+
 use garage_util::background::BackgroundRunner;
 use garage_util::data::*;
 
@@ -232,7 +234,12 @@ impl TableSchema for ObjectTable {
 	type E = Object;
 	type Filter = ObjectFilter;
 
-	fn updated(&self, old: Option<&Self::E>, new: Option<&Self::E>) {
+	fn updated(
+		&self,
+		_tx: &mut db::Transaction,
+		old: Option<&Self::E>,
+		new: Option<&Self::E>,
+	) -> db::TxOpResult<()> {
 		let version_table = self.version_table.clone();
 		let old = old.cloned();
 		let new = new.cloned();
@@ -259,7 +266,8 @@ impl TableSchema for ObjectTable {
 				}
 			}
 			Ok(())
-		})
+		});
+		Ok(())
 	}
 
 	fn matches_filter(entry: &Self::E, filter: &Self::Filter) -> bool {
diff --git a/src/model/s3/version_table.rs b/src/model/s3/version_table.rs
index ad096772..881c245a 100644
--- a/src/model/s3/version_table.rs
+++ b/src/model/s3/version_table.rs
@@ -1,6 +1,8 @@
 use serde::{Deserialize, Serialize};
 use std::sync::Arc;
 
+use garage_db as db;
+
 use garage_util::background::BackgroundRunner;
 use garage_util::data::*;
 
@@ -137,7 +139,12 @@ impl TableSchema for VersionTable {
 	type E = Version;
 	type Filter = DeletedFilter;
 
-	fn updated(&self, old: Option<&Self::E>, new: Option<&Self::E>) {
+	fn updated(
+		&self,
+		_tx: &mut db::Transaction,
+		old: Option<&Self::E>,
+		new: Option<&Self::E>,
+	) -> db::TxOpResult<()> {
 		let block_ref_table = self.block_ref_table.clone();
 		let old = old.cloned();
 		let new = new.cloned();
@@ -160,7 +167,9 @@ impl TableSchema for VersionTable {
 				}
 			}
 			Ok(())
-		})
+		});
+
+		Ok(())
 	}
 
 	fn matches_filter(entry: &Self::E, filter: &Self::Filter) -> bool {
diff --git a/src/table/Cargo.toml b/src/table/Cargo.toml
index ed1a213f..6de37cda 100644
--- a/src/table/Cargo.toml
+++ b/src/table/Cargo.toml
@@ -14,6 +14,7 @@ path = "lib.rs"
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
 [dependencies]
+garage_db = { version = "0.8.0", path = "../db" }
 garage_rpc = { version = "0.7.0", path = "../rpc" }
 garage_util = { version = "0.7.0", path = "../util" }
 
@@ -25,8 +26,6 @@ hexdump = "0.1"
 tracing = "0.1.30"
 rand = "0.8"
 
-sled = "0.34"
-
 rmp-serde = "0.15"
 serde = { version = "1.0", default-features = false, features = ["derive", "rc"] }
 serde_bytes = "0.11"
diff --git a/src/table/data.rs b/src/table/data.rs
index 5cb10066..3212e82b 100644
--- a/src/table/data.rs
+++ b/src/table/data.rs
@@ -3,12 +3,13 @@ use std::convert::TryInto;
 use std::sync::Arc;
 
 use serde_bytes::ByteBuf;
-use sled::{IVec, Transactional};
 use tokio::sync::Notify;
 
+use garage_db as db;
+use garage_db::counted_tree_hack::CountedTree;
+
 use garage_util::data::*;
 use garage_util::error::*;
-use garage_util::sled_counter::SledCountedTree;
 
 use garage_rpc::system::System;
 
@@ -25,12 +26,12 @@ pub struct TableData<F: TableSchema, R: TableReplication> {
 	pub instance: F,
 	pub replication: R,
 
-	pub store: sled::Tree,
+	pub store: db::Tree,
 
-	pub(crate) merkle_tree: sled::Tree,
-	pub(crate) merkle_todo: sled::Tree,
+	pub(crate) merkle_tree: db::Tree,
+	pub(crate) merkle_todo: db::Tree,
 	pub(crate) merkle_todo_notify: Notify,
-	pub(crate) gc_todo: SledCountedTree,
+	pub(crate) gc_todo: CountedTree,
 
 	pub(crate) metrics: TableMetrics,
 }
@@ -40,7 +41,7 @@ where
 	F: TableSchema,
 	R: TableReplication,
 {
-	pub fn new(system: Arc<System>, instance: F, replication: R, db: &sled::Db) -> Arc<Self> {
+	pub fn new(system: Arc<System>, instance: F, replication: R, db: &db::Db) -> Arc<Self> {
 		let store = db
 			.open_tree(&format!("{}:table", F::TABLE_NAME))
 			.expect("Unable to open DB tree");
@@ -55,7 +56,7 @@ where
 		let gc_todo = db
 			.open_tree(&format!("{}:gc_todo_v2", F::TABLE_NAME))
 			.expect("Unable to open DB tree");
-		let gc_todo = SledCountedTree::new(gc_todo);
+		let gc_todo = CountedTree::new(gc_todo).expect("Cannot count gc_todo_v2");
 
 		let metrics = TableMetrics::new(F::TABLE_NAME, merkle_todo.clone(), gc_todo.clone());
 
@@ -98,30 +99,30 @@ where
 					None => partition_hash.to_vec(),
 					Some(sk) => self.tree_key(partition_key, sk),
 				};
-				let range = self.store.range(first_key..);
+				let range = self.store.range(first_key..)?;
 				self.read_range_aux(partition_hash, range, filter, limit)
 			}
 			EnumerationOrder::Reverse => match start {
 				Some(sk) => {
 					let last_key = self.tree_key(partition_key, sk);
-					let range = self.store.range(..=last_key).rev();
+					let range = self.store.range_rev(..=last_key)?;
 					self.read_range_aux(partition_hash, range, filter, limit)
 				}
 				None => {
 					let mut last_key = partition_hash.to_vec();
 					let lower = u128::from_be_bytes(last_key[16..32].try_into().unwrap());
 					last_key[16..32].copy_from_slice(&u128::to_be_bytes(lower + 1));
-					let range = self.store.range(..last_key).rev();
+					let range = self.store.range_rev(..last_key)?;
 					self.read_range_aux(partition_hash, range, filter, limit)
 				}
 			},
 		}
 	}
 
-	fn read_range_aux(
+	fn read_range_aux<'a>(
 		&self,
 		partition_hash: Hash,
-		range: impl Iterator<Item = sled::Result<(IVec, IVec)>>,
+		range: db::ValueIter<'a>,
 		filter: &Option<F::Filter>,
 		limit: usize,
 	) -> Result<Vec<Arc<ByteBuf>>, Error> {
@@ -139,7 +140,7 @@ where
 				}
 			};
 			if keep {
-				ret.push(Arc::new(ByteBuf::from(value.as_ref())));
+				ret.push(Arc::new(ByteBuf::from(value)));
 			}
 			if ret.len() >= limit {
 				break;
@@ -183,12 +184,10 @@ where
 		tree_key: &[u8],
 		f: impl Fn(Option<F::E>) -> F::E,
 	) -> Result<Option<F::E>, Error> {
-		let changed = (&self.store, &self.merkle_todo).transaction(|(store, mkl_todo)| {
-			let (old_entry, old_bytes, new_entry) = match store.get(tree_key)? {
+		let changed = self.store.db().transaction(|mut tx| {
+			let (old_entry, old_bytes, new_entry) = match tx.get(&self.store, tree_key)? {
 				Some(old_bytes) => {
-					let old_entry = self
-						.decode_entry(&old_bytes)
-						.map_err(sled::transaction::ConflictableTransactionError::Abort)?;
+					let old_entry = self.decode_entry(&old_bytes).map_err(db::TxError::Abort)?;
 					let new_entry = f(Some(old_entry.clone()));
 					(Some(old_entry), Some(old_bytes), new_entry)
 				}
@@ -204,24 +203,28 @@ where
 			// the associated Merkle tree entry.
 			let new_bytes = rmp_to_vec_all_named(&new_entry)
 				.map_err(Error::RmpEncode)
-				.map_err(sled::transaction::ConflictableTransactionError::Abort)?;
+				.map_err(db::TxError::Abort)?;
 			let encoding_changed = Some(&new_bytes[..]) != old_bytes.as_ref().map(|x| &x[..]);
+			drop(old_bytes);
 
 			if value_changed || encoding_changed {
 				let new_bytes_hash = blake2sum(&new_bytes[..]);
-				mkl_todo.insert(tree_key.to_vec(), new_bytes_hash.as_slice())?;
-				store.insert(tree_key.to_vec(), new_bytes)?;
-				Ok(Some((old_entry, new_entry, new_bytes_hash)))
+				tx.insert(&self.merkle_todo, tree_key, new_bytes_hash.as_slice())?;
+				tx.insert(&self.store, tree_key, new_bytes)?;
+
+				self.instance
+					.updated(&mut tx, old_entry.as_ref(), Some(&new_entry))?;
+
+				Ok(Some((new_entry, new_bytes_hash)))
 			} else {
 				Ok(None)
 			}
 		})?;
 
-		if let Some((old_entry, new_entry, new_bytes_hash)) = changed {
+		if let Some((new_entry, new_bytes_hash)) = changed {
 			self.metrics.internal_update_counter.add(1);
 
 			let is_tombstone = new_entry.is_tombstone();
-			self.instance.updated(old_entry.as_ref(), Some(&new_entry));
 			self.merkle_todo_notify.notify_one();
 			if is_tombstone {
 				// We are only responsible for GC'ing this item if we are the
@@ -244,22 +247,23 @@ where
 	}
 
 	pub(crate) fn delete_if_equal(self: &Arc<Self>, k: &[u8], v: &[u8]) -> Result<bool, Error> {
-		let removed = (&self.store, &self.merkle_todo).transaction(|(store, mkl_todo)| {
-			if let Some(cur_v) = store.get(k)? {
-				if cur_v == v {
-					store.remove(k)?;
-					mkl_todo.insert(k, vec![])?;
-					return Ok(true);
+		let removed = self
+			.store
+			.db()
+			.transaction(|mut tx| match tx.get(&self.store, k)? {
+				Some(cur_v) if cur_v == v => {
+					tx.remove(&self.store, k)?;
+					tx.insert(&self.merkle_todo, k, vec![])?;
+
+					let old_entry = self.decode_entry(v).map_err(db::TxError::Abort)?;
+					self.instance.updated(&mut tx, Some(&old_entry), None)?;
+					Ok(true)
 				}
-			}
-			Ok(false)
-		})?;
+				_ => Ok(false),
+			})?;
 
 		if removed {
 			self.metrics.internal_delete_counter.add(1);
-
-			let old_entry = self.decode_entry(v)?;
-			self.instance.updated(Some(&old_entry), None);
 			self.merkle_todo_notify.notify_one();
 		}
 		Ok(removed)
@@ -270,25 +274,26 @@ where
 		k: &[u8],
 		vhash: Hash,
 	) -> Result<bool, Error> {
-		let removed = (&self.store, &self.merkle_todo).transaction(|(store, mkl_todo)| {
-			if let Some(cur_v) = store.get(k)? {
-				if blake2sum(&cur_v[..]) == vhash {
-					store.remove(k)?;
-					mkl_todo.insert(k, vec![])?;
-					return Ok(Some(cur_v));
+		let removed = self
+			.store
+			.db()
+			.transaction(|mut tx| match tx.get(&self.store, k)? {
+				Some(cur_v) if blake2sum(&cur_v[..]) == vhash => {
+					tx.remove(&self.store, k)?;
+					tx.insert(&self.merkle_todo, k, vec![])?;
+
+					let old_entry = self.decode_entry(&cur_v[..]).map_err(db::TxError::Abort)?;
+					self.instance.updated(&mut tx, Some(&old_entry), None)?;
+					Ok(true)
 				}
-			}
-			Ok(None)
-		})?;
+				_ => Ok(false),
+			})?;
 
-		if let Some(old_v) = removed {
-			let old_entry = self.decode_entry(&old_v[..])?;
-			self.instance.updated(Some(&old_entry), None);
+		if removed {
+			self.metrics.internal_delete_counter.add(1);
 			self.merkle_todo_notify.notify_one();
-			Ok(true)
-		} else {
-			Ok(false)
 		}
+		Ok(removed)
 	}
 
 	// ---- Utility functions ----
@@ -315,7 +320,7 @@ where
 		}
 	}
 
-	pub fn gc_todo_len(&self) -> usize {
-		self.gc_todo.len()
+	pub fn gc_todo_len(&self) -> Result<usize, Error> {
+		Ok(self.gc_todo.len())
 	}
 }
diff --git a/src/table/gc.rs b/src/table/gc.rs
index 2a05b6ae..e7fbbcb0 100644
--- a/src/table/gc.rs
+++ b/src/table/gc.rs
@@ -12,9 +12,10 @@ use futures::select;
 use futures_util::future::*;
 use tokio::sync::watch;
 
+use garage_db::counted_tree_hack::CountedTree;
+
 use garage_util::data::*;
 use garage_util::error::*;
-use garage_util::sled_counter::SledCountedTree;
 use garage_util::time::*;
 
 use garage_rpc::system::System;
@@ -100,18 +101,16 @@ where
 	async fn gc_loop_iter(&self) -> Result<Option<Duration>, Error> {
 		let now = now_msec();
 
-		let mut entries = vec![];
-		let mut excluded = vec![];
-
 		// List entries in the GC todo list
 		// These entries are put there when a tombstone is inserted in the table
 		// (see update_entry in data.rs)
-		for entry_kv in self.data.gc_todo.iter() {
+		let mut candidates = vec![];
+		for entry_kv in self.data.gc_todo.iter()? {
 			let (k, vhash) = entry_kv?;
-			let mut todo_entry = GcTodoEntry::parse(&k, &vhash);
+			let todo_entry = GcTodoEntry::parse(&k, &vhash);
 
 			if todo_entry.deletion_time() > now {
-				if entries.is_empty() && excluded.is_empty() {
+				if candidates.is_empty() {
 					// If the earliest entry in the todo list shouldn't yet be processed,
 					// return a duration to wait in the loop
 					return Ok(Some(Duration::from_millis(
@@ -123,15 +122,23 @@ where
 				}
 			}
 
-			let vhash = Hash::try_from(&vhash[..]).unwrap();
+			candidates.push(todo_entry);
+			if candidates.len() >= 2 * TABLE_GC_BATCH_SIZE {
+				break;
+			}
+		}
 
+		let mut entries = vec![];
+		let mut excluded = vec![];
+		for mut todo_entry in candidates {
 			// Check if the tombstone is still the current value of the entry.
 			// If not, we don't actually want to GC it, and we will remove it
 			// from the gc_todo table later (below).
+			let vhash = todo_entry.value_hash;
 			todo_entry.value = self
 				.data
 				.store
-				.get(&k[..])?
+				.get(&todo_entry.key[..])?
 				.filter(|v| blake2sum(&v[..]) == vhash)
 				.map(|v| v.to_vec());
 
@@ -353,17 +360,17 @@ impl GcTodoEntry {
 	}
 
 	/// Parses a GcTodoEntry from a (k, v) pair stored in the gc_todo tree
-	pub(crate) fn parse(sled_k: &[u8], sled_v: &[u8]) -> Self {
+	pub(crate) fn parse(db_k: &[u8], db_v: &[u8]) -> Self {
 		Self {
-			tombstone_timestamp: u64::from_be_bytes(sled_k[0..8].try_into().unwrap()),
-			key: sled_k[8..].to_vec(),
-			value_hash: Hash::try_from(sled_v).unwrap(),
+			tombstone_timestamp: u64::from_be_bytes(db_k[0..8].try_into().unwrap()),
+			key: db_k[8..].to_vec(),
+			value_hash: Hash::try_from(db_v).unwrap(),
 			value: None,
 		}
 	}
 
 	/// Saves the GcTodoEntry in the gc_todo tree
-	pub(crate) fn save(&self, gc_todo_tree: &SledCountedTree) -> Result<(), Error> {
+	pub(crate) fn save(&self, gc_todo_tree: &CountedTree) -> Result<(), Error> {
 		gc_todo_tree.insert(self.todo_table_key(), self.value_hash.as_slice())?;
 		Ok(())
 	}
@@ -373,9 +380,9 @@ impl GcTodoEntry {
 	/// This is usefull to remove a todo entry only under the condition
 	/// that it has not changed since the time it was read, i.e.
 	/// what we have to do is still the same
-	pub(crate) fn remove_if_equal(&self, gc_todo_tree: &SledCountedTree) -> Result<(), Error> {
-		let _ = gc_todo_tree.compare_and_swap::<_, _, Vec<u8>>(
-			&self.todo_table_key()[..],
+	pub(crate) fn remove_if_equal(&self, gc_todo_tree: &CountedTree) -> Result<(), Error> {
+		gc_todo_tree.compare_and_swap::<_, _, &[u8]>(
+			&self.todo_table_key(),
 			Some(self.value_hash),
 			None,
 		)?;
diff --git a/src/table/merkle.rs b/src/table/merkle.rs
index 93bf7e47..7685b193 100644
--- a/src/table/merkle.rs
+++ b/src/table/merkle.rs
@@ -4,11 +4,10 @@ use std::time::Duration;
 use futures::select;
 use futures_util::future::*;
 use serde::{Deserialize, Serialize};
-use sled::transaction::{
-	ConflictableTransactionError, ConflictableTransactionResult, TransactionalTree,
-};
 use tokio::sync::watch;
 
+use garage_db as db;
+
 use garage_util::background::BackgroundRunner;
 use garage_util::data::*;
 use garage_util::error::Error;
@@ -90,35 +89,35 @@ where
 
 	async fn updater_loop(self: Arc<Self>, mut must_exit: watch::Receiver<bool>) {
 		while !*must_exit.borrow() {
-			if let Some(x) = self.data.merkle_todo.iter().next() {
-				match x {
-					Ok((key, valhash)) => {
-						if let Err(e) = self.update_item(&key[..], &valhash[..]) {
-							warn!(
-								"({}) Error while updating Merkle tree item: {}",
-								F::TABLE_NAME,
-								e
-							);
-						}
-					}
-					Err(e) => {
-						warn!(
-							"({}) Error while iterating on Merkle todo tree: {}",
-							F::TABLE_NAME,
-							e
-						);
-						tokio::time::sleep(Duration::from_secs(10)).await;
+			match self.updater_loop_iter() {
+				Ok(true) => (),
+				Ok(false) => {
+					select! {
+						_ = self.data.merkle_todo_notify.notified().fuse() => {},
+						_ = must_exit.changed().fuse() => {},
 					}
 				}
-			} else {
-				select! {
-					_ = self.data.merkle_todo_notify.notified().fuse() => {},
-					_ = must_exit.changed().fuse() => {},
+				Err(e) => {
+					warn!(
+						"({}) Error while updating Merkle tree item: {}",
+						F::TABLE_NAME,
+						e
+					);
+					tokio::time::sleep(Duration::from_secs(10)).await;
 				}
 			}
 		}
 	}
 
+	fn updater_loop_iter(&self) -> Result<bool, Error> {
+		if let Some((key, valhash)) = self.data.merkle_todo.first()? {
+			self.update_item(&key, &valhash)?;
+			Ok(true)
+		} else {
+			Ok(false)
+		}
+	}
+
 	fn update_item(&self, k: &[u8], vhash_by: &[u8]) -> Result<(), Error> {
 		let khash = blake2sum(k);
 
@@ -137,13 +136,16 @@ where
 		};
 		self.data
 			.merkle_tree
-			.transaction(|tx| self.update_item_rec(tx, k, &khash, &key, new_vhash))?;
+			.db()
+			.transaction(|mut tx| self.update_item_rec(&mut tx, k, &khash, &key, new_vhash))?;
 
-		let deleted = self
-			.data
-			.merkle_todo
-			.compare_and_swap::<_, _, Vec<u8>>(k, Some(vhash_by), None)?
-			.is_ok();
+		let deleted = self.data.merkle_todo.db().transaction(|mut tx| {
+			let remove = matches!(tx.get(&self.data.merkle_todo, k)?, Some(ov) if ov == vhash_by);
+			if remove {
+				tx.remove(&self.data.merkle_todo, k)?;
+			}
+			Ok(remove)
+		})?;
 
 		if !deleted {
 			debug!(
@@ -157,12 +159,12 @@ where
 
 	fn update_item_rec(
 		&self,
-		tx: &TransactionalTree,
+		tx: &mut db::Transaction<'_>,
 		k: &[u8],
 		khash: &Hash,
 		key: &MerkleNodeKey,
 		new_vhash: Option<Hash>,
-	) -> ConflictableTransactionResult<Option<Hash>, Error> {
+	) -> db::TxResult<Option<Hash>, Error> {
 		let i = key.prefix.len();
 
 		// Read node at current position (defined by the prefix stored in key)
@@ -203,7 +205,7 @@ where
 							}
 							MerkleNode::Intermediate(_) => Some(MerkleNode::Intermediate(children)),
 							x @ MerkleNode::Leaf(_, _) => {
-								tx.remove(key_sub.encode())?;
+								tx.remove(&self.data.merkle_tree, key_sub.encode())?;
 								Some(x)
 							}
 						}
@@ -283,28 +285,27 @@ where
 
 	fn read_node_txn(
 		&self,
-		tx: &TransactionalTree,
+		tx: &mut db::Transaction<'_>,
 		k: &MerkleNodeKey,
-	) -> ConflictableTransactionResult<MerkleNode, Error> {
-		let ent = tx.get(k.encode())?;
-		MerkleNode::decode_opt(ent).map_err(ConflictableTransactionError::Abort)
+	) -> db::TxResult<MerkleNode, Error> {
+		let ent = tx.get(&self.data.merkle_tree, k.encode())?;
+		MerkleNode::decode_opt(&ent).map_err(db::TxError::Abort)
 	}
 
 	fn put_node_txn(
 		&self,
-		tx: &TransactionalTree,
+		tx: &mut db::Transaction<'_>,
 		k: &MerkleNodeKey,
 		v: &MerkleNode,
-	) -> ConflictableTransactionResult<Hash, Error> {
+	) -> db::TxResult<Hash, Error> {
 		trace!("Put Merkle node: {:?} => {:?}", k, v);
 		if *v == MerkleNode::Empty {
-			tx.remove(k.encode())?;
+			tx.remove(&self.data.merkle_tree, k.encode())?;
 			Ok(self.empty_node_hash)
 		} else {
-			let vby = rmp_to_vec_all_named(v)
-				.map_err(|e| ConflictableTransactionError::Abort(e.into()))?;
+			let vby = rmp_to_vec_all_named(v).map_err(|e| db::TxError::Abort(e.into()))?;
 			let rethash = blake2sum(&vby[..]);
-			tx.insert(k.encode(), vby)?;
+			tx.insert(&self.data.merkle_tree, k.encode(), vby)?;
 			Ok(rethash)
 		}
 	}
@@ -312,15 +313,15 @@ where
 	// Access a node in the Merkle tree, used by the sync protocol
 	pub(crate) fn read_node(&self, k: &MerkleNodeKey) -> Result<MerkleNode, Error> {
 		let ent = self.data.merkle_tree.get(k.encode())?;
-		MerkleNode::decode_opt(ent)
+		MerkleNode::decode_opt(&ent)
 	}
 
-	pub fn merkle_tree_len(&self) -> usize {
-		self.data.merkle_tree.len()
+	pub fn merkle_tree_len(&self) -> Result<usize, Error> {
+		Ok(self.data.merkle_tree.len()?)
 	}
 
-	pub fn todo_len(&self) -> usize {
-		self.data.merkle_todo.len()
+	pub fn todo_len(&self) -> Result<usize, Error> {
+		Ok(self.data.merkle_todo.len()?)
 	}
 }
 
@@ -347,7 +348,7 @@ impl MerkleNodeKey {
 }
 
 impl MerkleNode {
-	fn decode_opt(ent: Option<sled::IVec>) -> Result<Self, Error> {
+	fn decode_opt(ent: &Option<db::Value>) -> Result<Self, Error> {
 		match ent {
 			None => Ok(MerkleNode::Empty),
 			Some(v) => Ok(rmp_serde::decode::from_read_ref::<_, MerkleNode>(&v[..])?),
diff --git a/src/table/metrics.rs b/src/table/metrics.rs
index 752a2a6d..3a1783e0 100644
--- a/src/table/metrics.rs
+++ b/src/table/metrics.rs
@@ -1,6 +1,7 @@
 use opentelemetry::{global, metrics::*, KeyValue};
 
-use garage_util::sled_counter::SledCountedTree;
+use garage_db as db;
+use garage_db::counted_tree_hack::CountedTree;
 
 /// TableMetrics reference all counter used for metrics
 pub struct TableMetrics {
@@ -19,21 +20,19 @@ pub struct TableMetrics {
 	pub(crate) sync_items_received: Counter<u64>,
 }
 impl TableMetrics {
-	pub fn new(
-		table_name: &'static str,
-		merkle_todo: sled::Tree,
-		gc_todo: SledCountedTree,
-	) -> Self {
+	pub fn new(table_name: &'static str, merkle_todo: db::Tree, gc_todo: CountedTree) -> Self {
 		let meter = global::meter(table_name);
 		TableMetrics {
 			_merkle_todo_len: meter
 				.u64_value_observer(
 					"table.merkle_updater_todo_queue_length",
 					move |observer| {
-						observer.observe(
-							merkle_todo.len() as u64,
-							&[KeyValue::new("table_name", table_name)],
-						)
+						if let Ok(v) = merkle_todo.len() {
+							observer.observe(
+								v as u64,
+								&[KeyValue::new("table_name", table_name)],
+							);
+						}
 					},
 				)
 				.with_description("Merkle tree updater TODO queue length")
@@ -45,7 +44,7 @@ impl TableMetrics {
 						observer.observe(
 							gc_todo.len() as u64,
 							&[KeyValue::new("table_name", table_name)],
-						)
+						);
 					},
 				)
 				.with_description("Table garbage collector TODO queue length")
diff --git a/src/table/schema.rs b/src/table/schema.rs
index 37327037..74f57798 100644
--- a/src/table/schema.rs
+++ b/src/table/schema.rs
@@ -1,5 +1,6 @@
 use serde::{Deserialize, Serialize};
 
+use garage_db as db;
 use garage_util::data::*;
 
 use crate::crdt::Crdt;
@@ -82,11 +83,19 @@ pub trait TableSchema: Send + Sync {
 		None
 	}
 
-	// Updated triggers some stuff downstream, but it is not supposed to block or fail,
-	// as the update itself is an unchangeable fact that will never go back
-	// due to CRDT logic. Typically errors in propagation of info should be logged
-	// to stderr.
-	fn updated(&self, _old: Option<&Self::E>, _new: Option<&Self::E>) {}
+	/// Actions triggered by data changing in a table. If such actions
+	/// include updates to the local database that should be applied
+	/// atomically with the item update itself, a db transaction is
+	/// provided on which these changes should be done.
+	/// This function can return a DB error but that's all.
+	fn updated(
+		&self,
+		_tx: &mut db::Transaction,
+		_old: Option<&Self::E>,
+		_new: Option<&Self::E>,
+	) -> db::TxOpResult<()> {
+		Ok(())
+	}
 
 	fn matches_filter(entry: &Self::E, filter: &Self::Filter) -> bool;
 }
diff --git a/src/table/sync.rs b/src/table/sync.rs
index 08069ad0..4c83e991 100644
--- a/src/table/sync.rs
+++ b/src/table/sync.rs
@@ -258,9 +258,9 @@ where
 		while !*must_exit.borrow() {
 			let mut items = Vec::new();
 
-			for item in self.data.store.range(begin.to_vec()..end.to_vec()) {
+			for item in self.data.store.range(begin.to_vec()..end.to_vec())? {
 				let (key, value) = item?;
-				items.push((key.to_vec(), Arc::new(ByteBuf::from(value.as_ref()))));
+				items.push((key.to_vec(), Arc::new(ByteBuf::from(value))));
 
 				if items.len() >= 1024 {
 					break;
@@ -603,8 +603,16 @@ impl SyncTodo {
 			let retain = nodes.contains(&my_id);
 			if !retain {
 				// Check if we have some data to send, otherwise skip
-				if data.store.range(begin..end).next().is_none() {
-					continue;
+				match data.store.range(begin..end) {
+					Ok(mut iter) => {
+						if iter.next().is_none() {
+							continue;
+						}
+					}
+					Err(e) => {
+						warn!("DB error in add_full_sync: {}", e);
+						continue;
+					}
 				}
 			}
 
diff --git a/src/table/table.rs b/src/table/table.rs
index 2a167604..3c211728 100644
--- a/src/table/table.rs
+++ b/src/table/table.rs
@@ -13,6 +13,8 @@ use opentelemetry::{
 	Context,
 };
 
+use garage_db as db;
+
 use garage_util::data::*;
 use garage_util::error::Error;
 use garage_util::metrics::RecordDuration;
@@ -69,7 +71,7 @@ where
 {
 	// =============== PUBLIC INTERFACE FUNCTIONS (new, insert, get, etc) ===============
 
-	pub fn new(instance: F, replication: R, system: Arc<System>, db: &sled::Db) -> Arc<Self> {
+	pub fn new(instance: F, replication: R, system: Arc<System>, db: &db::Db) -> Arc<Self> {
 		let endpoint = system
 			.netapp
 			.endpoint(format!("garage_table/table.rs/Rpc:{}", F::TABLE_NAME));
diff --git a/src/util/Cargo.toml b/src/util/Cargo.toml
index 95cde531..5d073436 100644
--- a/src/util/Cargo.toml
+++ b/src/util/Cargo.toml
@@ -14,6 +14,8 @@ path = "lib.rs"
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
 [dependencies]
+garage_db = { version = "0.8.0", path = "../db" }
+
 blake2 = "0.9"
 err-derive = "0.3"
 xxhash-rust = { version = "0.8", default-features = false, features = ["xxh3"] }
@@ -22,8 +24,6 @@ tracing = "0.1.30"
 rand = "0.8"
 sha2 = "0.9"
 
-sled = "0.34"
-
 chrono = "0.4"
 rmp-serde = "0.15"
 serde = { version = "1.0", default-features = false, features = ["derive", "rc"] }
diff --git a/src/util/config.rs b/src/util/config.rs
index 99ebce31..e8ef4fdd 100644
--- a/src/util/config.rs
+++ b/src/util/config.rs
@@ -64,14 +64,19 @@ pub struct Config {
 	#[serde(default)]
 	pub kubernetes_skip_crd: bool,
 
+	// -- DB
+	/// Database engine to use for metadata (options: sled, sqlite, lmdb)
+	#[serde(default = "default_db_engine")]
+	pub db_engine: String,
+
 	/// Sled cache size, in bytes
 	#[serde(default = "default_sled_cache_capacity")]
 	pub sled_cache_capacity: u64,
-
 	/// Sled flush interval in milliseconds
 	#[serde(default = "default_sled_flush_every_ms")]
 	pub sled_flush_every_ms: u64,
 
+	// -- APIs
 	/// Configuration for S3 api
 	pub s3_api: S3ApiConfig,
 
@@ -129,6 +134,10 @@ pub struct AdminConfig {
 	pub trace_sink: Option<String>,
 }
 
+fn default_db_engine() -> String {
+	"sled".into()
+}
+
 fn default_sled_cache_capacity() -> u64 {
 	128 * 1024 * 1024
 }
diff --git a/src/util/error.rs b/src/util/error.rs
index 8734a0c8..9995c746 100644
--- a/src/util/error.rs
+++ b/src/util/error.rs
@@ -26,8 +26,8 @@ pub enum Error {
 	#[error(display = "Netapp error: {}", _0)]
 	Netapp(#[error(source)] netapp::error::Error),
 
-	#[error(display = "Sled error: {}", _0)]
-	Sled(#[error(source)] sled::Error),
+	#[error(display = "DB error: {}", _0)]
+	Db(#[error(source)] garage_db::Error),
 
 	#[error(display = "Messagepack encode error: {}", _0)]
 	RmpEncode(#[error(source)] rmp_serde::encode::Error),
@@ -78,11 +78,11 @@ impl Error {
 	}
 }
 
-impl From<sled::transaction::TransactionError<Error>> for Error {
-	fn from(e: sled::transaction::TransactionError<Error>) -> Error {
+impl From<garage_db::TxError<Error>> for Error {
+	fn from(e: garage_db::TxError<Error>) -> Error {
 		match e {
-			sled::transaction::TransactionError::Abort(x) => x,
-			sled::transaction::TransactionError::Storage(x) => Error::Sled(x),
+			garage_db::TxError::Abort(x) => x,
+			garage_db::TxError::Db(x) => Error::Db(x),
 		}
 	}
 }
diff --git a/src/util/lib.rs b/src/util/lib.rs
index d8ffdd0b..8ca6e310 100644
--- a/src/util/lib.rs
+++ b/src/util/lib.rs
@@ -11,7 +11,7 @@ pub mod error;
 pub mod formater;
 pub mod metrics;
 pub mod persister;
-pub mod sled_counter;
+//pub mod sled_counter;
 pub mod time;
 pub mod token_bucket;
 pub mod tranquilizer;
diff --git a/src/util/sled_counter.rs b/src/util/sled_counter.rs
deleted file mode 100644
index bc54cea0..00000000
--- a/src/util/sled_counter.rs
+++ /dev/null
@@ -1,100 +0,0 @@
-use std::sync::{
-	atomic::{AtomicUsize, Ordering},
-	Arc,
-};
-
-use sled::{CompareAndSwapError, IVec, Iter, Result, Tree};
-
-#[derive(Clone)]
-pub struct SledCountedTree(Arc<SledCountedTreeInternal>);
-
-struct SledCountedTreeInternal {
-	tree: Tree,
-	len: AtomicUsize,
-}
-
-impl SledCountedTree {
-	pub fn new(tree: Tree) -> Self {
-		let len = tree.len();
-		Self(Arc::new(SledCountedTreeInternal {
-			tree,
-			len: AtomicUsize::new(len),
-		}))
-	}
-
-	pub fn len(&self) -> usize {
-		self.0.len.load(Ordering::Relaxed)
-	}
-
-	pub fn is_empty(&self) -> bool {
-		self.0.tree.is_empty()
-	}
-
-	pub fn get<K: AsRef<[u8]>>(&self, key: K) -> Result<Option<IVec>> {
-		self.0.tree.get(key)
-	}
-
-	pub fn iter(&self) -> Iter {
-		self.0.tree.iter()
-	}
-
-	// ---- writing functions ----
-
-	pub fn insert<K, V>(&self, key: K, value: V) -> Result<Option<IVec>>
-	where
-		K: AsRef<[u8]>,
-		V: Into<IVec>,
-	{
-		let res = self.0.tree.insert(key, value);
-		if res == Ok(None) {
-			self.0.len.fetch_add(1, Ordering::Relaxed);
-		}
-		res
-	}
-
-	pub fn remove<K: AsRef<[u8]>>(&self, key: K) -> Result<Option<IVec>> {
-		let res = self.0.tree.remove(key);
-		if matches!(res, Ok(Some(_))) {
-			self.0.len.fetch_sub(1, Ordering::Relaxed);
-		}
-		res
-	}
-
-	pub fn pop_min(&self) -> Result<Option<(IVec, IVec)>> {
-		let res = self.0.tree.pop_min();
-		if let Ok(Some(_)) = &res {
-			self.0.len.fetch_sub(1, Ordering::Relaxed);
-		};
-		res
-	}
-
-	pub fn compare_and_swap<K, OV, NV>(
-		&self,
-		key: K,
-		old: Option<OV>,
-		new: Option<NV>,
-	) -> Result<std::result::Result<(), CompareAndSwapError>>
-	where
-		K: AsRef<[u8]>,
-		OV: AsRef<[u8]>,
-		NV: Into<IVec>,
-	{
-		let old_some = old.is_some();
-		let new_some = new.is_some();
-
-		let res = self.0.tree.compare_and_swap(key, old, new);
-
-		if res == Ok(Ok(())) {
-			match (old_some, new_some) {
-				(false, true) => {
-					self.0.len.fetch_add(1, Ordering::Relaxed);
-				}
-				(true, false) => {
-					self.0.len.fetch_sub(1, Ordering::Relaxed);
-				}
-				_ => (),
-			}
-		}
-		res
-	}
-}