Move block manager to separate module

4 files changed, 1237 insertions, 0 deletions

diff --git a/src/block/Cargo.toml b/src/block/Cargo.toml
new file mode 100644
index 00000000..9cba69ee
--- /dev/null
+++ b/src/block/Cargo.toml
@@ -0,0 +1,38 @@
+[package]
+name = "garage_block"
+version = "0.7.0"
+authors = ["Alex Auvolat <alex@adnab.me>"]
+edition = "2018"
+license = "AGPL-3.0"
+description = "Block manager for the Garage object store"
+repository = "https://git.deuxfleurs.fr/Deuxfleurs/garage"
+readme = "../../README.md"
+
+[lib]
+path = "lib.rs"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
+garage_rpc = { version = "0.7.0", path = "../rpc" }
+garage_util = { version = "0.7.0", path = "../util" }
+garage_table = { version = "0.7.0", path = "../table" }
+
+opentelemetry = "0.17"
+
+async-trait = "0.1.7"
+bytes = "1.0"
+hex = "0.4"
+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"
+
+futures = "0.3"
+futures-util = "0.3"
+tokio = { version = "1.0", default-features = false, features = ["rt", "rt-multi-thread", "io-util", "net", "time", "macros", "sync", "signal", "fs"] }
diff --git a/src/block/lib.rs b/src/block/lib.rs
new file mode 100644
index 00000000..47ff402d
--- /dev/null
+++ b/src/block/lib.rs
@@ -0,0 +1,6 @@
+#[macro_use]
+extern crate tracing;
+
+pub mod manager;
+
+mod metrics;
diff --git a/src/block/manager.rs b/src/block/manager.rs
new file mode 100644
index 00000000..f047e1d3
--- /dev/null
+++ b/src/block/manager.rs
@@ -0,0 +1,1091 @@
+use std::convert::TryInto;
+use std::path::{Path, PathBuf};
+use std::sync::Arc;
+use std::time::Duration;
+
+use async_trait::async_trait;
+use serde::{Deserialize, Serialize};
+use zstd::stream::{decode_all as zstd_decode, Encoder};
+
+use futures::future::*;
+use futures::select;
+use tokio::fs;
+use tokio::io::{AsyncReadExt, AsyncWriteExt};
+use tokio::sync::{watch, Mutex, Notify};
+
+use opentelemetry::{
+	trace::{FutureExt as OtelFutureExt, TraceContextExt, Tracer},
+	Context, KeyValue,
+};
+
+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;
+
+use garage_rpc::system::System;
+use garage_rpc::*;
+
+use garage_table::replication::{TableReplication, TableShardedReplication};
+
+use crate::metrics::*;
+
+/// Size under which data will be stored inlined in database instead of as files
+pub const INLINE_THRESHOLD: usize = 3072;
+
+pub const BACKGROUND_TRANQUILITY: u32 = 2;
+
+// Timeout for RPCs that read and write blocks to remote nodes
+const BLOCK_RW_TIMEOUT: Duration = Duration::from_secs(30);
+// Timeout for RPCs that ask other nodes whether they need a copy
+// of a given block before we delete it locally
+const NEED_BLOCK_QUERY_TIMEOUT: Duration = Duration::from_secs(5);
+
+// The delay between the time where a resync operation fails
+// and the time when it is retried, with exponential backoff
+// (multiplied by 2, 4, 8, 16, etc. for every consecutive failure).
+const RESYNC_RETRY_DELAY: Duration = Duration::from_secs(60);
+
+// The delay between the moment when the reference counter
+// drops to zero, and the moment where we allow ourselves
+// to delete the block locally.
+const BLOCK_GC_DELAY: Duration = Duration::from_secs(600);
+
+/// RPC messages used to share blocks of data between nodes
+#[derive(Debug, Serialize, Deserialize)]
+pub enum BlockRpc {
+	Ok,
+	/// Message to ask for a block of data, by hash
+	GetBlock(Hash),
+	/// Message to send a block of data, either because requested, of for first delivery of new
+	/// block
+	PutBlock {
+		hash: Hash,
+		data: DataBlock,
+	},
+	/// Ask other node if they should have this block, but don't actually have it
+	NeedBlockQuery(Hash),
+	/// Response : whether the node do require that block
+	NeedBlockReply(bool),
+}
+
+/// A possibly compressed block of data
+#[derive(Debug, Serialize, Deserialize)]
+pub enum DataBlock {
+	/// Uncompressed data
+	Plain(#[serde(with = "serde_bytes")] Vec<u8>),
+	/// Data compressed with zstd
+	Compressed(#[serde(with = "serde_bytes")] Vec<u8>),
+}
+
+impl DataBlock {
+	/// Query whether this block is compressed
+	pub fn is_compressed(&self) -> bool {
+		matches!(self, DataBlock::Compressed(_))
+	}
+
+	/// Get the inner, possibly compressed buffer. You should probably use [`DataBlock::verify_get`]
+	/// instead
+	pub fn inner_buffer(&self) -> &[u8] {
+		use DataBlock::*;
+		let (Plain(ref res) | Compressed(ref res)) = self;
+		res
+	}
+
+	/// Get the buffer, possibly decompressing it, and verify it's integrity.
+	/// For Plain block, data is compared to hash, for Compressed block, zstd checksumming system
+	/// is used instead.
+	pub fn verify_get(self, hash: Hash) -> Result<Vec<u8>, Error> {
+		match self {
+			DataBlock::Plain(data) => {
+				if blake2sum(&data) == hash {
+					Ok(data)
+				} else {
+					Err(Error::CorruptData(hash))
+				}
+			}
+			DataBlock::Compressed(data) => {
+				zstd_decode(&data[..]).map_err(|_| Error::CorruptData(hash))
+			}
+		}
+	}
+
+	/// Verify data integrity. Allocate less than [`DataBlock::verify_get`] and don't consume self, but
+	/// does not return the buffer content.
+	pub fn verify(&self, hash: Hash) -> Result<(), Error> {
+		match self {
+			DataBlock::Plain(data) => {
+				if blake2sum(data) == hash {
+					Ok(())
+				} else {
+					Err(Error::CorruptData(hash))
+				}
+			}
+			DataBlock::Compressed(data) => zstd::stream::copy_decode(&data[..], std::io::sink())
+				.map_err(|_| Error::CorruptData(hash)),
+		}
+	}
+
+	pub fn from_buffer(data: Vec<u8>, level: Option<i32>) -> DataBlock {
+		if let Some(level) = level {
+			if let Ok(data) = zstd_encode(&data[..], level) {
+				return DataBlock::Compressed(data);
+			}
+		}
+		DataBlock::Plain(data)
+	}
+}
+
+impl Rpc for BlockRpc {
+	type Response = Result<BlockRpc, Error>;
+}
+
+/// The block manager, handling block exchange between nodes, and block storage on local node
+pub struct BlockManager {
+	/// Replication strategy, allowing to find on which node blocks should be located
+	pub replication: TableShardedReplication,
+	/// Directory in which block are stored
+	pub data_dir: PathBuf,
+	/// Zstd compression level
+	compression_level: Option<i32>,
+
+	mutation_lock: Mutex<BlockManagerLocked>,
+
+	rc: sled::Tree,
+
+	resync_queue: SledCountedTree,
+	resync_notify: Notify,
+	resync_errors: SledCountedTree,
+
+	system: Arc<System>,
+	endpoint: Arc<Endpoint<BlockRpc, Self>>,
+
+	metrics: BlockManagerMetrics,
+}
+
+// This custom struct contains functions that must only be ran
+// when the lock is held. We ensure that it is the case by storing
+// it INSIDE a Mutex.
+struct BlockManagerLocked();
+
+impl BlockManager {
+	pub fn new(
+		db: &sled::Db,
+		data_dir: PathBuf,
+		compression_level: Option<i32>,
+		replication: TableShardedReplication,
+		system: Arc<System>,
+	) -> Arc<Self> {
+		let rc = db
+			.open_tree("block_local_rc")
+			.expect("Unable to open block_local_rc tree");
+
+		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_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 endpoint = system
+			.netapp
+			.endpoint("garage_model/block.rs/Rpc".to_string());
+
+		let manager_locked = BlockManagerLocked();
+
+		let metrics = BlockManagerMetrics::new(resync_queue.clone(), resync_errors.clone());
+
+		let block_manager = Arc::new(Self {
+			replication,
+			data_dir,
+			compression_level,
+			mutation_lock: Mutex::new(manager_locked),
+			rc,
+			resync_queue,
+			resync_notify: Notify::new(),
+			resync_errors,
+			system,
+			endpoint,
+			metrics,
+		});
+		block_manager.endpoint.set_handler(block_manager.clone());
+
+		block_manager
+	}
+
+	/// Ask nodes that might have a (possibly compressed) block for it
+	async fn rpc_get_raw_block(&self, hash: &Hash) -> Result<DataBlock, Error> {
+		let who = self.replication.read_nodes(hash);
+		let resps = self
+			.system
+			.rpc
+			.try_call_many(
+				&self.endpoint,
+				&who[..],
+				BlockRpc::GetBlock(*hash),
+				RequestStrategy::with_priority(PRIO_NORMAL)
+					.with_quorum(1)
+					.with_timeout(BLOCK_RW_TIMEOUT)
+					.interrupt_after_quorum(true),
+			)
+			.await?;
+
+		for resp in resps {
+			if let BlockRpc::PutBlock { data, .. } = resp {
+				return Ok(data);
+			}
+		}
+		Err(Error::Message(format!(
+			"Unable to read block {:?}: no valid blocks returned",
+			hash
+		)))
+	}
+
+	// ---- Public interface ----
+
+	/// Ask nodes that might have a block for it
+	pub async fn rpc_get_block(&self, hash: &Hash) -> Result<Vec<u8>, Error> {
+		self.rpc_get_raw_block(hash).await?.verify_get(*hash)
+	}
+
+	/// Send block to nodes that should have it
+	pub async fn rpc_put_block(&self, hash: Hash, data: Vec<u8>) -> Result<(), Error> {
+		let who = self.replication.write_nodes(&hash);
+		let data = DataBlock::from_buffer(data, self.compression_level);
+		self.system
+			.rpc
+			.try_call_many(
+				&self.endpoint,
+				&who[..],
+				BlockRpc::PutBlock { hash, data },
+				RequestStrategy::with_priority(PRIO_NORMAL)
+					.with_quorum(self.replication.write_quorum())
+					.with_timeout(BLOCK_RW_TIMEOUT),
+			)
+			.await?;
+		Ok(())
+	}
+
+	/// Launch the repair procedure on the data store
+	///
+	/// This will list all blocks locally present, as well as those
+	/// that are required because of refcount > 0, and will try
+	/// 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.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() {
+				return Ok(());
+			}
+		}
+
+		// 2. Repair blocks actually on disk
+		// Lists all blocks on disk and adds them to the resync queue.
+		// This allows us to find blocks we are storing but don't actually need,
+		// so that we can offload them if necessary and then delete them locally.
+		self.for_each_file(
+			(),
+			move |_, hash| async move { self.put_to_resync(&hash, Duration::from_secs(0)) },
+			must_exit,
+		)
+		.await
+	}
+
+	/// Verify integrity of each block on disk. Use `speed_limit` to limit the load generated by
+	/// this function.
+	pub async fn scrub_data_store(
+		&self,
+		must_exit: &watch::Receiver<bool>,
+		tranquility: u32,
+	) -> Result<(), Error> {
+		let tranquilizer = Tranquilizer::new(30);
+		self.for_each_file(
+			tranquilizer,
+			move |mut tranquilizer, hash| async move {
+				let _ = self.read_block(&hash).await;
+				tranquilizer.tranquilize(tranquility).await;
+				Ok(tranquilizer)
+			},
+			must_exit,
+		)
+		.await
+	}
+
+	/// Get lenght of resync queue
+	pub fn resync_queue_len(&self) -> usize {
+		self.resync_queue.len()
+	}
+
+	/// Get number of items in the refcount table
+	pub fn rc_len(&self) -> usize {
+		self.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> {
+		let old_rc = self
+			.rc
+			.fetch_and_update(&hash, |old| RcEntry::parse_opt(old).increment().serialize())?;
+		let old_rc = RcEntry::parse_opt(old_rc);
+		if old_rc.is_zero() {
+			// 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)?;
+		}
+		Ok(())
+	}
+
+	/// Decrement the number of time a block is used
+	pub fn block_decref(&self, hash: &Hash) -> Result<(), 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);
+		if let RcEntry::Deletable { .. } = new_rc {
+			self.put_to_resync(hash, BLOCK_GC_DELAY + Duration::from_secs(10))?;
+		}
+		Ok(())
+	}
+
+	/// Read a block's reference count
+	fn get_block_rc(&self, hash: &Hash) -> Result<RcEntry, Error> {
+		Ok(RcEntry::parse_opt(self.rc.get(hash.as_ref())?))
+	}
+
+	/// Delete an entry in the RC table if it is deletable and the
+	/// deletion time has passed
+	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,
+			};
+			updated.serialize()
+		})?;
+		Ok(())
+	}
+
+	// ---- Reading and writing blocks locally ----
+
+	/// Write a block to disk
+	async fn write_block(&self, hash: &Hash, data: &DataBlock) -> Result<BlockRpc, Error> {
+		let write_size = data.inner_buffer().len() as u64;
+
+		let res = self
+			.mutation_lock
+			.lock()
+			.await
+			.write_block(hash, data, self)
+			.bound_record_duration(&self.metrics.block_write_duration)
+			.await?;
+
+		self.metrics.bytes_written.add(write_size);
+
+		Ok(res)
+	}
+
+	/// Read block from disk, verifying it's integrity
+	async fn read_block(&self, hash: &Hash) -> Result<BlockRpc, Error> {
+		let data = self
+			.read_block_internal(hash)
+			.bound_record_duration(&self.metrics.block_read_duration)
+			.await?;
+
+		self.metrics
+			.bytes_read
+			.add(data.inner_buffer().len() as u64);
+
+		Ok(BlockRpc::PutBlock { hash: *hash, data })
+	}
+
+	async fn read_block_internal(&self, hash: &Hash) -> Result<DataBlock, Error> {
+		let mut path = self.block_path(hash);
+		let compressed = match self.is_block_compressed(hash).await {
+			Ok(c) => c,
+			Err(e) => {
+				// Not found but maybe we should have had it ??
+				self.put_to_resync(hash, 2 * BLOCK_RW_TIMEOUT)?;
+				return Err(Into::into(e));
+			}
+		};
+		if compressed {
+			path.set_extension("zst");
+		}
+		let mut f = fs::File::open(&path).await?;
+
+		let mut data = vec![];
+		f.read_to_end(&mut data).await?;
+		drop(f);
+
+		let data = if compressed {
+			DataBlock::Compressed(data)
+		} else {
+			DataBlock::Plain(data)
+		};
+
+		if data.verify(*hash).is_err() {
+			self.metrics.corruption_counter.add(1);
+
+			self.mutation_lock
+				.lock()
+				.await
+				.move_block_to_corrupted(hash, self)
+				.await?;
+			self.put_to_resync(hash, Duration::from_millis(0))?;
+			return Err(Error::CorruptData(*hash));
+		}
+
+		Ok(data)
+	}
+
+	/// Check if this node should have a block, but don't actually have it
+	async fn need_block(&self, hash: &Hash) -> Result<bool, Error> {
+		let BlockStatus { exists, needed } = self
+			.mutation_lock
+			.lock()
+			.await
+			.check_block_status(hash, self)
+			.await?;
+		Ok(needed.is_nonzero() && !exists)
+	}
+
+	/// Utility: gives the path of the directory in which a block should be found
+	fn block_dir(&self, hash: &Hash) -> PathBuf {
+		let mut path = self.data_dir.clone();
+		path.push(hex::encode(&hash.as_slice()[0..1]));
+		path.push(hex::encode(&hash.as_slice()[1..2]));
+		path
+	}
+
+	/// Utility: give the full path where a block should be found, minus extension if block is
+	/// compressed
+	fn block_path(&self, hash: &Hash) -> PathBuf {
+		let mut path = self.block_dir(hash);
+		path.push(hex::encode(hash.as_ref()));
+		path
+	}
+
+	/// Utility: check if block is stored compressed. Error if block is not stored
+	async fn is_block_compressed(&self, hash: &Hash) -> Result<bool, Error> {
+		let mut path = self.block_path(hash);
+		path.set_extension("zst");
+		if fs::metadata(&path).await.is_ok() {
+			return Ok(true);
+		}
+		path.set_extension("");
+		fs::metadata(&path).await.map(|_| false).map_err(Into::into)
+	}
+
+	// ---- Resync loop ----
+
+	pub fn spawn_background_worker(self: Arc<Self>) {
+		// Launch a background workers for background resync loop processing
+		let background = self.system.background.clone();
+		tokio::spawn(async move {
+			tokio::time::sleep(Duration::from_secs(10)).await;
+			background.spawn_worker("block resync worker".into(), move |must_exit| {
+				self.resync_loop(must_exit)
+			});
+		});
+	}
+
+	fn put_to_resync(&self, hash: &Hash, delay: Duration) -> Result<(), Error> {
+		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<(), Error> {
+		trace!("Put resync_queue: {} {:?}", when, hash);
+		let mut key = u64::to_be_bytes(when).to_vec();
+		key.extend(hash.as_ref());
+		self.resync_queue.insert(key, hash.as_ref())?;
+		self.resync_notify.notify_waiters();
+		Ok(())
+	}
+
+	async fn resync_loop(self: Arc<Self>, mut must_exit: watch::Receiver<bool>) {
+		let mut tranquilizer = Tranquilizer::new(30);
+
+		while !*must_exit.borrow() {
+			match self.resync_iter(&mut must_exit).await {
+				Ok(true) => {
+					tranquilizer.tranquilize(BACKGROUND_TRANQUILITY).await;
+				}
+				Ok(false) => {
+					tranquilizer.reset();
+				}
+				Err(e) => {
+					// The errors that we have here are only Sled errors
+					// We don't really know how to handle them so just ¯\_(ツ)_/¯
+					// (there is kind of an assumption that Sled won't error on us,
+					// if it does there is not much we can do -- TODO should we just panic?)
+					error!(
+						"Could not do a resync iteration: {} (this is a very bad error)",
+						e
+					);
+					tranquilizer.reset();
+				}
+			}
+		}
+	}
+
+	async fn resync_iter(&self, must_exit: &mut watch::Receiver<bool>) -> Result<bool, Error> {
+		if let Some(first_pair_res) = self.resync_queue.iter().next() {
+			let (time_bytes, hash_bytes) = first_pair_res?;
+
+			let time_msec = u64::from_be_bytes(time_bytes[0..8].try_into().unwrap());
+			let now = now_msec();
+
+			if now >= time_msec {
+				let hash = Hash::try_from(&hash_bytes[..]).unwrap();
+
+				if let Some(ec) = self.resync_errors.get(hash.as_slice())? {
+					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
+						// make sure the item is still in queue at expected time
+						self.put_to_resync_at(&hash, ec.next_try())?;
+						// ec.next_try() > now >= time_msec, so this remove
+						// is not removing the one we added just above
+						self.resync_queue.remove(time_bytes)?;
+						return Ok(false);
+					}
+				}
+
+				let tracer = opentelemetry::global::tracer("garage");
+				let trace_id = gen_uuid();
+				let span = tracer
+					.span_builder("Resync block")
+					.with_trace_id(
+						opentelemetry::trace::TraceId::from_hex(&hex::encode(
+							&trace_id.as_slice()[..16],
+						))
+						.unwrap(),
+					)
+					.with_attributes(vec![KeyValue::new("block", format!("{:?}", hash))])
+					.start(&tracer);
+
+				let res = self
+					.resync_block(&hash)
+					.with_context(Context::current_with_span(span))
+					.bound_record_duration(&self.metrics.resync_duration)
+					.await;
+
+				self.metrics.resync_counter.add(1);
+
+				if let Err(e) = &res {
+					self.metrics.resync_error_counter.add(1);
+					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),
+						None => ErrorCounter::new(now + 1),
+					};
+
+					self.resync_errors
+						.insert(hash.as_slice(), err_counter.encode())?;
+
+					self.put_to_resync_at(&hash, err_counter.next_try())?;
+					// err_counter.next_try() >= now + 1 > now,
+					// the entry we remove from the queue is not
+					// the entry we inserted with put_to_resync_at
+					self.resync_queue.remove(time_bytes)?;
+				} else {
+					self.resync_errors.remove(hash.as_slice())?;
+					self.resync_queue.remove(time_bytes)?;
+				}
+
+				Ok(true)
+			} else {
+				let delay = tokio::time::sleep(Duration::from_millis(time_msec - now));
+				select! {
+					_ = delay.fuse() => {},
+					_ = self.resync_notify.notified().fuse() => {},
+					_ = must_exit.changed().fuse() => {},
+				}
+				Ok(false)
+			}
+		} else {
+			select! {
+				_ = self.resync_notify.notified().fuse() => {},
+				_ = must_exit.changed().fuse() => {},
+			}
+			Ok(false)
+		}
+	}
+
+	async fn resync_block(&self, hash: &Hash) -> Result<(), Error> {
+		let BlockStatus { exists, needed } = self
+			.mutation_lock
+			.lock()
+			.await
+			.check_block_status(hash, self)
+			.await?;
+
+		if exists != needed.is_needed() || exists != needed.is_nonzero() {
+			debug!(
+				"Resync block {:?}: exists {}, nonzero rc {}, deletable {}",
+				hash,
+				exists,
+				needed.is_nonzero(),
+				needed.is_deletable(),
+			);
+		}
+
+		if exists && needed.is_deletable() {
+			info!("Resync block {:?}: offloading and deleting", hash);
+
+			let mut who = self.replication.write_nodes(hash);
+			if who.len() < self.replication.write_quorum() {
+				return Err(Error::Message("Not trying to offload block because we don't have a quorum of nodes to write to".to_string()));
+			}
+			who.retain(|id| *id != self.system.id);
+
+			let msg = Arc::new(BlockRpc::NeedBlockQuery(*hash));
+			let who_needs_fut = who.iter().map(|to| {
+				self.system.rpc.call_arc(
+					&self.endpoint,
+					*to,
+					msg.clone(),
+					RequestStrategy::with_priority(PRIO_BACKGROUND)
+						.with_timeout(NEED_BLOCK_QUERY_TIMEOUT),
+				)
+			});
+			let who_needs_resps = join_all(who_needs_fut).await;
+
+			let mut need_nodes = vec![];
+			for (node, needed) in who.iter().zip(who_needs_resps.into_iter()) {
+				match needed.err_context("NeedBlockQuery RPC")? {
+					BlockRpc::NeedBlockReply(needed) => {
+						if needed {
+							need_nodes.push(*node);
+						}
+					}
+					m => {
+						return Err(Error::unexpected_rpc_message(m));
+					}
+				}
+			}
+
+			if !need_nodes.is_empty() {
+				trace!(
+					"Block {:?} needed by {} nodes, sending",
+					hash,
+					need_nodes.len()
+				);
+
+				for node in need_nodes.iter() {
+					self.metrics
+						.resync_send_counter
+						.add(1, &[KeyValue::new("to", format!("{:?}", node))]);
+				}
+
+				let put_block_message = self.read_block(hash).await?;
+				self.system
+					.rpc
+					.try_call_many(
+						&self.endpoint,
+						&need_nodes[..],
+						put_block_message,
+						RequestStrategy::with_priority(PRIO_BACKGROUND)
+							.with_quorum(need_nodes.len())
+							.with_timeout(BLOCK_RW_TIMEOUT),
+					)
+					.await
+					.err_context("PutBlock RPC")?;
+			}
+			info!(
+				"Deleting unneeded block {:?}, offload finished ({} / {})",
+				hash,
+				need_nodes.len(),
+				who.len()
+			);
+
+			self.mutation_lock
+				.lock()
+				.await
+				.delete_if_unneeded(hash, self)
+				.await?;
+
+			self.clear_deleted_block_rc(hash)?;
+		}
+
+		if needed.is_nonzero() && !exists {
+			info!(
+				"Resync block {:?}: fetching absent but needed block (refcount > 0)",
+				hash
+			);
+
+			let block_data = self.rpc_get_raw_block(hash).await?;
+
+			self.metrics.resync_recv_counter.add(1);
+
+			self.write_block(hash, &block_data).await?;
+		}
+
+		Ok(())
+	}
+
+	// ---- Utility: iteration on files in the data directory ----
+
+	async fn for_each_file<F, Fut, State>(
+		&self,
+		state: State,
+		mut f: F,
+		must_exit: &watch::Receiver<bool>,
+	) -> Result<(), Error>
+	where
+		F: FnMut(State, Hash) -> Fut + Send,
+		Fut: Future<Output = Result<State, Error>> + Send,
+		State: Send,
+	{
+		self.for_each_file_rec(&self.data_dir, state, &mut f, must_exit)
+			.await
+			.map(|_| ())
+	}
+
+	fn for_each_file_rec<'a, F, Fut, State>(
+		&'a self,
+		path: &'a Path,
+		mut state: State,
+		f: &'a mut F,
+		must_exit: &'a watch::Receiver<bool>,
+	) -> BoxFuture<'a, Result<State, Error>>
+	where
+		F: FnMut(State, Hash) -> Fut + Send,
+		Fut: Future<Output = Result<State, Error>> + Send,
+		State: Send + 'a,
+	{
+		async move {
+			let mut ls_data_dir = fs::read_dir(path).await?;
+			while let Some(data_dir_ent) = ls_data_dir.next_entry().await? {
+				if *must_exit.borrow() {
+					break;
+				}
+
+				let name = data_dir_ent.file_name();
+				let name = if let Ok(n) = name.into_string() {
+					n
+				} else {
+					continue;
+				};
+				let ent_type = data_dir_ent.file_type().await?;
+
+				let name = name.strip_suffix(".zst").unwrap_or(&name);
+				if name.len() == 2 && hex::decode(&name).is_ok() && ent_type.is_dir() {
+					state = self
+						.for_each_file_rec(&data_dir_ent.path(), state, f, must_exit)
+						.await?;
+				} else if name.len() == 64 {
+					let hash_bytes = if let Ok(h) = hex::decode(&name) {
+						h
+					} else {
+						continue;
+					};
+					let mut hash = [0u8; 32];
+					hash.copy_from_slice(&hash_bytes[..]);
+					state = f(state, hash.into()).await?;
+				}
+			}
+			Ok(state)
+		}
+		.boxed()
+	}
+}
+
+#[async_trait]
+impl EndpointHandler<BlockRpc> for BlockManager {
+	async fn handle(
+		self: &Arc<Self>,
+		message: &BlockRpc,
+		_from: NodeID,
+	) -> Result<BlockRpc, Error> {
+		match message {
+			BlockRpc::PutBlock { hash, data } => self.write_block(hash, data).await,
+			BlockRpc::GetBlock(h) => self.read_block(h).await,
+			BlockRpc::NeedBlockQuery(h) => self.need_block(h).await.map(BlockRpc::NeedBlockReply),
+			m => Err(Error::unexpected_rpc_message(m)),
+		}
+	}
+}
+
+struct BlockStatus {
+	exists: bool,
+	needed: RcEntry,
+}
+
+impl BlockManagerLocked {
+	async fn check_block_status(
+		&self,
+		hash: &Hash,
+		mgr: &BlockManager,
+	) -> Result<BlockStatus, Error> {
+		let exists = mgr.is_block_compressed(hash).await.is_ok();
+		let needed = mgr.get_block_rc(hash)?;
+
+		Ok(BlockStatus { exists, needed })
+	}
+
+	async fn write_block(
+		&self,
+		hash: &Hash,
+		data: &DataBlock,
+		mgr: &BlockManager,
+	) -> Result<BlockRpc, Error> {
+		let compressed = data.is_compressed();
+		let data = data.inner_buffer();
+
+		let mut path = mgr.block_dir(hash);
+		let directory = path.clone();
+		path.push(hex::encode(hash));
+
+		fs::create_dir_all(&directory).await?;
+
+		let to_delete = match (mgr.is_block_compressed(hash).await, compressed) {
+			(Ok(true), _) => return Ok(BlockRpc::Ok),
+			(Ok(false), false) => return Ok(BlockRpc::Ok),
+			(Ok(false), true) => {
+				let path_to_delete = path.clone();
+				path.set_extension("zst");
+				Some(path_to_delete)
+			}
+			(Err(_), compressed) => {
+				if compressed {
+					path.set_extension("zst");
+				}
+				None
+			}
+		};
+
+		let mut path2 = path.clone();
+		path2.set_extension("tmp");
+		let mut f = fs::File::create(&path2).await?;
+		f.write_all(data).await?;
+		f.sync_all().await?;
+		drop(f);
+
+		fs::rename(path2, path).await?;
+		if let Some(to_delete) = to_delete {
+			fs::remove_file(to_delete).await?;
+		}
+
+		// We want to ensure that when this function returns, data is properly persisted
+		// to disk. The first step is the sync_all above that does an fsync on the data file.
+		// Now, we do an fsync on the containing directory, to ensure that the rename
+		// is persisted properly. See:
+		// http://thedjbway.b0llix.net/qmail/syncdir.html
+		let dir = fs::OpenOptions::new()
+			.read(true)
+			.mode(0)
+			.open(directory)
+			.await?;
+		dir.sync_all().await?;
+		drop(dir);
+
+		Ok(BlockRpc::Ok)
+	}
+
+	async fn move_block_to_corrupted(&self, hash: &Hash, mgr: &BlockManager) -> Result<(), Error> {
+		warn!(
+			"Block {:?} is corrupted. Renaming to .corrupted and resyncing.",
+			hash
+		);
+		let mut path = mgr.block_path(hash);
+		let mut path2 = path.clone();
+		if mgr.is_block_compressed(hash).await? {
+			path.set_extension("zst");
+			path2.set_extension("zst.corrupted");
+		} else {
+			path2.set_extension("corrupted");
+		}
+		fs::rename(path, path2).await?;
+		Ok(())
+	}
+
+	async fn delete_if_unneeded(&self, hash: &Hash, mgr: &BlockManager) -> Result<(), Error> {
+		let BlockStatus { exists, needed } = self.check_block_status(hash, mgr).await?;
+
+		if exists && needed.is_deletable() {
+			let mut path = mgr.block_path(hash);
+			if mgr.is_block_compressed(hash).await? {
+				path.set_extension("zst");
+			}
+			fs::remove_file(path).await?;
+			mgr.metrics.delete_counter.add(1);
+		}
+		Ok(())
+	}
+}
+
+/// Describes the state of the reference counter for a block
+#[derive(Clone, Copy, Debug)]
+enum RcEntry {
+	/// Present: the block has `count` references, with `count` > 0.
+	///
+	/// This is stored as u64::to_be_bytes(count)
+	Present { count: u64 },
+
+	/// Deletable: the block has zero references, and can be deleted
+	/// once time (returned by now_msec) is larger than at_time
+	/// (in millis since Unix epoch)
+	///
+	/// This is stored as [0u8; 8] followed by u64::to_be_bytes(at_time),
+	/// (this allows for the data format to be backwards compatible with
+	/// previous Garage versions that didn't have this intermediate state)
+	Deletable { at_time: u64 },
+
+	/// Absent: the block has zero references, and can be deleted
+	/// immediately
+	Absent,
+}
+
+impl RcEntry {
+	fn parse(bytes: &[u8]) -> Self {
+		if bytes.len() == 8 {
+			RcEntry::Present {
+				count: u64::from_be_bytes(bytes.try_into().unwrap()),
+			}
+		} else if bytes.len() == 16 {
+			RcEntry::Deletable {
+				at_time: u64::from_be_bytes(bytes[8..16].try_into().unwrap()),
+			}
+		} else {
+			panic!("Invalid RC entry: {:?}, database is corrupted. This is an error Garage is currently unable to recover from. Sorry, and also please report a bug.",
+				bytes
+			)
+		}
+	}
+
+	fn parse_opt<V: AsRef<[u8]>>(bytes: Option<V>) -> Self {
+		bytes
+			.map(|b| Self::parse(b.as_ref()))
+			.unwrap_or(Self::Absent)
+	}
+
+	fn serialize(self) -> Option<Vec<u8>> {
+		match self {
+			RcEntry::Present { count } => Some(u64::to_be_bytes(count).to_vec()),
+			RcEntry::Deletable { at_time } => {
+				Some([u64::to_be_bytes(0), u64::to_be_bytes(at_time)].concat())
+			}
+			RcEntry::Absent => None,
+		}
+	}
+
+	fn increment(self) -> Self {
+		let old_count = match self {
+			RcEntry::Present { count } => count,
+			_ => 0,
+		};
+		RcEntry::Present {
+			count: old_count + 1,
+		}
+	}
+
+	fn decrement(self) -> Self {
+		match self {
+			RcEntry::Present { count } => {
+				if count > 1 {
+					RcEntry::Present { count: count - 1 }
+				} else {
+					RcEntry::Deletable {
+						at_time: now_msec() + BLOCK_GC_DELAY.as_millis() as u64,
+					}
+				}
+			}
+			del => del,
+		}
+	}
+
+	fn is_zero(&self) -> bool {
+		matches!(self, RcEntry::Deletable { .. } | RcEntry::Absent)
+	}
+
+	fn is_nonzero(&self) -> bool {
+		!self.is_zero()
+	}
+
+	fn is_deletable(&self) -> bool {
+		match self {
+			RcEntry::Present { .. } => false,
+			RcEntry::Deletable { at_time } => now_msec() > *at_time,
+			RcEntry::Absent => true,
+		}
+	}
+
+	fn is_needed(&self) -> bool {
+		!self.is_deletable()
+	}
+}
+
+/// Counts the number of errors when resyncing a block,
+/// and the time of the last try.
+/// Used to implement exponential backoff.
+#[derive(Clone, Copy, Debug)]
+struct ErrorCounter {
+	errors: u64,
+	last_try: u64,
+}
+
+impl ErrorCounter {
+	fn new(now: u64) -> Self {
+		Self {
+			errors: 1,
+			last_try: now,
+		}
+	}
+
+	fn decode(data: sled::IVec) -> 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()),
+		}
+	}
+	fn encode(&self) -> Vec<u8> {
+		[
+			u64::to_be_bytes(self.errors),
+			u64::to_be_bytes(self.last_try),
+		]
+		.concat()
+	}
+
+	fn add1(self, now: u64) -> Self {
+		Self {
+			errors: self.errors + 1,
+			last_try: now,
+		}
+	}
+
+	fn delay_msec(&self) -> u64 {
+		(RESYNC_RETRY_DELAY.as_millis() as u64) << std::cmp::min(self.errors - 1, 10)
+	}
+	fn next_try(&self) -> u64 {
+		self.last_try + self.delay_msec()
+	}
+}
+
+fn zstd_encode<R: std::io::Read>(mut source: R, level: i32) -> std::io::Result<Vec<u8>> {
+	let mut result = Vec::<u8>::new();
+	let mut encoder = Encoder::new(&mut result, level)?;
+	encoder.include_checksum(true)?;
+	std::io::copy(&mut source, &mut encoder)?;
+	encoder.finish()?;
+	Ok(result)
+}
diff --git a/src/block/metrics.rs b/src/block/metrics.rs
new file mode 100644
index 00000000..f0f541a3
--- /dev/null
+++ b/src/block/metrics.rs
@@ -0,0 +1,102 @@
+use opentelemetry::{global, metrics::*};
+
+use garage_util::sled_counter::SledCountedTree;
+
+/// TableMetrics reference all counter used for metrics
+pub struct BlockManagerMetrics {
+	pub(crate) _resync_queue_len: ValueObserver<u64>,
+	pub(crate) _resync_errored_blocks: ValueObserver<u64>,
+
+	pub(crate) resync_counter: BoundCounter<u64>,
+	pub(crate) resync_error_counter: BoundCounter<u64>,
+	pub(crate) resync_duration: BoundValueRecorder<f64>,
+	pub(crate) resync_send_counter: Counter<u64>,
+	pub(crate) resync_recv_counter: BoundCounter<u64>,
+
+	pub(crate) bytes_read: BoundCounter<u64>,
+	pub(crate) block_read_duration: BoundValueRecorder<f64>,
+	pub(crate) bytes_written: BoundCounter<u64>,
+	pub(crate) block_write_duration: BoundValueRecorder<f64>,
+	pub(crate) delete_counter: BoundCounter<u64>,
+
+	pub(crate) corruption_counter: BoundCounter<u64>,
+}
+
+impl BlockManagerMetrics {
+	pub fn new(resync_queue: SledCountedTree, resync_errors: SledCountedTree) -> Self {
+		let meter = global::meter("garage_model/block");
+		Self {
+			_resync_queue_len: meter
+				.u64_value_observer("block.resync_queue_length", move |observer| {
+					observer.observe(resync_queue.len() as u64, &[])
+				})
+				.with_description(
+					"Number of block hashes queued for local check and possible resync",
+				)
+				.init(),
+			_resync_errored_blocks: meter
+				.u64_value_observer("block.resync_errored_blocks", move |observer| {
+					observer.observe(resync_errors.len() as u64, &[])
+				})
+				.with_description("Number of block hashes whose last resync resulted in an error")
+				.init(),
+
+			resync_counter: meter
+				.u64_counter("block.resync_counter")
+				.with_description("Number of calls to resync_block")
+				.init()
+				.bind(&[]),
+			resync_error_counter: meter
+				.u64_counter("block.resync_error_counter")
+				.with_description("Number of calls to resync_block that returned an error")
+				.init()
+				.bind(&[]),
+			resync_duration: meter
+				.f64_value_recorder("block.resync_duration")
+				.with_description("Duration of resync_block operations")
+				.init()
+				.bind(&[]),
+			resync_send_counter: meter
+				.u64_counter("block.resync_send_counter")
+				.with_description("Number of blocks sent to another node in resync operations")
+				.init(),
+			resync_recv_counter: meter
+				.u64_counter("block.resync_recv_counter")
+				.with_description("Number of blocks received from other nodes in resync operations")
+				.init()
+				.bind(&[]),
+
+			bytes_read: meter
+				.u64_counter("block.bytes_read")
+				.with_description("Number of bytes read from disk")
+				.init()
+				.bind(&[]),
+			block_read_duration: meter
+				.f64_value_recorder("block.read_duration")
+				.with_description("Duration of block read operations")
+				.init()
+				.bind(&[]),
+			bytes_written: meter
+				.u64_counter("block.bytes_written")
+				.with_description("Number of bytes written to disk")
+				.init()
+				.bind(&[]),
+			block_write_duration: meter
+				.f64_value_recorder("block.write_duration")
+				.with_description("Duration of block write operations")
+				.init()
+				.bind(&[]),
+			delete_counter: meter
+				.u64_counter("block.delete_counter")
+				.with_description("Number of blocks deleted")
+				.init()
+				.bind(&[]),
+
+			corruption_counter: meter
+				.u64_counter("block.corruption_counter")
+				.with_description("Data corruptions detected on block reads")
+				.init()
+				.bind(&[]),
+		}
+	}
+}