Merge pull request 'Make block resync speed dynamically configurable' (#369) from resync-ajustable-speed into main

Included in this PR:

- [x] Small refactor, resync code is moved to a separate `block/resync.rs` file
- [x] Block resync tranquility is no longer in config file, it is set dynamically using `garage worker set resync-tranquility` (this parameter is persisted over Garage restarts)
- [x] Up to 4 block resync workers can be activated to run simultaneously to speed up big resyncs, this parameter is set dynamically using `garage worker set resync-n-workers`

Reviewed-on: https://git.deuxfleurs.fr/Deuxfleurs/garage/pulls/369

8 files changed, 724 insertions, 527 deletions

diff --git a/src/block/lib.rs b/src/block/lib.rs
index ebdb95d8..d2814f77 100644
--- a/src/block/lib.rs
+++ b/src/block/lib.rs
@@ -3,6 +3,7 @@ extern crate tracing;
 
 pub mod manager;
 pub mod repair;
+pub mod resync;
 
 mod block;
 mod metrics;
diff --git a/src/block/manager.rs b/src/block/manager.rs
index 017ba9da..9240db25 100644
--- a/src/block/manager.rs
+++ b/src/block/manager.rs
@@ -1,32 +1,19 @@
-use std::convert::TryInto;
 use std::path::PathBuf;
 use std::sync::Arc;
 use std::time::Duration;
 
-use arc_swap::ArcSwapOption;
 use async_trait::async_trait;
 use serde::{Deserialize, Serialize};
 
-use futures::future::*;
 use tokio::fs;
 use tokio::io::{AsyncReadExt, AsyncWriteExt};
-use tokio::select;
-use tokio::sync::{mpsc, watch, Mutex, Notify};
-
-use opentelemetry::{
-	trace::{FutureExt as OtelFutureExt, TraceContextExt, Tracer},
-	Context, KeyValue,
-};
+use tokio::sync::{mpsc, Mutex};
 
 use garage_db as db;
-use garage_db::counted_tree_hack::CountedTree;
 
-use garage_util::background::*;
 use garage_util::data::*;
 use garage_util::error::*;
 use garage_util::metrics::RecordDuration;
-use garage_util::time::*;
-use garage_util::tranquilizer::Tranquilizer;
 
 use garage_rpc::system::System;
 use garage_rpc::*;
@@ -37,23 +24,13 @@ use crate::block::*;
 use crate::metrics::*;
 use crate::rc::*;
 use crate::repair::*;
+use crate::resync::*;
 
 /// Size under which data will be stored inlined in database instead of as files
 pub const INLINE_THRESHOLD: usize = 3072;
 
 // 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 minimum retry delay is 60 seconds = 1 minute
-// The maximum retry delay is 60 seconds * 2^6 = 60 seconds << 6 = 64 minutes (~1 hour)
-const RESYNC_RETRY_DELAY_MAX_BACKOFF_POWER: u64 = 6;
+pub(crate) const BLOCK_RW_TIMEOUT: Duration = Duration::from_secs(30);
 
 // The delay between the moment when the reference counter
 // drops to zero, and the moment where we allow ourselves
@@ -90,22 +67,18 @@ pub struct BlockManager {
 	pub data_dir: PathBuf,
 
 	compression_level: Option<i32>,
-	background_tranquility: u32,
 
 	mutation_lock: Mutex<BlockManagerLocked>,
 
 	pub(crate) rc: BlockRc,
-
-	resync_queue: CountedTree,
-	resync_notify: Notify,
-	resync_errors: CountedTree,
+	pub resync: BlockResyncManager,
 
 	pub(crate) system: Arc<System>,
-	endpoint: Arc<Endpoint<BlockRpc, Self>>,
+	pub(crate) endpoint: Arc<Endpoint<BlockRpc, Self>>,
 
-	metrics: BlockManagerMetrics,
+	pub(crate) metrics: BlockManagerMetrics,
 
-	tx_scrub_command: ArcSwapOption<mpsc::Sender<ScrubWorkerCommand>>,
+	tx_scrub_command: mpsc::Sender<ScrubWorkerCommand>,
 }
 
 // This custom struct contains functions that must only be ran
@@ -113,18 +86,11 @@ pub struct BlockManager {
 // it INSIDE a Mutex.
 struct BlockManagerLocked();
 
-enum ResyncIterResult {
-	BusyDidSomething,
-	BusyDidNothing,
-	IdleFor(Duration),
-}
-
 impl BlockManager {
 	pub fn new(
 		db: &db::Db,
 		data_dir: PathBuf,
 		compression_level: Option<i32>,
-		background_tranquility: u32,
 		replication: TableShardedReplication,
 		system: Arc<System>,
 	) -> Arc<Self> {
@@ -133,17 +99,7 @@ impl BlockManager {
 			.expect("Unable to open block_local_rc tree");
 		let rc = BlockRc::new(rc);
 
-		let resync_queue = db
-			.open_tree("block_local_resync_queue")
-			.expect("Unable to open block_local_resync_queue tree");
-		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 =
-			CountedTree::new(resync_errors).expect("Could not count block_local_resync_errors");
+		let resync = BlockResyncManager::new(db, &system);
 
 		let endpoint = system
 			.netapp
@@ -151,32 +107,39 @@ impl BlockManager {
 
 		let manager_locked = BlockManagerLocked();
 
-		let metrics = BlockManagerMetrics::new(resync_queue.clone(), resync_errors.clone());
+		let metrics = BlockManagerMetrics::new(resync.queue.clone(), resync.errors.clone());
+
+		let (scrub_tx, scrub_rx) = mpsc::channel(1);
 
 		let block_manager = Arc::new(Self {
 			replication,
 			data_dir,
 			compression_level,
-			background_tranquility,
 			mutation_lock: Mutex::new(manager_locked),
 			rc,
-			resync_queue,
-			resync_notify: Notify::new(),
-			resync_errors,
+			resync,
 			system,
 			endpoint,
 			metrics,
-			tx_scrub_command: ArcSwapOption::new(None),
+			tx_scrub_command: scrub_tx,
 		});
 		block_manager.endpoint.set_handler(block_manager.clone());
 
-		block_manager.clone().spawn_background_workers();
+		// Spawn a bunch of resync workers
+		for index in 0..MAX_RESYNC_WORKERS {
+			let worker = ResyncWorker::new(index, block_manager.clone());
+			block_manager.system.background.spawn_worker(worker);
+		}
+
+		// Spawn scrub worker
+		let scrub_worker = ScrubWorker::new(block_manager.clone(), scrub_rx);
+		block_manager.system.background.spawn_worker(scrub_worker);
 
 		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> {
+	pub(crate) async fn rpc_get_raw_block(&self, hash: &Hash) -> Result<DataBlock, Error> {
 		let who = self.replication.read_nodes(hash);
 		let resps = self
 			.system
@@ -228,20 +191,6 @@ impl BlockManager {
 		Ok(())
 	}
 
-	/// Get lenght of resync queue
-	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) -> 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) -> Result<usize, Error> {
 		Ok(self.rc.rc.len()?)
@@ -249,13 +198,7 @@ impl BlockManager {
 
 	/// Send command to start/stop/manager scrub worker
 	pub async fn send_scrub_command(&self, cmd: ScrubWorkerCommand) {
-		let _ = self
-			.tx_scrub_command
-			.load()
-			.as_ref()
-			.unwrap()
-			.send(cmd)
-			.await;
+		let _ = self.tx_scrub_command.send(cmd).await;
 	}
 
 	//// ----- Managing the reference counter ----
@@ -276,7 +219,7 @@ impl BlockManager {
 			// we will fecth it from someone.
 			let this = self.clone();
 			tokio::spawn(async move {
-				if let Err(e) = this.put_to_resync(&hash, 2 * BLOCK_RW_TIMEOUT) {
+				if let Err(e) = this.resync.put_to_resync(&hash, 2 * BLOCK_RW_TIMEOUT) {
 					error!("Block {:?} could not be put in resync queue: {}.", hash, e);
 				}
 			});
@@ -298,7 +241,9 @@ impl BlockManager {
 			// after that delay has passed.
 			let this = self.clone();
 			tokio::spawn(async move {
-				if let Err(e) = this.put_to_resync(&hash, BLOCK_GC_DELAY + Duration::from_secs(10))
+				if let Err(e) = this
+					.resync
+					.put_to_resync(&hash, BLOCK_GC_DELAY + Duration::from_secs(10))
 				{
 					error!("Block {:?} could not be put in resync queue: {}.", hash, e);
 				}
@@ -310,7 +255,11 @@ impl BlockManager {
 	// ---- Reading and writing blocks locally ----
 
 	/// Write a block to disk
-	async fn write_block(&self, hash: &Hash, data: &DataBlock) -> Result<BlockRpc, Error> {
+	pub(crate) async fn write_block(
+		&self,
+		hash: &Hash,
+		data: &DataBlock,
+	) -> Result<BlockRpc, Error> {
 		let write_size = data.inner_buffer().len() as u64;
 
 		let res = self
@@ -346,7 +295,7 @@ impl BlockManager {
 			Ok(c) => c,
 			Err(e) => {
 				// Not found but maybe we should have had it ??
-				self.put_to_resync(hash, 2 * BLOCK_RW_TIMEOUT)?;
+				self.resync.put_to_resync(hash, 2 * BLOCK_RW_TIMEOUT)?;
 				return Err(Into::into(e));
 			}
 		};
@@ -373,24 +322,37 @@ impl BlockManager {
 				.await
 				.move_block_to_corrupted(hash, self)
 				.await?;
-			self.put_to_resync(hash, Duration::from_millis(0))?;
+			self.resync.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
+	/// Check if this node has a block and whether it needs it
+	pub(crate) async fn check_block_status(&self, hash: &Hash) -> Result<BlockStatus, Error> {
+		self.mutation_lock
 			.lock()
 			.await
 			.check_block_status(hash, self)
-			.await?;
+			.await
+	}
+
+	/// 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.check_block_status(hash).await?;
 		Ok(needed.is_nonzero() && !exists)
 	}
 
+	/// Delete block if it is not needed anymore
+	pub(crate) async fn delete_if_unneeded(&self, hash: &Hash) -> Result<(), Error> {
+		self.mutation_lock
+			.lock()
+			.await
+			.delete_if_unneeded(hash, self)
+			.await
+	}
+
 	/// 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();
@@ -417,305 +379,6 @@ impl BlockManager {
 		path.set_extension("");
 		fs::metadata(&path).await.map(|_| false).map_err(Into::into)
 	}
-
-	// ---- Resync loop ----
-
-	// This part manages a queue of blocks that need to be
-	// "resynchronized", i.e. that need to have a check that
-	// they are at present if we need them, or that they are
-	// deleted once the garbage collection delay has passed.
-	//
-	// Here are some explanations on how the resync queue works.
-	// There are two Sled trees that are used to have information
-	// about the status of blocks that need to be resynchronized:
-	//
-	// - resync_queue: a tree that is ordered first by a timestamp
-	//   (in milliseconds since Unix epoch) that is the time at which
-	//   the resync must be done, and second by block hash.
-	//   The key in this tree is just:
-	//       concat(timestamp (8 bytes), hash (32 bytes))
-	//   The value is the same 32-byte hash.
-	//
-	// - resync_errors: a tree that indicates for each block
-	//   if the last resync resulted in an error, and if so,
-	//   the following two informations (see the ErrorCounter struct):
-	//   - how many consecutive resync errors for this block?
-	//   - when was the last try?
-	//   These two informations are used to implement an
-	//   exponential backoff retry strategy.
-	//   The key in this tree is the 32-byte hash of the block,
-	//   and the value is the encoded ErrorCounter value.
-	//
-	// We need to have these two trees, because the resync queue
-	// is not just a queue of items to process, but a set of items
-	// that are waiting a specific delay until we can process them
-	// (the delay being necessary both internally for the exponential
-	// backoff strategy, and exposed as a parameter when adding items
-	// to the queue, e.g. to wait until the GC delay has passed).
-	// This is why we need one tree ordered by time, and one
-	// ordered by identifier of item to be processed (block hash).
-	//
-	// When the worker wants to process an item it takes from
-	// resync_queue, it checks in resync_errors that if there is an
-	// exponential back-off delay to await, it has passed before we
-	// process the item. If not, the item in the queue is skipped
-	// (but added back for later processing after the time of the
-	// delay).
-	//
-	// An alternative that would have seemed natural is to
-	// only add items to resync_queue with a processing time that is
-	// after the delay, but there are several issues with this:
-	// - This requires to synchronize updates to resync_queue and
-	//   resync_errors (with the current model, there is only one thread,
-	//   the worker thread, that accesses resync_errors,
-	//   so no need to synchronize) by putting them both in a lock.
-	//   This would mean that block_incref might need to take a lock
-	//   before doing its thing, meaning it has much more chances of
-	//   not completing successfully if something bad happens to Garage.
-	//   Currently Garage is not able to recover from block_incref that
-	//   doesn't complete successfully, because it is necessary to ensure
-	//   the consistency between the state of the block manager and
-	//   information in the BlockRef table.
-	// - If a resync fails, we put that block in the resync_errors table,
-	//   and also add it back to resync_queue to be processed after
-	//   the exponential back-off delay,
-	//   but maybe the block is already scheduled to be resynced again
-	//   at another time that is before the exponential back-off delay,
-	//   and we have no way to check that easily. This means that
-	//   in all cases, we need to check the resync_errors table
-	//   in the resync loop at the time when a block is popped from
-	//   the resync_queue.
-	// Overall, the current design is therefore simpler and more robust
-	// because it tolerates inconsistencies between the resync_queue
-	// and resync_errors table (items being scheduled in resync_queue
-	// for times that are earlier than the exponential back-off delay
-	// is a natural condition that is handled properly).
-
-	fn spawn_background_workers(self: Arc<Self>) {
-		// Launch a background workers for background resync loop processing
-		let background = self.system.background.clone();
-		let worker = ResyncWorker::new(self.clone());
-		tokio::spawn(async move {
-			tokio::time::sleep(Duration::from_secs(10)).await;
-			background.spawn_worker(worker);
-		});
-
-		// Launch a background worker for data store scrubs
-		let (scrub_tx, scrub_rx) = mpsc::channel(1);
-		self.tx_scrub_command.store(Some(Arc::new(scrub_tx)));
-		let scrub_worker = ScrubWorker::new(self.clone(), scrub_rx);
-		self.system.background.spawn_worker(scrub_worker);
-	}
-
-	pub(crate) 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) -> db::Result<()> {
-		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_iter(&self) -> Result<ResyncIterResult, 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();
-
-			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
-						// (we want to do the remove after the insert to ensure
-						// that the item is not lost if we crash in-between)
-						self.resync_queue.remove(time_bytes)?;
-						return Ok(ResyncIterResult::BusyDidNothing);
-					}
-				}
-
-				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(ResyncIterResult::BusyDidSomething)
-			} else {
-				Ok(ResyncIterResult::IdleFor(Duration::from_millis(
-					time_msec - now,
-				)))
-			}
-		} else {
-			// Here we wait either for a notification that an item has been
-			// added to the queue, or for a constant delay of 10 secs to expire.
-			// The delay avoids a race condition where the notification happens
-			// between the time we checked the queue and the first poll
-			// to resync_notify.notified(): if that happens, we'll just loop
-			// back 10 seconds later, which is fine.
-			Ok(ResyncIterResult::IdleFor(Duration::from_secs(10)))
-		}
-	}
-
-	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.rc.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(())
-	}
 }
 
 #[async_trait]
@@ -734,80 +397,9 @@ impl EndpointHandler<BlockRpc> for BlockManager {
 	}
 }
 
-struct ResyncWorker {
-	manager: Arc<BlockManager>,
-	tranquilizer: Tranquilizer,
-	next_delay: Duration,
-}
-
-impl ResyncWorker {
-	fn new(manager: Arc<BlockManager>) -> Self {
-		Self {
-			manager,
-			tranquilizer: Tranquilizer::new(30),
-			next_delay: Duration::from_secs(10),
-		}
-	}
-}
-
-#[async_trait]
-impl Worker for ResyncWorker {
-	fn name(&self) -> String {
-		"Block resync worker".into()
-	}
-
-	fn info(&self) -> Option<String> {
-		let mut ret = vec![];
-		let qlen = self.manager.resync_queue_len().unwrap_or(0);
-		let elen = self.manager.resync_errors_len().unwrap_or(0);
-		if qlen > 0 {
-			ret.push(format!("{} blocks in queue", qlen));
-		}
-		if elen > 0 {
-			ret.push(format!("{} blocks in error state", elen));
-		}
-		if !ret.is_empty() {
-			Some(ret.join(", "))
-		} else {
-			None
-		}
-	}
-
-	async fn work(&mut self, _must_exit: &mut watch::Receiver<bool>) -> Result<WorkerState, Error> {
-		self.tranquilizer.reset();
-		match self.manager.resync_iter().await {
-			Ok(ResyncIterResult::BusyDidSomething) => Ok(self
-				.tranquilizer
-				.tranquilize_worker(self.manager.background_tranquility)),
-			Ok(ResyncIterResult::BusyDidNothing) => Ok(WorkerState::Busy),
-			Ok(ResyncIterResult::IdleFor(delay)) => {
-				self.next_delay = delay;
-				Ok(WorkerState::Idle)
-			}
-			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?)
-				// Here we just give the error to the worker manager,
-				// it will print it to the logs and increment a counter
-				Err(e.into())
-			}
-		}
-	}
-
-	async fn wait_for_work(&mut self, _must_exit: &watch::Receiver<bool>) -> WorkerState {
-		select! {
-			_ = tokio::time::sleep(self.next_delay) => (),
-			_ = self.manager.resync_notify.notified() => (),
-		};
-		WorkerState::Busy
-	}
-}
-
-struct BlockStatus {
-	exists: bool,
-	needed: RcEntry,
+pub(crate) struct BlockStatus {
+	pub(crate) exists: bool,
+	pub(crate) needed: RcEntry,
 }
 
 impl BlockManagerLocked {
@@ -912,50 +504,3 @@ impl BlockManagerLocked {
 		Ok(())
 	}
 }
-
-/// 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: &[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()),
-		}
-	}
-	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, RESYNC_RETRY_DELAY_MAX_BACKOFF_POWER)
-	}
-	fn next_try(&self) -> u64 {
-		self.last_try + self.delay_msec()
-	}
-}
diff --git a/src/block/repair.rs b/src/block/repair.rs
index 07ff6772..e2884b69 100644
--- a/src/block/repair.rs
+++ b/src/block/repair.rs
@@ -19,7 +19,17 @@ use garage_util::tranquilizer::Tranquilizer;
 
 use crate::manager::*;
 
-const SCRUB_INTERVAL: Duration = Duration::from_secs(3600 * 24 * 30); // full scrub every 30 days
+// Full scrub every 30 days
+const SCRUB_INTERVAL: Duration = Duration::from_secs(3600 * 24 * 30);
+// Scrub tranquility is initially set to 4, but can be changed in the CLI
+// and the updated version is persisted over Garage restarts
+const INITIAL_SCRUB_TRANQUILITY: u32 = 4;
+
+// ---- ---- ----
+// FIRST KIND OF REPAIR: FINDING MISSING BLOCKS/USELESS BLOCKS
+// This is a one-shot repair operation that can be launched,
+// checks everything, and then exits.
+// ---- ---- ----
 
 pub struct RepairWorker {
 	manager: Arc<BlockManager>,
@@ -102,7 +112,9 @@ impl Worker for RepairWorker {
 				}
 
 				for hash in batch_of_hashes.into_iter() {
-					self.manager.put_to_resync(&hash, Duration::from_secs(0))?;
+					self.manager
+						.resync
+						.put_to_resync(&hash, Duration::from_secs(0))?;
 					self.next_start = Some(hash)
 				}
 
@@ -114,7 +126,9 @@ impl Worker for RepairWorker {
 				// 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.
 				if let Some(hash) = bi.next().await? {
-					self.manager.put_to_resync(&hash, Duration::from_secs(0))?;
+					self.manager
+						.resync
+						.put_to_resync(&hash, Duration::from_secs(0))?;
 					Ok(WorkerState::Busy)
 				} else {
 					Ok(WorkerState::Done)
@@ -128,7 +142,13 @@ impl Worker for RepairWorker {
 	}
 }
 
-// ----
+// ---- ---- ----
+// SECOND KIND OF REPAIR: SCRUBBING THE DATASTORE
+// This is significantly more complex than the process above,
+// as it is a continuously-running task that triggers automatically
+// every SCRUB_INTERVAL, but can also be triggered manually
+// and whose parameter (esp. speed) can be controlled at runtime.
+// ---- ---- ----
 
 pub struct ScrubWorker {
 	manager: Arc<BlockManager>,
@@ -176,7 +196,7 @@ impl ScrubWorker {
 			Ok(v) => v,
 			Err(_) => ScrubWorkerPersisted {
 				time_last_complete_scrub: 0,
-				tranquility: 4,
+				tranquility: INITIAL_SCRUB_TRANQUILITY,
 				corruptions_detected: 0,
 			},
 		};
@@ -343,7 +363,9 @@ impl Worker for ScrubWorker {
 	}
 }
 
-// ----
+// ---- ---- ----
+// UTILITY FOR ENUMERATING THE BLOCK STORE
+// ---- ---- ----
 
 struct BlockStoreIterator {
 	path: Vec<ReadingDir>,
diff --git a/src/block/resync.rs b/src/block/resync.rs
new file mode 100644
index 00000000..39e4d50f
--- /dev/null
+++ b/src/block/resync.rs
@@ -0,0 +1,590 @@
+use std::collections::HashSet;
+use std::convert::TryInto;
+use std::sync::{Arc, Mutex};
+use std::time::Duration;
+
+use arc_swap::ArcSwap;
+use async_trait::async_trait;
+use serde::{Deserialize, Serialize};
+
+use futures::future::*;
+use tokio::select;
+use tokio::sync::{watch, Notify};
+
+use opentelemetry::{
+	trace::{FutureExt as OtelFutureExt, TraceContextExt, Tracer},
+	Context, KeyValue,
+};
+
+use garage_db as db;
+use garage_db::counted_tree_hack::CountedTree;
+
+use garage_util::background::*;
+use garage_util::data::*;
+use garage_util::error::*;
+use garage_util::metrics::RecordDuration;
+use garage_util::persister::Persister;
+use garage_util::time::*;
+use garage_util::tranquilizer::Tranquilizer;
+
+use garage_rpc::system::System;
+use garage_rpc::*;
+
+use garage_table::replication::TableReplication;
+
+use crate::manager::*;
+
+// Timeout for RPCs that ask other nodes whether they need a copy
+// of a given block before we delete it locally
+pub(crate) 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).
+pub(crate) const RESYNC_RETRY_DELAY: Duration = Duration::from_secs(60);
+// The minimum retry delay is 60 seconds = 1 minute
+// The maximum retry delay is 60 seconds * 2^6 = 60 seconds << 6 = 64 minutes (~1 hour)
+pub(crate) const RESYNC_RETRY_DELAY_MAX_BACKOFF_POWER: u64 = 6;
+
+// No more than 4 resync workers can be running in the system
+pub(crate) const MAX_RESYNC_WORKERS: usize = 4;
+// Resync tranquility is initially set to 2, but can be changed in the CLI
+// and the updated version is persisted over Garage restarts
+const INITIAL_RESYNC_TRANQUILITY: u32 = 2;
+
+pub struct BlockResyncManager {
+	pub(crate) queue: CountedTree,
+	pub(crate) notify: Notify,
+	pub(crate) errors: CountedTree,
+
+	busy_set: BusySet,
+
+	persister: Persister<ResyncPersistedConfig>,
+	persisted: ArcSwap<ResyncPersistedConfig>,
+}
+
+#[derive(Serialize, Deserialize, Clone, Copy)]
+struct ResyncPersistedConfig {
+	n_workers: usize,
+	tranquility: u32,
+}
+
+enum ResyncIterResult {
+	BusyDidSomething,
+	BusyDidNothing,
+	IdleFor(Duration),
+}
+
+type BusySet = Arc<Mutex<HashSet<Vec<u8>>>>;
+
+struct BusyBlock {
+	time_bytes: Vec<u8>,
+	hash_bytes: Vec<u8>,
+	busy_set: BusySet,
+}
+
+impl BlockResyncManager {
+	pub(crate) fn new(db: &db::Db, system: &System) -> Self {
+		let queue = db
+			.open_tree("block_local_resync_queue")
+			.expect("Unable to open block_local_resync_queue tree");
+		let queue = CountedTree::new(queue).expect("Could not count block_local_resync_queue");
+
+		let errors = db
+			.open_tree("block_local_resync_errors")
+			.expect("Unable to open block_local_resync_errors tree");
+		let errors = CountedTree::new(errors).expect("Could not count block_local_resync_errors");
+
+		let persister = Persister::new(&system.metadata_dir, "resync_cfg");
+		let persisted = match persister.load() {
+			Ok(v) => v,
+			Err(_) => ResyncPersistedConfig {
+				n_workers: 1,
+				tranquility: INITIAL_RESYNC_TRANQUILITY,
+			},
+		};
+
+		Self {
+			queue,
+			notify: Notify::new(),
+			errors,
+			busy_set: Arc::new(Mutex::new(HashSet::new())),
+			persister,
+			persisted: ArcSwap::new(Arc::new(persisted)),
+		}
+	}
+
+	/// Get lenght of resync queue
+	pub fn 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.queue.len())
+	}
+
+	/// Get number of blocks that have an error
+	pub fn errors_len(&self) -> Result<usize, Error> {
+		// (see queue_len comment)
+		Ok(self.errors.len())
+	}
+
+	// ---- Resync loop ----
+
+	// This part manages a queue of blocks that need to be
+	// "resynchronized", i.e. that need to have a check that
+	// they are at present if we need them, or that they are
+	// deleted once the garbage collection delay has passed.
+	//
+	// Here are some explanations on how the resync queue works.
+	// There are two Sled trees that are used to have information
+	// about the status of blocks that need to be resynchronized:
+	//
+	// - resync.queue: a tree that is ordered first by a timestamp
+	//   (in milliseconds since Unix epoch) that is the time at which
+	//   the resync must be done, and second by block hash.
+	//   The key in this tree is just:
+	//       concat(timestamp (8 bytes), hash (32 bytes))
+	//   The value is the same 32-byte hash.
+	//
+	// - resync.errors: a tree that indicates for each block
+	//   if the last resync resulted in an error, and if so,
+	//   the following two informations (see the ErrorCounter struct):
+	//   - how many consecutive resync errors for this block?
+	//   - when was the last try?
+	//   These two informations are used to implement an
+	//   exponential backoff retry strategy.
+	//   The key in this tree is the 32-byte hash of the block,
+	//   and the value is the encoded ErrorCounter value.
+	//
+	// We need to have these two trees, because the resync queue
+	// is not just a queue of items to process, but a set of items
+	// that are waiting a specific delay until we can process them
+	// (the delay being necessary both internally for the exponential
+	// backoff strategy, and exposed as a parameter when adding items
+	// to the queue, e.g. to wait until the GC delay has passed).
+	// This is why we need one tree ordered by time, and one
+	// ordered by identifier of item to be processed (block hash).
+	//
+	// When the worker wants to process an item it takes from
+	// resync.queue, it checks in resync.errors that if there is an
+	// exponential back-off delay to await, it has passed before we
+	// process the item. If not, the item in the queue is skipped
+	// (but added back for later processing after the time of the
+	// delay).
+	//
+	// An alternative that would have seemed natural is to
+	// only add items to resync.queue with a processing time that is
+	// after the delay, but there are several issues with this:
+	// - This requires to synchronize updates to resync.queue and
+	//   resync.errors (with the current model, there is only one thread,
+	//   the worker thread, that accesses resync.errors,
+	//   so no need to synchronize) by putting them both in a lock.
+	//   This would mean that block_incref might need to take a lock
+	//   before doing its thing, meaning it has much more chances of
+	//   not completing successfully if something bad happens to Garage.
+	//   Currently Garage is not able to recover from block_incref that
+	//   doesn't complete successfully, because it is necessary to ensure
+	//   the consistency between the state of the block manager and
+	//   information in the BlockRef table.
+	// - If a resync fails, we put that block in the resync.errors table,
+	//   and also add it back to resync.queue to be processed after
+	//   the exponential back-off delay,
+	//   but maybe the block is already scheduled to be resynced again
+	//   at another time that is before the exponential back-off delay,
+	//   and we have no way to check that easily. This means that
+	//   in all cases, we need to check the resync.errors table
+	//   in the resync loop at the time when a block is popped from
+	//   the resync.queue.
+	// Overall, the current design is therefore simpler and more robust
+	// because it tolerates inconsistencies between the resync.queue
+	// and resync.errors table (items being scheduled in resync.queue
+	// for times that are earlier than the exponential back-off delay
+	// is a natural condition that is handled properly).
+
+	pub(crate) 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)
+	}
+
+	pub(crate) 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());
+		self.queue.insert(key, hash.as_ref())?;
+		self.notify.notify_waiters();
+		Ok(())
+	}
+
+	async fn resync_iter(&self, manager: &BlockManager) -> Result<ResyncIterResult, db::Error> {
+		if let Some(block) = self.get_block_to_resync()? {
+			let time_msec = u64::from_be_bytes(block.time_bytes[0..8].try_into().unwrap());
+			let now = now_msec();
+
+			if now >= time_msec {
+				let hash = Hash::try_from(&block.hash_bytes[..]).unwrap();
+
+				if let Some(ec) = self.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
+						// (we want to do the remove after the insert to ensure
+						// that the item is not lost if we crash in-between)
+						self.queue.remove(&block.time_bytes)?;
+						return Ok(ResyncIterResult::BusyDidNothing);
+					}
+				}
+
+				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(manager, &hash)
+					.with_context(Context::current_with_span(span))
+					.bound_record_duration(&manager.metrics.resync_duration)
+					.await;
+
+				manager.metrics.resync_counter.add(1);
+
+				if let Err(e) = &res {
+					manager.metrics.resync_error_counter.add(1);
+					warn!("Error when resyncing {:?}: {}", hash, e);
+
+					let err_counter = match self.errors.get(hash.as_slice())? {
+						Some(ec) => ErrorCounter::decode(&ec).add1(now + 1),
+						None => ErrorCounter::new(now + 1),
+					};
+
+					self.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.queue.remove(&block.time_bytes)?;
+				} else {
+					self.errors.remove(hash.as_slice())?;
+					self.queue.remove(&block.time_bytes)?;
+				}
+
+				Ok(ResyncIterResult::BusyDidSomething)
+			} else {
+				Ok(ResyncIterResult::IdleFor(Duration::from_millis(
+					time_msec - now,
+				)))
+			}
+		} else {
+			// Here we wait either for a notification that an item has been
+			// added to the queue, or for a constant delay of 10 secs to expire.
+			// The delay avoids a race condition where the notification happens
+			// between the time we checked the queue and the first poll
+			// to resync_notify.notified(): if that happens, we'll just loop
+			// back 10 seconds later, which is fine.
+			Ok(ResyncIterResult::IdleFor(Duration::from_secs(10)))
+		}
+	}
+
+	fn get_block_to_resync(&self) -> Result<Option<BusyBlock>, db::Error> {
+		let mut busy = self.busy_set.lock().unwrap();
+		for it in self.queue.iter()? {
+			let (time_bytes, hash_bytes) = it?;
+			if !busy.contains(&time_bytes) {
+				busy.insert(time_bytes.clone());
+				return Ok(Some(BusyBlock {
+					time_bytes,
+					hash_bytes,
+					busy_set: self.busy_set.clone(),
+				}));
+			}
+		}
+		Ok(None)
+	}
+
+	async fn resync_block(&self, manager: &BlockManager, hash: &Hash) -> Result<(), Error> {
+		let BlockStatus { exists, needed } = manager.check_block_status(hash).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 = manager.replication.write_nodes(hash);
+			if who.len() < manager.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 != manager.system.id);
+
+			let msg = Arc::new(BlockRpc::NeedBlockQuery(*hash));
+			let who_needs_fut = who.iter().map(|to| {
+				manager.system.rpc.call_arc(
+					&manager.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() {
+					manager
+						.metrics
+						.resync_send_counter
+						.add(1, &[KeyValue::new("to", format!("{:?}", node))]);
+				}
+
+				let put_block_message = manager.read_block(hash).await?;
+				manager
+					.system
+					.rpc
+					.try_call_many(
+						&manager.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()
+			);
+
+			manager.delete_if_unneeded(hash).await?;
+
+			manager.rc.clear_deleted_block_rc(hash)?;
+		}
+
+		if needed.is_nonzero() && !exists {
+			info!(
+				"Resync block {:?}: fetching absent but needed block (refcount > 0)",
+				hash
+			);
+
+			let block_data = manager.rpc_get_raw_block(hash).await?;
+
+			manager.metrics.resync_recv_counter.add(1);
+
+			manager.write_block(hash, &block_data).await?;
+		}
+
+		Ok(())
+	}
+
+	async fn update_persisted(
+		&self,
+		update: impl Fn(&mut ResyncPersistedConfig),
+	) -> Result<(), Error> {
+		let mut cfg: ResyncPersistedConfig = *self.persisted.load().as_ref();
+		update(&mut cfg);
+		self.persister.save_async(&cfg).await?;
+		self.persisted.store(Arc::new(cfg));
+		self.notify.notify_waiters();
+		Ok(())
+	}
+
+	pub async fn set_n_workers(&self, n_workers: usize) -> Result<(), Error> {
+		if !(1..=MAX_RESYNC_WORKERS).contains(&n_workers) {
+			return Err(Error::Message(format!(
+				"Invalid number of resync workers, must be between 1 and {}",
+				MAX_RESYNC_WORKERS
+			)));
+		}
+		self.update_persisted(|cfg| cfg.n_workers = n_workers).await
+	}
+
+	pub async fn set_tranquility(&self, tranquility: u32) -> Result<(), Error> {
+		self.update_persisted(|cfg| cfg.tranquility = tranquility)
+			.await
+	}
+}
+
+impl Drop for BusyBlock {
+	fn drop(&mut self) {
+		let mut busy = self.busy_set.lock().unwrap();
+		busy.remove(&self.time_bytes);
+	}
+}
+
+pub(crate) struct ResyncWorker {
+	index: usize,
+	manager: Arc<BlockManager>,
+	tranquilizer: Tranquilizer,
+	next_delay: Duration,
+}
+
+impl ResyncWorker {
+	pub(crate) fn new(index: usize, manager: Arc<BlockManager>) -> Self {
+		Self {
+			index,
+			manager,
+			tranquilizer: Tranquilizer::new(30),
+			next_delay: Duration::from_secs(10),
+		}
+	}
+}
+
+#[async_trait]
+impl Worker for ResyncWorker {
+	fn name(&self) -> String {
+		format!("Block resync worker #{}", self.index + 1)
+	}
+
+	fn info(&self) -> Option<String> {
+		let persisted = self.manager.resync.persisted.load();
+
+		if self.index >= persisted.n_workers {
+			return Some("(unused)".into());
+		}
+
+		let mut ret = vec![];
+		ret.push(format!("tranquility = {}", persisted.tranquility));
+
+		let qlen = self.manager.resync.queue_len().unwrap_or(0);
+		if qlen > 0 {
+			ret.push(format!("{} blocks in queue", qlen));
+		}
+
+		let elen = self.manager.resync.errors_len().unwrap_or(0);
+		if elen > 0 {
+			ret.push(format!("{} blocks in error state", elen));
+		}
+
+		Some(ret.join(", "))
+	}
+
+	async fn work(&mut self, _must_exit: &mut watch::Receiver<bool>) -> Result<WorkerState, Error> {
+		if self.index >= self.manager.resync.persisted.load().n_workers {
+			return Ok(WorkerState::Idle);
+		}
+
+		self.tranquilizer.reset();
+		match self.manager.resync.resync_iter(&self.manager).await {
+			Ok(ResyncIterResult::BusyDidSomething) => Ok(self
+				.tranquilizer
+				.tranquilize_worker(self.manager.resync.persisted.load().tranquility)),
+			Ok(ResyncIterResult::BusyDidNothing) => Ok(WorkerState::Busy),
+			Ok(ResyncIterResult::IdleFor(delay)) => {
+				self.next_delay = delay;
+				Ok(WorkerState::Idle)
+			}
+			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?)
+				// Here we just give the error to the worker manager,
+				// it will print it to the logs and increment a counter
+				Err(e.into())
+			}
+		}
+	}
+
+	async fn wait_for_work(&mut self, _must_exit: &watch::Receiver<bool>) -> WorkerState {
+		while self.index >= self.manager.resync.persisted.load().n_workers {
+			self.manager.resync.notify.notified().await
+		}
+
+		select! {
+			_ = tokio::time::sleep(self.next_delay) => (),
+			_ = self.manager.resync.notify.notified() => (),
+		};
+
+		WorkerState::Busy
+	}
+}
+
+/// 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: &[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()),
+		}
+	}
+	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, RESYNC_RETRY_DELAY_MAX_BACKOFF_POWER)
+	}
+	fn next_try(&self) -> u64 {
+		self.last_try + self.delay_msec()
+	}
+}
diff --git a/src/garage/admin.rs b/src/garage/admin.rs
index 71ee608c..76261050 100644
--- a/src/garage/admin.rs
+++ b/src/garage/admin.rs
@@ -15,6 +15,8 @@ use garage_table::*;
 
 use garage_rpc::*;
 
+use garage_block::repair::ScrubWorkerCommand;
+
 use garage_model::bucket_alias_table::*;
 use garage_model::bucket_table::*;
 use garage_model::garage::Garage;
@@ -779,13 +781,13 @@ impl AdminRpcHandler {
 		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();
 
@@ -836,6 +838,32 @@ impl AdminRpcHandler {
 				let workers = self.garage.background.get_worker_info();
 				Ok(AdminRpc::WorkerList(workers, opt))
 			}
+			WorkerCmd::Set { opt } => match opt {
+				WorkerSetCmd::ScrubTranquility { tranquility } => {
+					let scrub_command = ScrubWorkerCommand::SetTranquility(tranquility);
+					self.garage
+						.block_manager
+						.send_scrub_command(scrub_command)
+						.await;
+					Ok(AdminRpc::Ok("Scrub tranquility updated".into()))
+				}
+				WorkerSetCmd::ResyncNWorkers { n_workers } => {
+					self.garage
+						.block_manager
+						.resync
+						.set_n_workers(n_workers)
+						.await?;
+					Ok(AdminRpc::Ok("Number of resync workers updated".into()))
+				}
+				WorkerSetCmd::ResyncTranquility { tranquility } => {
+					self.garage
+						.block_manager
+						.resync
+						.set_tranquility(tranquility)
+						.await?;
+					Ok(AdminRpc::Ok("Resync tranquility updated".into()))
+				}
+			},
 		}
 	}
 }
diff --git a/src/garage/cli/structs.rs b/src/garage/cli/structs.rs
index 9274f80f..0388cef5 100644
--- a/src/garage/cli/structs.rs
+++ b/src/garage/cli/structs.rs
@@ -501,6 +501,12 @@ pub enum WorkerCmd {
 		#[structopt(flatten)]
 		opt: WorkerListOpt,
 	},
+	/// Set worker parameter
+	#[structopt(name = "set", version = version::garage())]
+	Set {
+		#[structopt(subcommand)]
+		opt: WorkerSetCmd,
+	},
 }
 
 #[derive(Serialize, Deserialize, StructOpt, Debug, Eq, PartialEq, Clone, Copy)]
@@ -512,3 +518,16 @@ pub struct WorkerListOpt {
 	#[structopt(short = "e", long = "errors")]
 	pub errors: bool,
 }
+
+#[derive(Serialize, Deserialize, StructOpt, Debug, Eq, PartialEq, Clone)]
+pub enum WorkerSetCmd {
+	/// Set tranquility of scrub operations
+	#[structopt(name = "scrub-tranquility", version = version::garage())]
+	ScrubTranquility { tranquility: u32 },
+	/// Set number of concurrent block resync workers
+	#[structopt(name = "resync-n-workers", version = version::garage())]
+	ResyncNWorkers { n_workers: usize },
+	/// Set tranquility of block resync operations
+	#[structopt(name = "resync-tranquility", version = version::garage())]
+	ResyncTranquility { tranquility: u32 },
+}
diff --git a/src/model/garage.rs b/src/model/garage.rs
index 15769a17..4dd24582 100644
--- a/src/model/garage.rs
+++ b/src/model/garage.rs
@@ -159,7 +159,6 @@ impl Garage {
 			&db,
 			config.data_dir.clone(),
 			config.compression_level,
-			config.block_manager_background_tranquility,
 			data_rep_param,
 			system.clone(),
 		);
diff --git a/src/util/config.rs b/src/util/config.rs
index e8ef4fdd..a2bb8fb3 100644
--- a/src/util/config.rs
+++ b/src/util/config.rs
@@ -23,10 +23,6 @@ pub struct Config {
 	#[serde(default = "default_block_size")]
 	pub block_size: usize,
 
-	/// Size of data blocks to save to disk
-	#[serde(default = "default_block_manager_background_tranquility")]
-	pub block_manager_background_tranquility: u32,
-
 	/// Replication mode. Supported values:
 	/// - none, 1 -> no replication
 	/// - 2 -> 2-way replication
@@ -147,9 +143,6 @@ fn default_sled_flush_every_ms() -> u64 {
 fn default_block_size() -> usize {
 	1048576
 }
-fn default_block_manager_background_tranquility() -> u32 {
-	2
-}
 
 /// Read and parse configuration
 pub fn read_config(config_file: PathBuf) -> Result<Config, Error> {