Implement exponential backoff for resync retries

src/model/block.rs

@@ -39,7 +39,7 @@ use crate::garage::Garage;
 pub const INLINE_THRESHOLD: usize = 3072;
 
 pub const BACKGROUND_WORKERS: u64 = 1;
-pub const BACKGROUND_TRANQUILITY: u32 = 3;
+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);
@@ -48,7 +48,8 @@ const BLOCK_RW_TIMEOUT: Duration = Duration::from_secs(30);
 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.
+// 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
@@ -158,6 +159,7 @@ pub struct BlockManager {
 
 	resync_queue: SledCountedTree,
 	resync_notify: Notify,
+	resync_errors: SledCountedTree,
 
 	system: Arc<System>,
 	endpoint: Arc<Endpoint<BlockRpc, Self>>,
@@ -187,13 +189,18 @@ impl BlockManager {
 			.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());
+		let metrics = BlockManagerMetrics::new(resync_queue.clone(), resync_errors.clone());
 
 		let block_manager = Arc::new(Self {
 			replication,
@@ -202,6 +209,7 @@ impl BlockManager {
 			rc,
 			resync_queue,
 			resync_notify: Notify::new(),
+			resync_errors,
 			system,
 			endpoint,
 			garage: ArcSwapOption::from(None),
@@ -519,6 +527,10 @@ impl BlockManager {
 
 	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());
@@ -560,6 +572,17 @@ impl BlockManager {
 			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())?;
+						return Ok(false);
+					}
+				}
+
 				let tracer = opentelemetry::global::tracer("garage");
 				let trace_id = gen_uuid();
 				let span = tracer
@@ -584,8 +607,19 @@ impl BlockManager {
 				if let Err(e) = &res {
 					self.metrics.resync_error_counter.add(1);
 					warn!("Error when resyncing {:?}: {}", hash, e);
-					self.put_to_resync(&hash, RESYNC_RETRY_DELAY)?;
+
+					let err_counter = match self.resync_errors.get(hash.as_slice())? {
+						Some(ec) => ErrorCounter::decode(ec).add1(),
+						None => ErrorCounter::new(),
+					};
+
+					self.put_to_resync_at(&hash, err_counter.next_try())?;
+					self.resync_errors
+						.insert(hash.as_slice(), err_counter.encode())?;
+				} else {
+					self.resync_errors.remove(hash.as_slice())?;
 				}
+
 				Ok(true)
 			} else {
 				self.resync_queue.insert(time_bytes, hash_bytes)?;
@@ -994,6 +1028,58 @@ impl RcEntry {
 	}
 }
 
+/// 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 Default for ErrorCounter {
+	fn default() -> Self {
+		Self {
+			errors: 1,
+			last_try: now_msec(),
+		}
+	}
+}
+
+impl ErrorCounter {
+	fn new() -> Self {
+		Self::default()
+	}
+
+	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) -> Self {
+		Self {
+			errors: self.errors + 1,
+			last_try: now_msec(),
+		}
+	}
+
+	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)?;

src/model/block_metrics.rs

@@ -5,6 +5,7 @@ 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>,
@@ -22,7 +23,7 @@ pub struct BlockManagerMetrics {
 }
 
 impl BlockManagerMetrics {
-	pub fn new(resync_queue: SledCountedTree) -> Self {
+	pub fn new(resync_queue: SledCountedTree, resync_errors: SledCountedTree) -> Self {
 		let meter = global::meter("garage_model/block");
 		Self {
 			_resync_queue_len: meter
@@ -33,6 +34,12 @@ impl BlockManagerMetrics {
 					"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")

src/util/sled_counter.rs

@@ -52,6 +52,14 @@ impl SledCountedTree {
 		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 {