layout: add helper for cached/external values to centralize recomputation

10 files changed, 222 insertions, 181 deletions

diff --git a/src/api/admin/cluster.rs b/src/api/admin/cluster.rs
index d912b58f..593bd778 100644
--- a/src/api/admin/cluster.rs
+++ b/src/api/admin/cluster.rs
@@ -240,7 +240,6 @@ pub async fn handle_update_cluster_layout(
 			.merge(&roles.update_mutator(node, layout::NodeRoleV(new_role)));
 	}
 
-	layout.update_hashes();
 	garage
 		.system
 		.layout_manager
diff --git a/src/api/k2v/index.rs b/src/api/k2v/index.rs
index c189232a..e8cd1fba 100644
--- a/src/api/k2v/index.rs
+++ b/src/api/k2v/index.rs
@@ -29,7 +29,7 @@ pub async fn handle_read_index(
 		.system
 		.cluster_layout()
 		.all_nongateway_nodes()
-		.into_owned();
+		.to_vec();
 
 	let (partition_keys, more, next_start) = read_range(
 		&garage.k2v.counter_table.table,
diff --git a/src/garage/cli/layout.rs b/src/garage/cli/layout.rs
index 0f01a37a..51774314 100644
--- a/src/garage/cli/layout.rs
+++ b/src/garage/cli/layout.rs
@@ -49,7 +49,7 @@ pub async fn cmd_assign_role(
 	};
 
 	let mut layout = fetch_layout(rpc_cli, rpc_host).await?;
-	let all_nodes = layout.all_nodes().into_owned();
+	let all_nodes = layout.get_all_nodes();
 
 	let added_nodes = args
 		.node_ids
@@ -331,7 +331,6 @@ pub async fn send_layout(
 	rpc_host: NodeID,
 	mut layout: LayoutHistory,
 ) -> Result<(), Error> {
-	layout.update_hashes();
 	rpc_cli
 		.call(
 			&rpc_host,
diff --git a/src/model/helper/bucket.rs b/src/model/helper/bucket.rs
index 2cb53424..efa3e27b 100644
--- a/src/model/helper/bucket.rs
+++ b/src/model/helper/bucket.rs
@@ -455,7 +455,7 @@ impl<'a> BucketHelper<'a> {
 				.system
 				.cluster_layout()
 				.all_nongateway_nodes()
-				.into_owned();
+				.to_vec();
 			let k2vindexes = self
 				.0
 				.k2v
diff --git a/src/model/index_counter.rs b/src/model/index_counter.rs
index 2d968733..e8702bf1 100644
--- a/src/model/index_counter.rs
+++ b/src/model/index_counter.rs
@@ -7,7 +7,7 @@ use serde::{Deserialize, Serialize};
 
 use garage_db as db;
 
-use garage_rpc::layout::LayoutHistory;
+use garage_rpc::layout::LayoutHelper;
 use garage_rpc::system::System;
 use garage_util::background::BackgroundRunner;
 use garage_util::data::*;
@@ -83,7 +83,7 @@ impl<T: CountedItem> Entry<T::CP, T::CS> for CounterEntry<T> {
 }
 
 impl<T: CountedItem> CounterEntry<T> {
-	pub fn filtered_values(&self, layout: &LayoutHistory) -> HashMap<String, i64> {
+	pub fn filtered_values(&self, layout: &LayoutHelper) -> HashMap<String, i64> {
 		let nodes = layout.all_nongateway_nodes();
 		self.filtered_values_with_nodes(&nodes)
 	}
diff --git a/src/rpc/layout/history.rs b/src/rpc/layout/history.rs
index 1684918e..b6f0e495 100644
--- a/src/rpc/layout/history.rs
+++ b/src/rpc/layout/history.rs
@@ -1,5 +1,5 @@
-use std::borrow::Cow;
 use std::collections::HashSet;
+use std::ops::Deref;
 
 use garage_util::crdt::{Crdt, Lww, LwwMap};
 use garage_util::data::*;
@@ -9,95 +9,106 @@ use garage_util::error::*;
 use super::schema::*;
 use super::*;
 
+pub struct LayoutHelper {
+	layout: Option<LayoutHistory>,
 
-impl LayoutHistory {
-	pub fn new(replication_factor: usize) -> Self {
-		let version = LayoutVersion::new(replication_factor);
+	// cached values
+	ack_map_min: u64,
+	sync_map_min: u64,
 
-		let staging = LayoutStaging {
-			parameters: Lww::<LayoutParameters>::new(version.parameters),
-			roles: LwwMap::new(),
-		};
+	all_nodes: Vec<Uuid>,
+	all_nongateway_nodes: Vec<Uuid>,
 
-		let mut ret = LayoutHistory {
-			versions: vec![version],
-			update_trackers: Default::default(),
-			trackers_hash: [0u8; 32].into(),
-			staging: Lww::raw(0, staging),
-			staging_hash: [0u8; 32].into(),
-		};
-		ret.update_hashes();
-		ret
-	}
+	trackers_hash: Hash,
+	staging_hash: Hash,
+}
 
-	pub fn current(&self) -> &LayoutVersion {
-		self.versions.last().as_ref().unwrap()
+impl Deref for LayoutHelper {
+	type Target = LayoutHistory;
+	fn deref(&self) -> &LayoutHistory {
+		self.layout()
 	}
+}
 
-	pub fn update_hashes(&mut self) {
-		self.trackers_hash = self.calculate_trackers_hash();
-		self.staging_hash = self.calculate_staging_hash();
+impl LayoutHelper {
+	pub fn new(mut layout: LayoutHistory) -> Self {
+		layout.cleanup_old_versions();
+
+		let all_nongateway_nodes = layout.get_all_nongateway_nodes();
+		layout.clamp_update_trackers(&all_nongateway_nodes);
+
+		let min_version = layout.min_stored();
+		let ack_map_min = layout
+			.update_trackers
+			.ack_map
+			.min(&all_nongateway_nodes, min_version);
+		let sync_map_min = layout
+			.update_trackers
+			.sync_map
+			.min(&all_nongateway_nodes, min_version);
+
+		let all_nodes = layout.get_all_nodes();
+		let trackers_hash = layout.calculate_trackers_hash();
+		let staging_hash = layout.calculate_staging_hash();
+
+		LayoutHelper {
+			layout: Some(layout),
+			ack_map_min,
+			sync_map_min,
+			all_nodes,
+			all_nongateway_nodes,
+			trackers_hash,
+			staging_hash,
+		}
 	}
 
-	pub(crate) fn calculate_trackers_hash(&self) -> Hash {
-		blake2sum(&nonversioned_encode(&self.update_trackers).unwrap()[..])
-	}
+	// ------------------ single updating function --------------
 
-	pub(crate) fn calculate_staging_hash(&self) -> Hash {
-		blake2sum(&nonversioned_encode(&self.staging).unwrap()[..])
+	fn layout(&self) -> &LayoutHistory {
+		self.layout.as_ref().unwrap()
 	}
 
-	// ------------------ who stores what now? ---------------
-
-	pub fn all_ack(&self) -> u64 {
-		self.update_trackers.ack_map.current_min
+	pub(crate) fn update<F>(&mut self, f: F) -> bool
+	where
+		F: FnOnce(&mut LayoutHistory) -> bool,
+	{
+		let changed = f(&mut self.layout.as_mut().unwrap());
+		if changed {
+			*self = Self::new(self.layout.take().unwrap());
+		}
+		changed
 	}
 
-	pub fn min_stored(&self) -> u64 {
-		self.versions.first().as_ref().unwrap().version
+	// ------------------ read helpers ---------------
+
+	pub fn all_nodes(&self) -> &[Uuid] {
+		&self.all_nodes
 	}
 
-	pub fn sync_versions(&self) -> (u64, u64, u64) {
-		(self.current().version, self.all_ack(), self.min_stored())
+	pub fn all_nongateway_nodes(&self) -> &[Uuid] {
+		&self.all_nongateway_nodes
 	}
 
-	pub fn all_nodes(&self) -> Cow<'_, [Uuid]> {
-		// TODO: cache this
-		if self.versions.len() == 1 {
-			self.versions[0].all_nodes().into()
-		} else {
-			let set = self
-				.versions
-				.iter()
-				.map(|x| x.all_nodes())
-				.flatten()
-				.collect::<HashSet<_>>();
-			set.into_iter().copied().collect::<Vec<_>>().into()
-		}
+	pub fn all_ack(&self) -> u64 {
+		self.ack_map_min
 	}
 
-	pub fn all_nongateway_nodes(&self) -> Cow<'_, [Uuid]> {
-		// TODO: cache this
-		if self.versions.len() == 1 {
-			self.versions[0].nongateway_nodes().into()
-		} else {
-			let set = self
-				.versions
-				.iter()
-				.map(|x| x.nongateway_nodes())
-				.flatten()
-				.collect::<HashSet<_>>();
-			set.into_iter().copied().collect::<Vec<_>>().into()
-		}
+	pub fn sync_versions(&self) -> (u64, u64, u64) {
+		(
+			self.layout().current().version,
+			self.all_ack(),
+			self.layout().min_stored(),
+		)
 	}
 
 	pub fn read_nodes_of(&self, position: &Hash) -> Vec<Uuid> {
-		let sync_min = self.update_trackers.sync_map.current_min;
+		let sync_min = self.sync_map_min;
 		let version = self
+			.layout()
 			.versions
 			.iter()
 			.find(|x| x.version == sync_min)
-			.or(self.versions.last())
+			.or(self.layout().versions.last())
 			.unwrap();
 		version
 			.nodes_of(position, version.replication_factor)
@@ -105,7 +116,8 @@ impl LayoutHistory {
 	}
 
 	pub fn write_sets_of(&self, position: &Hash) -> Vec<Vec<Uuid>> {
-		self.versions
+		self.layout()
+			.versions
 			.iter()
 			.map(|x| x.nodes_of(position, x.replication_factor).collect())
 			.collect()
@@ -113,7 +125,7 @@ impl LayoutHistory {
 
 	pub fn storage_nodes_of(&self, position: &Hash) -> Vec<Uuid> {
 		let mut ret = vec![];
-		for version in self.versions.iter() {
+		for version in self.layout().versions.iter() {
 			ret.extend(version.nodes_of(position, version.replication_factor));
 		}
 		ret.sort();
@@ -121,7 +133,35 @@ impl LayoutHistory {
 		ret
 	}
 
-	// ------------------ update tracking ---------------
+	pub fn trackers_hash(&self) -> Hash {
+		self.trackers_hash
+	}
+
+	pub fn staging_hash(&self) -> Hash {
+		self.staging_hash
+	}
+
+	// ------------------ helpers for update tracking ---------------
+
+	pub(crate) fn sync_first(&mut self, node: Uuid) {
+		let first_version = self.versions.first().as_ref().unwrap().version;
+		self.update(|layout| layout.update_trackers.sync_map.set_max(node, first_version));
+	}
+
+	pub(crate) fn sync_ack(&mut self, node: Uuid) {
+		let sync_map_min = self.sync_map_min;
+		self.update(|layout| {
+			layout
+				.update_trackers
+				.sync_ack_map
+				.set_max(node, sync_map_min)
+		});
+	}
+
+	pub(crate) fn ack_last(&mut self, node: Uuid) {
+		let last_version = self.current().version;
+		self.update(|layout| layout.update_trackers.ack_map.set_max(node, last_version));
+	}
 
 	pub(crate) fn update_trackers_of(&mut self, node_id: Uuid) {
 		// Ensure trackers for this node's values are up-to-date
@@ -136,55 +176,104 @@ impl LayoutHistory {
 		// 3. Acknowledge everyone has synced up to min(self.sync_map)
 		self.sync_ack(node_id);
 
-		// 4. Cleanup layout versions that are not needed anymore
-		self.cleanup_old_versions();
-
-		// 5. Recalculate global minima
-		self.update_trackers_min();
-
 		info!("ack_map: {:?}", self.update_trackers.ack_map);
 		info!("sync_map: {:?}", self.update_trackers.sync_map);
 		info!("sync_ack_map: {:?}", self.update_trackers.sync_ack_map);
+	}
+}
 
-		// Finally, update hashes
-		self.update_hashes();
+// ----
+
+impl LayoutHistory {
+	pub fn new(replication_factor: usize) -> Self {
+		let version = LayoutVersion::new(replication_factor);
+
+		let staging = LayoutStaging {
+			parameters: Lww::<LayoutParameters>::new(version.parameters),
+			roles: LwwMap::new(),
+		};
+
+		LayoutHistory {
+			versions: vec![version],
+			update_trackers: Default::default(),
+			staging: Lww::raw(0, staging),
+		}
 	}
 
-	fn update_trackers_min(&mut self) {
-		// TODO: for TableFullReplication, counting gateway nodes might be
-		// necessary? Think about this more.
-		let storage_nodes = self.all_nongateway_nodes().into_owned();
-		let min_version = self.versions.first().unwrap().version;
-		self.update_trackers.update_min(&storage_nodes, min_version);
+	// ------------------ who stores what now? ---------------
+
+	pub fn current(&self) -> &LayoutVersion {
+		self.versions.last().as_ref().unwrap()
 	}
 
-	pub(crate) fn ack_last(&mut self, node: Uuid) {
-		let last_version = self.current().version;
-		self.update_trackers.ack_map.set_max(node, last_version);
-		self.update_trackers_min();
+	pub fn min_stored(&self) -> u64 {
+		self.versions.first().as_ref().unwrap().version
 	}
 
-	pub(crate) fn sync_first(&mut self, node: Uuid) {
-		let first_version = self.versions.first().as_ref().unwrap().version;
-		self.update_trackers.sync_map.set_max(node, first_version);
-		self.update_trackers_min();
+	pub fn get_all_nodes(&self) -> Vec<Uuid> {
+		if self.versions.len() == 1 {
+			self.versions[0].all_nodes().to_vec()
+		} else {
+			let set = self
+				.versions
+				.iter()
+				.map(|x| x.all_nodes())
+				.flatten()
+				.collect::<HashSet<_>>();
+			set.into_iter().copied().collect::<Vec<_>>()
+		}
 	}
 
-	pub(crate) fn sync_ack(&mut self, node: Uuid) {
-		self.update_trackers
-			.sync_ack_map
-			.set_max(node, self.update_trackers.sync_map.current_min);
-		self.update_trackers_min();
+	fn get_all_nongateway_nodes(&self) -> Vec<Uuid> {
+		if self.versions.len() == 1 {
+			self.versions[0].nongateway_nodes().to_vec()
+		} else {
+			let set = self
+				.versions
+				.iter()
+				.map(|x| x.nongateway_nodes())
+				.flatten()
+				.collect::<HashSet<_>>();
+			set.into_iter().copied().collect::<Vec<_>>()
+		}
 	}
 
-	pub(crate) fn cleanup_old_versions(&mut self) {
-		let min_sync_ack = self.update_trackers.sync_ack_map.current_min;
-		while self.versions.first().as_ref().unwrap().version < min_sync_ack {
-			let removed = self.versions.remove(0);
-			info!("Layout history: pruning old version {}", removed.version);
+	// ---- housekeeping (all invoked by LayoutHelper) ----
+
+	fn cleanup_old_versions(&mut self) {
+		loop {
+			let all_nongateway_nodes = self.get_all_nongateway_nodes();
+			let min_version = self.min_stored();
+			let sync_ack_map_min = self
+				.update_trackers
+				.sync_ack_map
+				.min(&all_nongateway_nodes, min_version);
+			if self.min_stored() < sync_ack_map_min {
+				let removed = self.versions.remove(0);
+				info!("Layout history: pruning old version {}", removed.version);
+			} else {
+				break;
+			}
 		}
 	}
 
+	fn clamp_update_trackers(&mut self, nodes: &[Uuid]) {
+		let min_v = self.min_stored();
+		for node in nodes {
+			self.update_trackers.ack_map.set_max(*node, min_v);
+			self.update_trackers.sync_map.set_max(*node, min_v);
+			self.update_trackers.sync_ack_map.set_max(*node, min_v);
+		}
+	}
+
+	fn calculate_trackers_hash(&self) -> Hash {
+		blake2sum(&nonversioned_encode(&self.update_trackers).unwrap()[..])
+	}
+
+	fn calculate_staging_hash(&self) -> Hash {
+		blake2sum(&nonversioned_encode(&self.staging).unwrap()[..])
+	}
+
 	// ================== updates to layout, public interface ===================
 
 	pub fn merge(&mut self, other: &LayoutHistory) -> bool {
@@ -221,20 +310,6 @@ impl LayoutHistory {
 				self.versions.remove(0);
 				changed = true;
 			}
-			if changed {
-				let min_v = self.versions.first().unwrap().version;
-				let nodes = self.all_nongateway_nodes().into_owned();
-				for node in nodes {
-					self.update_trackers.ack_map.set_max(node, min_v);
-					self.update_trackers.sync_map.set_max(node, min_v);
-					self.update_trackers.sync_ack_map.set_max(node, min_v);
-				}
-			}
-		}
-
-		// Update the current_min value in trackers if anything changed
-		if changed {
-			self.update_trackers_min();
 		}
 
 		// Merge staged layout changes
@@ -280,7 +355,6 @@ To know the correct value of the new layout version, invoke `garage layout show`
 			parameters: self.staging.get().parameters.clone(),
 			roles: LwwMap::new(),
 		});
-		self.update_hashes();
 
 		Ok((self, msg))
 	}
@@ -290,20 +364,11 @@ To know the correct value of the new layout version, invoke `garage layout show`
 			parameters: Lww::new(self.current().parameters.clone()),
 			roles: LwwMap::new(),
 		});
-		self.update_hashes();
 
 		Ok(self)
 	}
 
 	pub fn check(&self) -> Result<(), String> {
-		// Check that the hash of the staging data is correct
-		if self.trackers_hash != self.calculate_trackers_hash() {
-			return Err("trackers_hash is incorrect".into());
-		}
-		if self.staging_hash != self.calculate_staging_hash() {
-			return Err("staging_hash is incorrect".into());
-		}
-
 		for version in self.versions.iter() {
 			version.check()?;
 		}
diff --git a/src/rpc/layout/manager.rs b/src/rpc/layout/manager.rs
index 21ec2d8d..e270ad21 100644
--- a/src/rpc/layout/manager.rs
+++ b/src/rpc/layout/manager.rs
@@ -24,7 +24,7 @@ pub struct LayoutManager {
 	replication_factor: usize,
 	persist_cluster_layout: Persister<LayoutHistory>,
 
-	layout: Arc<RwLock<LayoutHistory>>,
+	layout: Arc<RwLock<LayoutHelper>>,
 	pub(crate) change_notify: Arc<Notify>,
 
 	table_sync_version: Mutex<HashMap<String, u64>>,
@@ -54,7 +54,7 @@ impl LayoutManager {
 		let persist_cluster_layout: Persister<LayoutHistory> =
 			Persister::new(&config.metadata_dir, "cluster_layout");
 
-		let mut cluster_layout = match persist_cluster_layout.load() {
+		let cluster_layout = match persist_cluster_layout.load() {
 			Ok(x) => {
 				if x.current().replication_factor != replication_factor {
 					return Err(Error::Message(format!(
@@ -74,6 +74,7 @@ impl LayoutManager {
 			}
 		};
 
+		let mut cluster_layout = LayoutHelper::new(cluster_layout);
 		cluster_layout.update_trackers_of(node_id.into());
 
 		let layout = Arc::new(RwLock::new(cluster_layout));
@@ -100,7 +101,7 @@ impl LayoutManager {
 
 	// ---- PUBLIC INTERFACE ----
 
-	pub fn layout(&self) -> RwLockReadGuard<'_, LayoutHistory> {
+	pub fn layout(&self) -> RwLockReadGuard<'_, LayoutHelper> {
 		self.layout.read().unwrap()
 	}
 
@@ -108,8 +109,8 @@ impl LayoutManager {
 		let layout = self.layout();
 		LayoutStatus {
 			cluster_layout_version: layout.current().version,
-			cluster_layout_trackers_hash: layout.trackers_hash,
-			cluster_layout_staging_hash: layout.staging_hash,
+			cluster_layout_trackers_hash: layout.trackers_hash(),
+			cluster_layout_staging_hash: layout.staging_hash(),
 		}
 	}
 
@@ -137,13 +138,8 @@ impl LayoutManager {
 		drop(table_sync_version);
 
 		let mut layout = self.layout.write().unwrap();
-		if layout
-			.update_trackers
-			.sync_map
-			.set_max(self.node_id, sync_until)
-		{
+		if layout.update(|l| l.update_trackers.sync_map.set_max(self.node_id, sync_until)) {
 			debug!("sync_until updated to {}", sync_until);
-			layout.update_hashes();
 			self.broadcast_update(SystemRpc::AdvertiseClusterLayoutTrackers(
 				layout.update_trackers.clone(),
 			));
@@ -157,7 +153,7 @@ impl LayoutManager {
 		let prev_layout_check = layout.check().is_ok();
 
 		if !prev_layout_check || adv.check().is_ok() {
-			if layout.merge(adv) {
+			if layout.update(|l| l.merge(adv)) {
 				layout.update_trackers_of(self.node_id);
 				if prev_layout_check && layout.check().is_err() {
 					panic!("Merged two correct layouts and got an incorrect layout.");
@@ -171,7 +167,7 @@ impl LayoutManager {
 	fn merge_layout_trackers(&self, adv: &UpdateTrackers) -> Option<UpdateTrackers> {
 		let mut layout = self.layout.write().unwrap();
 		if layout.update_trackers != *adv {
-			if layout.update_trackers.merge(adv) {
+			if layout.update(|l| l.update_trackers.merge(adv)) {
 				layout.update_trackers_of(self.node_id);
 				return Some(layout.update_trackers.clone());
 			}
diff --git a/src/rpc/layout/schema.rs b/src/rpc/layout/schema.rs
index 969f5a0b..00a2c017 100644
--- a/src/rpc/layout/schema.rs
+++ b/src/rpc/layout/schema.rs
@@ -188,7 +188,7 @@ mod v010 {
 	use super::v09;
 	use crate::layout::CompactNodeType;
 	use garage_util::crdt::{Lww, LwwMap};
-	use garage_util::data::{Hash, Uuid};
+	use garage_util::data::Uuid;
 	use serde::{Deserialize, Serialize};
 	use std::collections::BTreeMap;
 	pub use v09::{LayoutParameters, NodeRole, NodeRoleV, ZoneRedundancy};
@@ -202,13 +202,9 @@ mod v010 {
 
 		/// Update trackers
 		pub update_trackers: UpdateTrackers,
-		/// Hash of the update trackers
-		pub trackers_hash: Hash,
 
 		/// Staged changes for the next version
 		pub staging: Lww<LayoutStaging>,
-		/// Hash of the serialized staging_parameters + staging_roles
-		pub staging_hash: Hash,
 	}
 
 	/// A version of the layout of the cluster, i.e. the list of roles
@@ -260,10 +256,7 @@ mod v010 {
 
 	/// The history of cluster layouts
 	#[derive(Clone, Debug, Serialize, Deserialize, Default, PartialEq)]
-	pub struct UpdateTracker {
-		pub values: BTreeMap<Uuid, u64>,
-		pub current_min: u64,
-	}
+	pub struct UpdateTracker(pub BTreeMap<Uuid, u64>);
 
 	impl garage_util::migrate::Migrate for LayoutHistory {
 		const VERSION_MARKER: &'static [u8] = b"G010lh";
@@ -293,32 +286,27 @@ mod v010 {
 				nongateway_node_count,
 				ring_assignment_data: previous.ring_assignment_data,
 			};
-			let update_tracker = UpdateTracker {
-				values: version
+			let update_tracker = UpdateTracker(
+				version
 					.nongateway_nodes()
 					.iter()
 					.copied()
 					.map(|x| (x, version.version))
 					.collect::<BTreeMap<Uuid, u64>>(),
-				current_min: 0,
-			};
+			);
 			let staging = LayoutStaging {
 				parameters: previous.staging_parameters,
 				roles: previous.staging_roles,
 			};
-			let mut ret = Self {
+			Self {
 				versions: vec![version],
 				update_trackers: UpdateTrackers {
 					ack_map: update_tracker.clone(),
 					sync_map: update_tracker.clone(),
 					sync_ack_map: update_tracker.clone(),
 				},
-				trackers_hash: [0u8; 32].into(),
 				staging: Lww::raw(previous.version, staging),
-				staging_hash: [0u8; 32].into(),
-			};
-			ret.update_hashes();
-			ret
+			}
 		}
 	}
 }
@@ -382,14 +370,14 @@ impl core::str::FromStr for ZoneRedundancy {
 impl UpdateTracker {
 	fn merge(&mut self, other: &UpdateTracker) -> bool {
 		let mut changed = false;
-		for (k, v) in other.values.iter() {
-			if let Some(v_mut) = self.values.get_mut(k) {
+		for (k, v) in other.0.iter() {
+			if let Some(v_mut) = self.0.get_mut(k) {
 				if *v > *v_mut {
 					*v_mut = *v;
 					changed = true;
 				}
 			} else {
-				self.values.insert(*k, *v);
+				self.0.insert(*k, *v);
 				changed = true;
 			}
 		}
@@ -397,23 +385,23 @@ impl UpdateTracker {
 	}
 
 	pub(crate) fn set_max(&mut self, peer: Uuid, value: u64) -> bool {
-		match self.values.get_mut(&peer) {
+		match self.0.get_mut(&peer) {
 			Some(e) if *e < value => {
 				*e = value;
 				true
 			}
 			None => {
-				self.values.insert(peer, value);
+				self.0.insert(peer, value);
 				true
 			}
 			_ => false,
 		}
 	}
 
-	fn update_min(&mut self, storage_nodes: &[Uuid], min_version: u64) {
-		self.current_min = storage_nodes
+	pub(crate) fn min(&self, storage_nodes: &[Uuid], min_version: u64) -> u64 {
+		storage_nodes
 			.iter()
-			.map(|x| self.values.get(x).copied().unwrap_or(min_version))
+			.map(|x| self.0.get(x).copied().unwrap_or(min_version))
 			.min()
 			.unwrap_or(min_version)
 	}
@@ -426,10 +414,4 @@ impl UpdateTrackers {
 		let c3 = self.sync_ack_map.merge(&other.sync_ack_map);
 		c1 || c2 || c3
 	}
-
-	pub(crate) fn update_min(&mut self, storage_nodes: &[Uuid], min_version: u64) {
-		self.ack_map.update_min(&storage_nodes, min_version);
-		self.sync_map.update_min(&storage_nodes, min_version);
-		self.sync_ack_map.update_min(&storage_nodes, min_version);
-	}
 }
diff --git a/src/rpc/rpc_helper.rs b/src/rpc/rpc_helper.rs
index 1bad495b..e269ddaa 100644
--- a/src/rpc/rpc_helper.rs
+++ b/src/rpc/rpc_helper.rs
@@ -26,7 +26,7 @@ use garage_util::data::*;
 use garage_util::error::Error;
 use garage_util::metrics::RecordDuration;
 
-use crate::layout::LayoutHistory;
+use crate::layout::LayoutHelper;
 use crate::metrics::RpcMetrics;
 
 // Default RPC timeout = 5 minutes
@@ -90,7 +90,7 @@ pub struct RpcHelper(Arc<RpcHelperInner>);
 struct RpcHelperInner {
 	our_node_id: Uuid,
 	fullmesh: Arc<FullMeshPeeringStrategy>,
-	layout: Arc<RwLock<LayoutHistory>>,
+	layout: Arc<RwLock<LayoutHelper>>,
 	metrics: RpcMetrics,
 	rpc_timeout: Duration,
 }
@@ -99,7 +99,7 @@ impl RpcHelper {
 	pub(crate) fn new(
 		our_node_id: Uuid,
 		fullmesh: Arc<FullMeshPeeringStrategy>,
-		layout: Arc<RwLock<LayoutHistory>>,
+		layout: Arc<RwLock<LayoutHelper>>,
 		rpc_timeout: Option<Duration>,
 	) -> Self {
 		let metrics = RpcMetrics::new();
diff --git a/src/rpc/system.rs b/src/rpc/system.rs
index 31d78bf6..d74dc2a1 100644
--- a/src/rpc/system.rs
+++ b/src/rpc/system.rs
@@ -34,7 +34,7 @@ use crate::consul::ConsulDiscovery;
 #[cfg(feature = "kubernetes-discovery")]
 use crate::kubernetes::*;
 use crate::layout::manager::{LayoutManager, LayoutStatus};
-use crate::layout::{self, LayoutHistory, NodeRoleV};
+use crate::layout::{self, LayoutHelper, LayoutHistory, NodeRoleV};
 use crate::replication_mode::*;
 use crate::rpc_helper::*;
 
@@ -350,7 +350,7 @@ impl System {
 
 	// ---- Public utilities / accessors ----
 
-	pub fn cluster_layout(&self) -> RwLockReadGuard<'_, LayoutHistory> {
+	pub fn cluster_layout(&self) -> RwLockReadGuard<'_, LayoutHelper> {
 		self.layout_manager.layout()
 	}