Improve freezer DB efficiency with periodic restore points (#649)

* Draft of checkpoint freezer DB

* Fix bugs

* Adjust root iterators for checkpoint database

* Fix freezer state lookups with no slot hint

* Fix split comment

* Use "restore point" to refer to frozen states

* Resolve some FIXMEs

* Configurable slots per restore point

* Document new freezer DB functions

* Fix up StoreConfig

* Fix new test for merge

* Document SPRP default CLI flag, clarify tests

beacon_node/store/src/config.rs

@@ -0,0 +1,32 @@
+use serde_derive::{Deserialize, Serialize};
+use std::path::PathBuf;
+use types::{EthSpec, MinimalEthSpec};
+
+/// Default directory name for the freezer database under the top-level data dir.
+const DEFAULT_FREEZER_DB_DIR: &str = "freezer_db";
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct StoreConfig {
+    /// Name of the directory inside the data directory where the main "hot" DB is located.
+    pub db_name: String,
+    /// Path where the freezer database will be located.
+    pub freezer_db_path: Option<PathBuf>,
+    /// Number of slots to wait between storing restore points in the freezer database.
+    pub slots_per_restore_point: u64,
+}
+
+impl Default for StoreConfig {
+    fn default() -> Self {
+        Self {
+            db_name: "chain_db".to_string(),
+            freezer_db_path: None,
+            slots_per_restore_point: MinimalEthSpec::slots_per_historical_root() as u64,
+        }
+    }
+}
+
+impl StoreConfig {
+    pub fn default_freezer_db_dir(&self) -> &'static str {
+        DEFAULT_FREEZER_DB_DIR
+    }
+}

beacon_node/store/src/hot_cold_store.rs

@@ -1,30 +1,43 @@
 use crate::chunked_vector::{
     store_updated_vector, BlockRoots, HistoricalRoots, RandaoMixes, StateRoots,
 };
-use crate::iter::StateRootsIterator;
+use crate::iter::{ParentRootBlockIterator, StateRootsIterator};
 use crate::{
     leveldb_store::LevelDB, DBColumn, Error, PartialBeaconState, SimpleStoreItem, Store, StoreItem,
 };
 use parking_lot::RwLock;
-use slog::{info, trace, Logger};
+use slog::{debug, trace, warn, Logger};
 use ssz::{Decode, Encode};
+use ssz_derive::{Decode, Encode};
+use state_processing::{
+    per_block_processing, per_slot_processing, BlockProcessingError, BlockSignatureStrategy,
+    SlotProcessingError,
+};
 use std::convert::TryInto;
 use std::path::Path;
 use std::sync::Arc;
 use types::*;
 
-/// 32-byte key for accessing the `split_slot` of the freezer DB.
-pub const SPLIT_SLOT_DB_KEY: &str = "FREEZERDBSPLITSLOTFREEZERDBSPLIT";
+/// 32-byte key for accessing the `split` of the freezer DB.
+pub const SPLIT_DB_KEY: &str = "FREEZERDBSPLITFREEZERDBSPLITFREE";
 
+/// On-disk database that stores finalized states efficiently.
+///
+/// Stores vector fields like the `block_roots` and `state_roots` separately, and only stores
+/// intermittent "restore point" states pre-finalization.
 pub struct HotColdDB {
-    /// The slot before which all data is stored in the cold database.
+    /// The slot and state root at the point where the database is split between hot and cold.
     ///
-    /// Data for slots less than `split_slot` is in the cold DB, while data for slots
-    /// greater than or equal is in the hot DB.
-    split_slot: RwLock<Slot>,
+    /// States with slots less than `split.slot` are in the cold DB, while states with slots
+    /// greater than or equal are in the hot DB.
+    split: RwLock<Split>,
+    /// Number of slots per restore point state in the freezer database.
+    slots_per_restore_point: u64,
     /// Cold database containing compact historical data.
     cold_db: LevelDB,
     /// Hot database containing duplicated but quick-to-access recent data.
+    ///
+    /// The hot database also contains all blocks.
     hot_db: LevelDB,
     /// Chain spec.
     spec: ChainSpec,
@@ -38,6 +51,24 @@ pub enum HotColdDbError {
         current_split_slot: Slot,
         proposed_split_slot: Slot,
     },
+    MissingStateToFreeze(Hash256),
+    MissingRestorePointHash(u64),
+    MissingRestorePoint(Hash256),
+    MissingStateSlot(Hash256),
+    MissingSplitState(Hash256, Slot),
+    RestorePointDecodeError(ssz::DecodeError),
+    RestorePointReplayFailure {
+        expected_state_root: Hash256,
+        observed_state_root: Hash256,
+    },
+    BlockReplayBeaconError(BeaconStateError),
+    BlockReplaySlotError(SlotProcessingError),
+    BlockReplayBlockError(BlockProcessingError),
+    InvalidSlotsPerRestorePoint {
+        slots_per_restore_point: u64,
+        slots_per_historical_root: u64,
+    },
+    RestorePointBlockHashError(BeaconStateError),
 }
 
 impl Store for HotColdDB {
@@ -79,24 +110,31 @@ impl Store for HotColdDB {
     ) -> Result<Option<BeaconState<E>>, Error> {
         if let Some(slot) = slot {
             if slot < self.get_split_slot() {
-                self.load_archive_state(state_root)
+                self.load_archive_state(state_root, slot).map(Some)
             } else {
                 self.hot_db.get_state(state_root, None)
             }
         } else {
             match self.hot_db.get_state(state_root, None)? {
                 Some(state) => Ok(Some(state)),
-                None => self.load_archive_state(state_root),
+                None => {
+                    // Look-up the state in the freezer DB. We don't know the slot, so we must
+                    // look it up separately and then use it to reconstruct the state from a
+                    // restore point.
+                    let slot = self.load_state_slot(state_root)?;
+                    self.load_archive_state(state_root, slot).map(Some)
+                }
             }
         }
     }
 
+    /// Advance the split point of the store, moving new finalized states to the freezer.
     fn freeze_to_state<E: EthSpec>(
         store: Arc<Self>,
-        _frozen_head_root: Hash256,
+        frozen_head_root: Hash256,
         frozen_head: &BeaconState<E>,
     ) -> Result<(), Error> {
-        info!(
+        debug!(
             store.log,
             "Freezer migration started";
             "slot" => frozen_head.slot
@@ -119,25 +157,28 @@ impl Store for HotColdDB {
         for (state_root, slot) in
             state_root_iter.take_while(|&(_, slot)| slot >= current_split_slot)
         {
-            trace!(store.log, "Freezing";
-                   "slot" => slot,
-                   "state_root" => format!("{}", state_root));
+            if slot % store.slots_per_restore_point == 0 {
+                let state: BeaconState<E> = store
+                    .hot_db
+                    .get_state(&state_root, None)?
+                    .ok_or_else(|| HotColdDbError::MissingStateToFreeze(state_root))?;
 
-            let state: BeaconState<E> = match store.hot_db.get_state(&state_root, None)? {
-                Some(s) => s,
-                // If there's no state it could be a skip slot, which is fine, our job is just
-                // to move everything that was in the hot DB to the cold.
-                None => continue,
-            };
+                store.store_archive_state(&state_root, &state)?;
+            }
+
+            // Store a pointer from this state root to its slot, so we can later reconstruct states
+            // from their state root alone.
+            store.store_state_slot(&state_root, slot)?;
 
             to_delete.push(state_root);
-
-            store.store_archive_state(&state_root, &state)?;
         }
 
         // 2. Update the split slot
-        *store.split_slot.write() = frozen_head.slot;
-        store.store_split_slot()?;
+        *store.split.write() = Split {
+            slot: frozen_head.slot,
+            state_root: frozen_head_root,
+        };
+        store.store_split()?;
 
         // 3. Delete from the hot DB
         for state_root in to_delete {
@@ -146,7 +187,7 @@ impl Store for HotColdDB {
                 .key_delete(DBColumn::BeaconState.into(), state_root.as_bytes())?;
         }
 
-        info!(
+        debug!(
             store.log,
             "Freezer migration complete";
             "slot" => frozen_head.slot
@@ -157,38 +198,60 @@ impl Store for HotColdDB {
 }
 
 impl HotColdDB {
-    pub fn open(
+    /// Open a new or existing database, with the given paths to the hot and cold DBs.
+    ///
+    /// The `slots_per_restore_point` parameter must be a divisor of `SLOTS_PER_HISTORICAL_ROOT`.
+    pub fn open<E: EthSpec>(
         hot_path: &Path,
         cold_path: &Path,
+        slots_per_restore_point: u64,
         spec: ChainSpec,
         log: Logger,
     ) -> Result<Self, Error> {
+        Self::verify_slots_per_restore_point::<E>(slots_per_restore_point)?;
+
         let db = HotColdDB {
-            split_slot: RwLock::new(Slot::new(0)),
+            split: RwLock::new(Split::default()),
+            slots_per_restore_point,
             cold_db: LevelDB::open(cold_path)?,
             hot_db: LevelDB::open(hot_path)?,
             spec,
             log,
         };
+
         // Load the previous split slot from the database (if any). This ensures we can
         // stop and restart correctly.
-        if let Some(split_slot) = db.load_split_slot()? {
-            *db.split_slot.write() = split_slot;
+        if let Some(split) = db.load_split()? {
+            *db.split.write() = split;
         }
         Ok(db)
     }
 
+    /// Store a pre-finalization state in the freezer database.
+    ///
+    /// Will return an error if the state does not lie on a restore point boundary.
     pub fn store_archive_state<E: EthSpec>(
         &self,
         state_root: &Hash256,
         state: &BeaconState<E>,
     ) -> Result<(), Error> {
+        if state.slot % self.slots_per_restore_point != 0 {
+            warn!(
+                self.log,
+                "Not storing non-restore point state in freezer";
+                "slot" => state.slot.as_u64(),
+                "state_root" => format!("{:?}", state_root)
+            );
+            return Ok(());
+        }
+
         trace!(
             self.log,
-            "Freezing state";
-            "slot" => state.slot.as_u64(),
+            "Creating restore point";
+            "slot" => state.slot,
             "state_root" => format!("{:?}", state_root)
         );
+
         // 1. Convert to PartialBeaconState and store that in the DB.
         let partial_state = PartialBeaconState::from_state_forgetful(state);
         partial_state.db_put(&self.cold_db, state_root)?;
@@ -200,17 +263,35 @@ impl HotColdDB {
         store_updated_vector(HistoricalRoots, db, state, &self.spec)?;
         store_updated_vector(RandaoMixes, db, state, &self.spec)?;
 
+        // 3. Store restore point.
+        let restore_point_index = state.slot.as_u64() / self.slots_per_restore_point;
+        self.store_restore_point_hash(restore_point_index, *state_root)?;
+
         Ok(())
     }
 
+    /// Load a pre-finalization state from the freezer database.
+    ///
+    /// Will reconstruct the state if it lies between restore points.
     pub fn load_archive_state<E: EthSpec>(
         &self,
         state_root: &Hash256,
-    ) -> Result<Option<BeaconState<E>>, Error> {
-        let mut partial_state = match PartialBeaconState::db_get(&self.cold_db, state_root)? {
-            Some(s) => s,
-            None => return Ok(None),
-        };
+        slot: Slot,
+    ) -> Result<BeaconState<E>, Error> {
+        if slot % self.slots_per_restore_point == 0 {
+            self.load_restore_point(state_root)
+        } else {
+            self.load_intermediate_state(state_root, slot)
+        }
+    }
+
+    /// Load a restore point state by its `state_root`.
+    fn load_restore_point<E: EthSpec>(
+        &self,
+        state_root: &Hash256,
+    ) -> Result<BeaconState<E>, Error> {
+        let mut partial_state = PartialBeaconState::db_get(&self.cold_db, state_root)?
+            .ok_or_else(|| HotColdDbError::MissingRestorePoint(*state_root))?;
 
         // Fill in the fields of the partial state.
         partial_state.load_block_roots(&self.cold_db, &self.spec)?;
@@ -218,43 +299,277 @@ impl HotColdDB {
         partial_state.load_historical_roots(&self.cold_db, &self.spec)?;
         partial_state.load_randao_mixes(&self.cold_db, &self.spec)?;
 
-        let state: BeaconState<E> = partial_state.try_into()?;
-
-        Ok(Some(state))
+        Ok(partial_state.try_into()?)
     }
 
+    /// Load a restore point state by its `restore_point_index`.
+    fn load_restore_point_by_index<E: EthSpec>(
+        &self,
+        restore_point_index: u64,
+    ) -> Result<BeaconState<E>, Error> {
+        let state_root = self.load_restore_point_hash(restore_point_index)?;
+        self.load_restore_point(&state_root)
+    }
+
+    /// Load a state that lies between restore points.
+    fn load_intermediate_state<E: EthSpec>(
+        &self,
+        state_root: &Hash256,
+        slot: Slot,
+    ) -> Result<BeaconState<E>, Error> {
+        // 1. Load the restore points either side of the intermediate state.
+        let low_restore_point_idx = slot.as_u64() / self.slots_per_restore_point;
+        let high_restore_point_idx = low_restore_point_idx + 1;
+
+        // Acquire the read lock, so that the split can't change while this is happening.
+        let split = self.split.read();
+
+        let low_restore_point = self.load_restore_point_by_index(low_restore_point_idx)?;
+        // If the slot of the high point lies outside the freezer, use the split state
+        // as the upper restore point.
+        let high_restore_point = if high_restore_point_idx * self.slots_per_restore_point
+            >= split.slot.as_u64()
+        {
+            self.get_state::<E>(&split.state_root, Some(split.slot))?
+                .ok_or_else(|| HotColdDbError::MissingSplitState(split.state_root, split.slot))?
+        } else {
+            self.load_restore_point_by_index(high_restore_point_idx)?
+        };
+
+        // 2. Load the blocks from the high restore point back to the low restore point.
+        let blocks = self.load_blocks_to_replay(
+            low_restore_point.slot,
+            slot,
+            self.get_high_restore_point_block_root(&high_restore_point, slot)?,
+        )?;
+
+        // 3. Replay the blocks on top of the low restore point.
+        let mut state = self.replay_blocks(low_restore_point, blocks, slot)?;
+
+        // 4. Check that the state root is correct (should be quick with the cache already built).
+        // TODO: we could optimise out *all* the tree hashing when replaying blocks,
+        // in which case we could also drop this check.
+        let observed_state_root = state.update_tree_hash_cache()?;
+
+        if observed_state_root == *state_root {
+            Ok(state)
+        } else {
+            Err(HotColdDbError::RestorePointReplayFailure {
+                expected_state_root: *state_root,
+                observed_state_root,
+            }
+            .into())
+        }
+    }
+
+    /// Get a suitable block root for backtracking from `high_restore_point` to the state at `slot`.
+    ///
+    /// Defaults to the block root for `slot`, which *should* be in range.
+    fn get_high_restore_point_block_root<E: EthSpec>(
+        &self,
+        high_restore_point: &BeaconState<E>,
+        slot: Slot,
+    ) -> Result<Hash256, HotColdDbError> {
+        high_restore_point
+            .get_block_root(slot)
+            .or_else(|_| high_restore_point.get_oldest_block_root())
+            .map(|x| *x)
+            .map_err(HotColdDbError::RestorePointBlockHashError)
+    }
+
+    /// Load the blocks between `start_slot` and `end_slot` by backtracking from `end_block_hash`.
+    ///
+    /// Blocks are returned in slot-ascending order, suitable for replaying on a state with slot
+    /// equal to `start_slot`, to reach a state with slot equal to `end_slot`.
+    fn load_blocks_to_replay<E: EthSpec>(
+        &self,
+        start_slot: Slot,
+        end_slot: Slot,
+        end_block_hash: Hash256,
+    ) -> Result<Vec<BeaconBlock<E>>, Error> {
+        let mut blocks = ParentRootBlockIterator::new(self, end_block_hash)
+            // Include the block at the end slot (if any), it needs to be
+            // replayed in order to construct the canonical state at `end_slot`.
+            .filter(|block| block.slot <= end_slot)
+            // Exclude the block at the start slot (if any), because it has already
+            // been applied to the starting state.
+            .take_while(|block| block.slot > start_slot)
+            .collect::<Vec<_>>();
+        blocks.reverse();
+        Ok(blocks)
+    }
+
+    /// Replay `blocks` on top of `state` until `target_slot` is reached.
+    ///
+    /// Will skip slots as necessary.
+    fn replay_blocks<E: EthSpec>(
+        &self,
+        mut state: BeaconState<E>,
+        blocks: Vec<BeaconBlock<E>>,
+        target_slot: Slot,
+    ) -> Result<BeaconState<E>, Error> {
+        state
+            .build_all_caches(&self.spec)
+            .map_err(HotColdDbError::BlockReplayBeaconError)?;
+
+        for block in blocks {
+            while state.slot < block.slot {
+                per_slot_processing(&mut state, &self.spec)
+                    .map_err(HotColdDbError::BlockReplaySlotError)?;
+            }
+            per_block_processing(
+                &mut state,
+                &block,
+                None,
+                BlockSignatureStrategy::NoVerification,
+                &self.spec,
+            )
+            .map_err(HotColdDbError::BlockReplayBlockError)?;
+        }
+
+        while state.slot < target_slot {
+            per_slot_processing(&mut state, &self.spec)
+                .map_err(HotColdDbError::BlockReplaySlotError)?;
+        }
+
+        Ok(state)
+    }
+
+    /// Fetch a copy of the current split slot from memory.
     pub fn get_split_slot(&self) -> Slot {
-        *self.split_slot.read()
+        self.split.read().slot
     }
 
-    fn load_split_slot(&self) -> Result<Option<Slot>, Error> {
-        let key = Hash256::from_slice(SPLIT_SLOT_DB_KEY.as_bytes());
-        let split_slot: Option<SplitSlot> = self.hot_db.get(&key)?;
-        Ok(split_slot.map(|s| Slot::new(s.0)))
+    /// Load the split point from disk.
+    fn load_split(&self) -> Result<Option<Split>, Error> {
+        let key = Hash256::from_slice(SPLIT_DB_KEY.as_bytes());
+        let split: Option<Split> = self.hot_db.get(&key)?;
+        Ok(split)
     }
 
-    fn store_split_slot(&self) -> Result<(), Error> {
-        let key = Hash256::from_slice(SPLIT_SLOT_DB_KEY.as_bytes());
-        self.hot_db
-            .put(&key, &SplitSlot(self.get_split_slot().as_u64()))?;
+    /// Store the split point on disk.
+    fn store_split(&self) -> Result<(), Error> {
+        let key = Hash256::from_slice(SPLIT_DB_KEY.as_bytes());
+        self.hot_db.put(&key, &*self.split.read())?;
         Ok(())
     }
+
+    /// Load the state root of a restore point.
+    fn load_restore_point_hash(&self, restore_point_index: u64) -> Result<Hash256, Error> {
+        let key = Self::restore_point_key(restore_point_index);
+        RestorePointHash::db_get(&self.cold_db, &key)?
+            .map(|r| r.state_root)
+            .ok_or(HotColdDbError::MissingRestorePointHash(restore_point_index).into())
+    }
+
+    /// Store the state root of a restore point.
+    fn store_restore_point_hash(
+        &self,
+        restore_point_index: u64,
+        state_root: Hash256,
+    ) -> Result<(), Error> {
+        let key = Self::restore_point_key(restore_point_index);
+        RestorePointHash { state_root }
+            .db_put(&self.cold_db, &key)
+            .map_err(Into::into)
+    }
+
+    /// Convert a `restore_point_index` into a database key.
+    fn restore_point_key(restore_point_index: u64) -> Hash256 {
+        Hash256::from_low_u64_be(restore_point_index)
+    }
+
+    /// Load a frozen state's slot, given its root.
+    fn load_state_slot(&self, state_root: &Hash256) -> Result<Slot, Error> {
+        StateSlot::db_get(&self.cold_db, state_root)?
+            .map(|s| s.slot)
+            .ok_or_else(|| HotColdDbError::MissingStateSlot(*state_root).into())
+    }
+
+    /// Store the slot of a frozen state.
+    fn store_state_slot(&self, state_root: &Hash256, slot: Slot) -> Result<(), Error> {
+        StateSlot { slot }
+            .db_put(&self.cold_db, state_root)
+            .map_err(Into::into)
+    }
+
+    /// Check that the restore point frequency is a divisor of the slots per historical root.
+    ///
+    /// This ensures that we have at least one restore point within range of our state
+    /// root history when iterating backwards (and allows for more frequent restore points if
+    /// desired).
+    fn verify_slots_per_restore_point<E: EthSpec>(
+        slots_per_restore_point: u64,
+    ) -> Result<(), HotColdDbError> {
+        let slots_per_historical_root = E::SlotsPerHistoricalRoot::to_u64();
+        if slots_per_restore_point > 0 && slots_per_historical_root % slots_per_restore_point == 0 {
+            Ok(())
+        } else {
+            Err(HotColdDbError::InvalidSlotsPerRestorePoint {
+                slots_per_restore_point,
+                slots_per_historical_root,
+            })
+        }
+    }
 }
 
-/// Struct for storing the split slot in the database.
-#[derive(Clone, Copy)]
-struct SplitSlot(u64);
+/// Struct for storing the split slot and state root in the database.
+#[derive(Clone, Copy, Default, Encode, Decode)]
+struct Split {
+    slot: Slot,
+    state_root: Hash256,
+}
 
-impl SimpleStoreItem for SplitSlot {
+impl SimpleStoreItem for Split {
     fn db_column() -> DBColumn {
         DBColumn::BeaconMeta
     }
 
     fn as_store_bytes(&self) -> Vec<u8> {
-        self.0.as_ssz_bytes()
+        self.as_ssz_bytes()
     }
 
     fn from_store_bytes(bytes: &[u8]) -> Result<Self, Error> {
-        Ok(SplitSlot(u64::from_ssz_bytes(bytes)?))
+        Ok(Self::from_ssz_bytes(bytes)?)
+    }
+}
+
+/// Struct for storing the slot of a state root in the database.
+#[derive(Clone, Copy, Default, Encode, Decode)]
+struct StateSlot {
+    slot: Slot,
+}
+
+impl SimpleStoreItem for StateSlot {
+    fn db_column() -> DBColumn {
+        DBColumn::BeaconStateSlot
+    }
+
+    fn as_store_bytes(&self) -> Vec<u8> {
+        self.as_ssz_bytes()
+    }
+
+    fn from_store_bytes(bytes: &[u8]) -> Result<Self, Error> {
+        Ok(Self::from_ssz_bytes(bytes)?)
+    }
+}
+
+/// Struct for storing the state root of a restore point in the database.
+#[derive(Clone, Copy, Default, Encode, Decode)]
+struct RestorePointHash {
+    state_root: Hash256,
+}
+
+impl SimpleStoreItem for RestorePointHash {
+    fn db_column() -> DBColumn {
+        DBColumn::BeaconRestorePoint
+    }
+
+    fn as_store_bytes(&self) -> Vec<u8> {
+        self.as_ssz_bytes()
+    }
+
+    fn from_store_bytes(bytes: &[u8]) -> Result<Self, Error> {
+        Ok(Self::from_ssz_bytes(bytes)?)
     }
 }

beacon_node/store/src/iter.rs

@@ -1,7 +1,10 @@
 use crate::Store;
 use std::borrow::Cow;
+use std::marker::PhantomData;
 use std::sync::Arc;
-use types::{BeaconBlock, BeaconState, BeaconStateError, EthSpec, Hash256, Slot};
+use types::{
+    typenum::Unsigned, BeaconBlock, BeaconState, BeaconStateError, EthSpec, Hash256, Slot,
+};
 
 /// Implemented for types that have ancestors (e.g., blocks, states) that may be iterated over.
 ///
@@ -80,12 +83,9 @@ impl<'a, T: EthSpec, U: Store> Iterator for StateRootsIterator<'a, T, U> {
         match self.beacon_state.get_state_root(self.slot) {
             Ok(root) => Some((*root, self.slot)),
             Err(BeaconStateError::SlotOutOfBounds) => {
-                // Read a `BeaconState` from the store that has access to prior historical root.
-                let beacon_state: BeaconState<T> = {
-                    let new_state_root = self.beacon_state.get_oldest_state_root().ok()?;
-
-                    self.store.get_state(&new_state_root, None).ok()?
-                }?;
+                // Read a `BeaconState` from the store that has access to prior historical roots.
+                let beacon_state =
+                    next_historical_root_backtrack_state(&*self.store, &self.beacon_state)?;
 
                 self.beacon_state = Cow::Owned(beacon_state);
 
@@ -98,6 +98,39 @@ impl<'a, T: EthSpec, U: Store> Iterator for StateRootsIterator<'a, T, U> {
     }
 }
 
+/// Block iterator that uses the `parent_root` of each block to backtrack.
+pub struct ParentRootBlockIterator<'a, E: EthSpec, S: Store> {
+    store: &'a S,
+    next_block_root: Hash256,
+    _phantom: PhantomData<E>,
+}
+
+impl<'a, E: EthSpec, S: Store> ParentRootBlockIterator<'a, E, S> {
+    pub fn new(store: &'a S, start_block_root: Hash256) -> Self {
+        Self {
+            store,
+            next_block_root: start_block_root,
+            _phantom: PhantomData,
+        }
+    }
+}
+
+impl<'a, E: EthSpec, S: Store> Iterator for ParentRootBlockIterator<'a, E, S> {
+    type Item = BeaconBlock<E>;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        // Stop once we reach the zero parent, otherwise we'll keep returning the genesis
+        // block forever.
+        if self.next_block_root.is_zero() {
+            None
+        } else {
+            let block: BeaconBlock<E> = self.store.get(&self.next_block_root).ok()??;
+            self.next_block_root = block.parent_root;
+            Some(block)
+        }
+    }
+}
+
 #[derive(Clone)]
 /// Extends `BlockRootsIterator`, returning `BeaconBlock` instances, instead of their roots.
 pub struct BlockIterator<'a, T: EthSpec, U> {
@@ -177,7 +210,7 @@ impl<'a, T: EthSpec, U: Store> Iterator for BlockRootsIterator<'a, T, U> {
     type Item = (Hash256, Slot);
 
     fn next(&mut self) -> Option<Self::Item> {
-        if (self.slot == 0) || (self.slot > self.beacon_state.slot) {
+        if self.slot == 0 || self.slot > self.beacon_state.slot {
             return None;
         }
 
@@ -186,13 +219,9 @@ impl<'a, T: EthSpec, U: Store> Iterator for BlockRootsIterator<'a, T, U> {
         match self.beacon_state.get_block_root(self.slot) {
             Ok(root) => Some((*root, self.slot)),
             Err(BeaconStateError::SlotOutOfBounds) => {
-                // Read a `BeaconState` from the store that has access to prior historical root.
-                let beacon_state: BeaconState<T> = {
-                    // Load the earliest state from disk.
-                    let new_state_root = self.beacon_state.get_oldest_state_root().ok()?;
-
-                    self.store.get_state(&new_state_root, None).ok()?
-                }?;
+                // Read a `BeaconState` from the store that has access to prior historical roots.
+                let beacon_state =
+                    next_historical_root_backtrack_state(&*self.store, &self.beacon_state)?;
 
                 self.beacon_state = Cow::Owned(beacon_state);
 
@@ -205,6 +234,26 @@ impl<'a, T: EthSpec, U: Store> Iterator for BlockRootsIterator<'a, T, U> {
     }
 }
 
+/// Fetch the next state to use whilst backtracking in `*RootsIterator`.
+fn next_historical_root_backtrack_state<E: EthSpec, S: Store>(
+    store: &S,
+    current_state: &BeaconState<E>,
+) -> Option<BeaconState<E>> {
+    // For compatibility with the freezer database's restore points, we load a state at
+    // a restore point slot (thus avoiding replaying blocks). In the case where we're
+    // not frozen, this just means we might not jump back by the maximum amount on
+    // our first jump (i.e. at most 1 extra state load).
+    let new_state_slot = slot_of_prev_restore_point::<E>(current_state.slot);
+    let new_state_root = current_state.get_state_root(new_state_slot).ok()?;
+    store.get_state(new_state_root, Some(new_state_slot)).ok()?
+}
+
+/// Compute the slot of the last guaranteed restore point in the freezer database.
+fn slot_of_prev_restore_point<E: EthSpec>(current_slot: Slot) -> Slot {
+    let slots_per_historical_root = E::SlotsPerHistoricalRoot::to_u64();
+    (current_slot - 1) / slots_per_historical_root * slots_per_historical_root
+}
+
 pub type ReverseBlockRootIterator<'a, E, S> =
     ReverseHashAndSlotIterator<BlockRootsIterator<'a, E, S>>;
 pub type ReverseStateRootIterator<'a, E, S> =

beacon_node/store/src/lib.rs

@@ -12,6 +12,7 @@ extern crate lazy_static;
 
 mod block_at_slot;
 pub mod chunked_vector;
+pub mod config;
 mod errors;
 mod hot_cold_store;
 mod impls;
@@ -25,6 +26,7 @@ pub mod migrate;
 
 use std::sync::Arc;
 
+pub use self::config::StoreConfig;
 pub use self::hot_cold_store::HotColdDB as DiskStore;
 pub use self::leveldb_store::LevelDB as SimpleDiskStore;
 pub use self::memory_store::MemoryStore;
@@ -117,6 +119,10 @@ pub enum DBColumn {
     BeaconBlock,
     BeaconState,
     BeaconChain,
+    /// For the table mapping restore point numbers to state roots.
+    BeaconRestorePoint,
+    /// For the mapping from state roots to their slots.
+    BeaconStateSlot,
     BeaconBlockRoots,
     BeaconStateRoots,
     BeaconHistoricalRoots,
@@ -131,6 +137,8 @@ impl Into<&'static str> for DBColumn {
             DBColumn::BeaconBlock => "blk",
             DBColumn::BeaconState => "ste",
             DBColumn::BeaconChain => "bch",
+            DBColumn::BeaconRestorePoint => "brp",
+            DBColumn::BeaconStateSlot => "bss",
             DBColumn::BeaconBlockRoots => "bbr",
             DBColumn::BeaconStateRoots => "bsr",
             DBColumn::BeaconHistoricalRoots => "bhr",
@@ -263,9 +271,17 @@ mod tests {
 
         let hot_dir = tempdir().unwrap();
         let cold_dir = tempdir().unwrap();
+        let slots_per_restore_point = MinimalEthSpec::slots_per_historical_root() as u64;
         let spec = MinimalEthSpec::default_spec();
         let log = NullLoggerBuilder.build().unwrap();
-        let store = DiskStore::open(&hot_dir.path(), &cold_dir.path(), spec, log).unwrap();
+        let store = DiskStore::open::<MinimalEthSpec>(
+            &hot_dir.path(),
+            &cold_dir.path(),
+            slots_per_restore_point,
+            spec,
+            log,
+        )
+        .unwrap();
 
         test_impl(store);
     }