Conserve disk space by raising default SPRP (#3137)

## Proposed Changes

Increase the default `--slots-per-restore-point` to 8192 for a 4x reduction in freezer DB disk usage.

Existing nodes that use the previous default of 2048 will be left unchanged. Newly synced nodes (with or without checkpoint sync) will use the new 8192 default. 

Long-term we could do away with the freezer DB entirely for validator-only nodes, but this change is much simpler and grants us some extra space in the short term. We can also roll it out gradually across our nodes by purging databases one by one, while keeping the Ansible config the same.

## Additional Info

We ignore a change from 2048 to 8192 if the user hasn't set the 8192 explicitly. We fire a debug log in the case where we do ignore:

```
DEBG Ignoring slots-per-restore-point config in favour of on-disk value, on_disk: 2048, config: 8192
```

beacon_node/http_api/src/database.rs

@@ -9,10 +9,12 @@ pub fn info<T: BeaconChainTypes>(
 ) -> Result<DatabaseInfo, warp::Rejection> {
     let store = &chain.store;
     let split = store.get_split_info();
+    let config = store.get_config().clone();
     let anchor = store.get_anchor_info();
 
     Ok(DatabaseInfo {
         schema_version: CURRENT_SCHEMA_VERSION.as_u64(),
+        config,
         split,
         anchor,
     })

beacon_node/src/config.rs

@@ -284,7 +284,9 @@ pub fn get_config<E: EthSpec>(
         client_config.freezer_db_path = Some(PathBuf::from(freezer_dir));
     }
 
-    client_config.store.slots_per_restore_point = get_slots_per_restore_point::<E>(cli_args)?;
+    let (sprp, sprp_explicit) = get_slots_per_restore_point::<E>(cli_args)?;
+    client_config.store.slots_per_restore_point = sprp;
+    client_config.store.slots_per_restore_point_set_explicitly = sprp_explicit;
 
     if let Some(block_cache_size) = cli_args.value_of("block-cache-size") {
         client_config.store.block_cache_size = block_cache_size
@@ -813,15 +815,20 @@ pub fn get_data_dir(cli_args: &ArgMatches) -> PathBuf {
 }
 
 /// Get the `slots_per_restore_point` value to use for the database.
-pub fn get_slots_per_restore_point<E: EthSpec>(cli_args: &ArgMatches) -> Result<u64, String> {
+///
+/// Return `(sprp, set_explicitly)` where `set_explicitly` is `true` if the user provided the value.
+pub fn get_slots_per_restore_point<E: EthSpec>(
+    cli_args: &ArgMatches,
+) -> Result<(u64, bool), String> {
     if let Some(slots_per_restore_point) =
         clap_utils::parse_optional(cli_args, "slots-per-restore-point")?
     {
-        Ok(slots_per_restore_point)
+        Ok((slots_per_restore_point, true))
     } else {
-        Ok(std::cmp::min(
+        let default = std::cmp::min(
             E::slots_per_historical_root() as u64,
             store::config::DEFAULT_SLOTS_PER_RESTORE_POINT,
-        ))
+        );
+        Ok((default, false))
     }
 }

beacon_node/store/src/config.rs

@@ -4,7 +4,8 @@ use ssz::{Decode, Encode};
 use ssz_derive::{Decode, Encode};
 use types::{EthSpec, MinimalEthSpec};
 
-pub const DEFAULT_SLOTS_PER_RESTORE_POINT: u64 = 2048;
+pub const PREV_DEFAULT_SLOTS_PER_RESTORE_POINT: u64 = 2048;
+pub const DEFAULT_SLOTS_PER_RESTORE_POINT: u64 = 8192;
 pub const DEFAULT_BLOCK_CACHE_SIZE: usize = 5;
 
 /// Database configuration parameters.
@@ -12,6 +13,8 @@ pub const DEFAULT_BLOCK_CACHE_SIZE: usize = 5;
 pub struct StoreConfig {
     /// Number of slots to wait between storing restore points in the freezer database.
     pub slots_per_restore_point: u64,
+    /// Flag indicating whether the `slots_per_restore_point` was set explicitly by the user.
+    pub slots_per_restore_point_set_explicitly: bool,
     /// Maximum number of blocks to store in the in-memory block cache.
     pub block_cache_size: usize,
     /// Whether to compact the database on initialization.
@@ -36,6 +39,7 @@ impl Default for StoreConfig {
         Self {
             // Safe default for tests, shouldn't ever be read by a CLI node.
             slots_per_restore_point: MinimalEthSpec::slots_per_historical_root() as u64,
+            slots_per_restore_point_set_explicitly: false,
             block_cache_size: DEFAULT_BLOCK_CACHE_SIZE,
             compact_on_init: false,
             compact_on_prune: true,

beacon_node/store/src/hot_cold_store.rs

@@ -1,7 +1,10 @@
 use crate::chunked_vector::{
     store_updated_vector, BlockRoots, HistoricalRoots, RandaoMixes, StateRoots,
 };
-use crate::config::{OnDiskStoreConfig, StoreConfig};
+use crate::config::{
+    OnDiskStoreConfig, StoreConfig, DEFAULT_SLOTS_PER_RESTORE_POINT,
+    PREV_DEFAULT_SLOTS_PER_RESTORE_POINT,
+};
 use crate::forwards_iter::{HybridForwardsBlockRootsIterator, HybridForwardsStateRootsIterator};
 use crate::impls::beacon_state::{get_full_state, store_full_state};
 use crate::iter::{ParentRootBlockIterator, StateRootsIterator};
@@ -150,7 +153,7 @@ impl<E: EthSpec> HotColdDB<E, LevelDB<E>, LevelDB<E>> {
     ) -> Result<Arc<Self>, Error> {
         Self::verify_slots_per_restore_point(config.slots_per_restore_point)?;
 
-        let db = Arc::new(HotColdDB {
+        let mut db = HotColdDB {
             split: RwLock::new(Split::default()),
             anchor_info: RwLock::new(None),
             cold_db: LevelDB::open(cold_path)?,
@@ -160,10 +163,31 @@ impl<E: EthSpec> HotColdDB<E, LevelDB<E>, LevelDB<E>> {
             spec,
             log,
             _phantom: PhantomData,
-        });
+        };
+
+        // Allow the slots-per-restore-point value to stay at the previous default if the config
+        // uses the new default. Don't error on a failed read because the config itself may need
+        // migrating.
+        if let Ok(Some(disk_config)) = db.load_config() {
+            if !db.config.slots_per_restore_point_set_explicitly
+                && disk_config.slots_per_restore_point == PREV_DEFAULT_SLOTS_PER_RESTORE_POINT
+                && db.config.slots_per_restore_point == DEFAULT_SLOTS_PER_RESTORE_POINT
+            {
+                debug!(
+                    db.log,
+                    "Ignoring slots-per-restore-point config in favour of on-disk value";
+                    "config" => db.config.slots_per_restore_point,
+                    "on_disk" => disk_config.slots_per_restore_point,
+                );
+
+                // Mutate the in-memory config so that it's compatible.
+                db.config.slots_per_restore_point = PREV_DEFAULT_SLOTS_PER_RESTORE_POINT;
+            }
+        }
 
         // Ensure that the schema version of the on-disk database matches the software.
         // If the version is mismatched, an automatic migration will be attempted.
+        let db = Arc::new(db);
         if let Some(schema_version) = db.load_schema_version()? {
             debug!(
                 db.log,
@@ -1108,6 +1132,11 @@ impl<E: EthSpec, Hot: ItemStore<E>, Cold: ItemStore<E>> HotColdDB<E, Hot, Cold>
             .map_or(self.spec.genesis_slot, |anchor| anchor.oldest_block_slot)
     }
 
+    /// Return the in-memory configuration used by the database.
+    pub fn get_config(&self) -> &StoreConfig {
+        &self.config
+    }
+
     /// Load previously-stored config from disk.
     fn load_config(&self) -> Result<Option<OnDiskStoreConfig>, Error> {
         self.hot_db.get(&CONFIG_KEY)