Database Configuration

Lighthouse uses an efficient "split" database schema, whereby finalized states are stored separately from recent, unfinalized states. We refer to the portion of the database storing finalized states as the freezer or cold DB, and the portion storing recent states as the hot DB.

In both the hot and cold DBs, full BeaconState data structures are only stored periodically, and intermediate states are reconstructed by quickly replaying blocks on top of the nearest state. For example, to fetch a state at slot 7 the database might fetch a full state from slot 0, and replay blocks from slots 1-7 while omitting redundant signature checks and Merkle root calculations. The full states upon which blocks are replayed are referred to as restore points in the case of the freezer DB, and epoch boundary states in the case of the hot DB.

The frequency at which the hot database stores full BeaconStates is fixed to one-state-per-epoch in order to keep loads of recent states performant. For the freezer DB, the frequency is configurable via the --slots-per-restore-point CLI flag, which is the topic of the next section.

Freezer DB Space-time Trade-offs

Frequent restore points use more disk space but accelerate the loading of historical states. Conversely, infrequent restore points use much less space, but cause the loading of historical states to slow down dramatically. A lower slots per restore point value (SPRP) corresponds to more frequent restore points, while a higher SPRP corresponds to less frequent. The table below shows some example values.

As you can see, it's a high-stakes trade-off! The relationships to disk usage and historical state load time are both linear – doubling SPRP halves disk usage and doubles load time. The minimum SPRP is 32, and the maximum is 8192.

The values shown in the table are approximate, calculated using a simple heuristic: each BeaconState consumes around 5MB of disk space, and each block replayed takes around 3ms. The Yearly Disk Usage column shows the approx size of the freezer DB alone (hot DB not included), and the Load Historical State time is the worst-case load time for a state in the last slot of an epoch.

To configure your Lighthouse node's database with a non-default SPRP, run your Beacon Node with the --slots-per-restore-point flag:

lighthouse beacon_node --slots-per-restore-point 8192

Glossary

• Freezer DB: part of the database storing finalized states. States are stored in a sparser format, and usually less frequently than in the hot DB.
• Cold DB: see Freezer DB.
• Hot DB: part of the database storing recent states, all blocks, and other runtime data. Full states are stored every epoch.
• Restore Point: a full BeaconState stored periodically in the freezer DB.
• Slots Per Restore Point (SPRP): the number of slots between restore points in the freezer DB.
• Split Slot: the slot at which states are divided between the hot and the cold DBs. All states from slots less than the split slot are in the freezer, while all states with slots greater than or equal to the split slot are in the hot DB.