# 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 `BeaconState`s 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. | Use Case | SPRP | Yearly Disk Usage | Load Historical State | |--------------------------|------|-------------------|-----------------------| | Block explorer/analysis | 32 | 1.4 TB | 155 ms | | Hobbyist (prev. default) | 2048 | 23.1 GB | 10.2 s | | Validator only (default) | 8192 | 5.7 GB | 41 s | 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 default value is 8192 for databases synced from scratch using Lighthouse v2.2.0 or later, or 2048 for prior versions. Please see the section on [Defaults](#defaults) below. The values shown in the table are approximate, calculated using a simple heuristic: each `BeaconState` consumes around 18MB of disk space, and each block replayed takes around 5ms. 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 before a restore point. ### Defaults As of Lighthouse v2.2.0, the default slots-per-restore-point value has been increased from 2048 to 8192 in order to conserve disk space. Existing nodes will continue to use SPRP=2048 unless re-synced. Note that it is currently not possible to change the SPRP without re-syncing, although fast re-syncing may be achieved with [Checkpoint Sync](./checkpoint-sync.md). ### CLI Configuration To configure your Lighthouse node's database with a non-default SPRP, run your Beacon Node with the `--slots-per-restore-point` flag: ```bash lighthouse beacon_node --slots-per-restore-point 32 ``` ## 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.