Glossary

Core Identifiers

Volume A sparse data object consisting of Pages located at PageIdxs starting from 1. Volumes are referred to by their Volume ID. Each Volume tracks a local Log and a remote Log for replication. See Volumes for details.
Page A fixed-length block of storage. The default size is 4 KiB (4096 bytes).
PageIdx The index of a page within a volume. The first page of a volume has a page index of 1.
PageCount The number of logical pages in a Volume. This does not account for sparseness. Writing to PageIdx(1000) in an empty Volume immediately sets the page count to 1000.
Segment A file composed of one or more ZStd compressed frames containing pages. Segments are tracked by Commits and uploaded to Remote storage.

LSN (Log Sequence Number) A sequentially increasing number that tracks changes to a Volume. Each transaction results in a new LSN, which is greater than all previous LSNs for the Volume. LSNs never have gaps and are monotonic.
Snapshot An immutable logical view of a Volume at a particular point in time.
Commit A transaction that advances a Volume’s LSN and records changed pages.

Tag A human-readable name that references a Volume. Stored as a name → VolumeId mapping.
SyncPoint Tracks synchronization state between local and remote Logs. Contains the remote LSN (attachment point) and optional local watermark (last pushed LSN). Determines ahead/behind status.
VolumeReader Read-only interface to a Volume at a specific snapshot. Immutable - always sees a consistent snapshot. Multiple VolumeReaders can operate in parallel without locking.
VolumeWriter Write interface for writing transactionally to a Volume. Provides read-your-writes semantics on top of an immutable Snapshot.

FjallStorage Local LSM-tree based storage layer. Stores Tags, Volumes, Logs, and Pages. Provides fast local cache and transaction staging.
Remote Storage Object storage backend (S3, filesystem, or memory). Stores commits and segments. The shared source of truth for replication.