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Graft

Glossary

  • GID A 128 bit Graft Identifier. See GIDs for details.

  • Volume A sparse data object consisting of Pages located at PageIdxs starting from 1. Volumes are referred to primarily by a Volume ID.

  • Volume ID A 16 byte GID used to uniquely identify a Volume.

  • Page A fixed-length block of storage. The default size is 4KiB (4096 bytes).

  • PageIdx The index of a page within a volume. The first page of a volume has a page index of 1.

  • Graft A set of PageIdxs corresponding to a single Volume. Used to track which PageIdxs are contained in a Segment or which PageIdxs have changed between two Snapshots.

  • PageCount The number of logical pages in a Volume. This does not take into account sparseness. This means that if a page is written to PageIdx(1000) in an empty Volume, the Volume’s size will immediately jump to 1000 pages.

  • 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. The commit process ensures that the sequence of LSNs never has gaps and is monotonic.

  • Snapshot A tuple (volume id, lsn, PageCount) that defines a fixed point in time for the state of a volume.

  • Metastore A service which stores Volume metadata including the log of segments per Volume. This service is also responsible for coordinating GC, authn, authz, and background tasks.

  • Pagestore A service which stores pages keyed by [volume id]/[pageidx]/[lsn]. It can efficiently retrieve the latest LSN for a given PageIdx that is less than or equal to a specified LSN, allowing the Pagestore to read the state of a Volume at any Snapshot.

  • Replica Client A node that keeps up with changes to a Volume over time. May subscribe the Metastore to receive Grafts, or periodically poll for updates. Notably, Graft Replicas lazily retrieve Pages they want rather than downloading all changes.

  • Lite Client An embedded client optimized for reading or writing to a volume without any state. Generally has a very small (or non-existent) cache and does not subscribe to updates. Used in “fire and forget” workloads.

  • Segment An object stored in blob storage containing Pages and an index mapping from (Volume ID, PageIdx) to each Page.

  • Segment ID A 16 byte GID used to uniquely identify a Segment.