Module Granary_sample.Mem_walSource

In-memory, byte-addressed WAL device (#403).

Store.open_block_wal expects a WAL accessed by positioned byte I/O at non-sector-aligned offsets, so the WAL cannot ride a page-addressed Mirage_block device directly. This is the shared in-memory backing that both the sample unikernel (mirage/unikernel.ml) and the host smoke test (test/test_mirage_unikernel_smoke.ml) wire into Store.open_block_wal, so the one piece of arch-relevant glue is exercised from a single source.

It is not durable across process runs (the buffer is lost on exit), so a file-backed main DB written with this WAL must be re-created fresh each run.

Sourcetype t
Sourceval create : ?initial_size:int -> unit -> t

create ?initial_size () is a fresh zero-filled WAL buffer (initial_size bytes, default 65536). The buffer grows on demand as the WAL is written.

Sourceval size_bytes : t -> int64

Current buffer size in bytes — pass as ~wal_size_bytes to Store.open_block_wal.

Sourceval read_at : t -> offset:int64 -> Cstruct.t -> (unit, string) result Lwt.t

read_at t ~offset buf fills buf from offset; bytes past the written region read back as zero (the WAL grows lazily).

Sourceval write_at : t -> offset:int64 -> Cstruct.t -> (unit, string) result Lwt.t

write_at t ~offset src writes src at offset, growing the buffer to cover offset + length src.

Sourceval sync : unit -> (unit, string) result Lwt.t

No-op flush: an in-memory buffer has nothing to fsync. Pass as ~wal_sync. Db.wal_sync_count still counts the engine's calls to this callback.

Sourceval pp : Format.formatter -> t -> unit

Pretty-print the device (its current buffer size).