Struct DoubleWriteBuffer 

pub struct DoubleWriteBuffer { /* private fields */ }

Double-write buffer file.

Implementations

impl DoubleWriteBuffer

pub fn open(path: &Path, mode: DwbMode) -> Result<Self>

Open or create the double-write buffer file in the requested I/O mode.

Callers that want the DWB disabled must not call this at all; DwbMode::Off returns WalError::DwbOffNotOpenable.

pub fn mode(&self) -> DwbMode

I/O mode this buffer was opened with.

pub fn path(&self) -> &Path

Path to the double-write buffer file.

pub fn write_record(&mut self, record: &WalRecord) -> Result<DwbMirror>

Write a WAL record to the double-write buffer before WAL append.

The record is written at the current circular position and the file is fsynced immediately. Use write_record_deferred + flush for batch mode (multiple records per fsync).

pub fn write_record_deferred(&mut self, record: &WalRecord) -> Result<DwbMirror>

Stage a WAL record in its ring slot without any disk write.

The slot image stays in memory until flush() writes it and fsyncs the file. Use this in batch mode: stage all records in a group commit batch, then call flush() once. A flush writes at most DWB_CAPACITY slots, however many records it covers.

Returns DwbMirror::Skipped for a record that cannot be mirrored, so the caller can account for the lost protection instead of assuming the record is covered.

pub fn staged_slots(&self) -> u32

Records staged since the last flush whose slots will reach disk at the next flush. At most DWB_CAPACITY.

pub fn slots_written(&self) -> u64

Slots this buffer wrote to disk since it was opened.

pub fn write_staged(&mut self) -> Result<u32>

Write the staged slots that survive in the ring, oldest first, with no fsync. Returns the number of slots written.

flush() calls this before writing the header. A caller that must tell a failed slot write from a failed fsync calls it first.

pub fn flush(&mut self) -> Result<()>

Write the staged slots and the DWB header, then fsync the file.

Must be called after one or more write_record_deferred calls to make the records durable. The single fsync covers all deferred writes since the last flush — amortizing the cost across the group commit batch.

impl DoubleWriteBuffer

pub fn recover_record(&mut self, target_lsn: u64) -> Result<Option<WalRecord>>

Try to recover a WAL record by LSN from the double-write buffer.

Reads the slots on disk only. A record staged since the last flush is not visible here until that flush writes its slot.

When two slots hold the same LSN — a crash between the DWB fsync and the WAL fsync leaves an LSN durable here but absent from the WAL, so recovery hands that LSN to a different record — the copy with the highest slot sequence number wins. Returning the older one would resurrect a payload that was never acknowledged to any client.

Trait Implementations

impl Debug for DoubleWriteBuffer

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.