write-ahead-log · shipping · also WAL

Write-ahead log

A write-ahead log is a recovery log whose relevant records must become durable before the corresponding changes are persisted in database data files.

Write-ahead logging names the ordering protocol; the write-ahead log is its record stream. PostgreSQL can replay logged changes after a crash, so committing a transaction need not flush every modified data page. A record in a memory buffer is not yet a durable record.

Relations

related

database-checkpoint, full-page-write, fsync, persistence-ordering, crash-consistency, log-sequence-number

Advantages

• Allows changed data pages to be written later while preserving logged recovery information. [1]

Limitations

• Recovery still depends on storage honoring synchronization requests. [2]

Citations

1. PostgreSQL 18: Write-Ahead Logging (accessed 2026-10-11)
2. PostgreSQL 18: Reliability (accessed 2026-10-11)
3. WAL, checkpoints and torn pages: three different recovery guarantees (accessed 2026-10-11)

JSON: /data/terms/write-ahead-log.json