Understanding Write-Ahead Logging (WAL) in PostgreSQL
summary
Explains how WAL stores a sequential change log in 16 MB segments split into 8 KB pages, with LSNs formed from timeline/file/segment hex fields; WAL records include headers (LSN, XID, type), full-page images for first post-checkpoint writes, and commit markers. Details wal_writer, checkpoints (default timeout 30s), replication via wal sender/receiver and replication slots, and logical decoding currently decodes every record. Shows diagnostic tools pg_waldump and pg_walinspect and usage for recovery and incremental backups.
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chapters (20)
- 0:00Introduction to WAL
- 0:57History of WAL in PostgreSQL
- 3:03Why WAL was introduced
- 4:27WAL buffer and commit process
- 7:17Key components of WAL
- 9:26WAL segment file structure
- 10:30WAL pages and records
- 11:32WAL record components
- 14:24Checkpoint process and WAL
- 19:50Recovery after crash
- 23:29WAL retention policies
- 27:25WAL in streaming replication
- 29:56WAL in logical replication
- 34:14WAL summarizer for backups
- 35:41WAL archiving for point-in-time recovery
- 36:02WAL sync commit levels
- 40:03WAL levels and performance
- 41:05Diagnosing WAL with pg_waldump
- 42:12Diagnosing WAL with pg_walinspect
- 42:56Summary and Q&A
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