Log Sequence Number (Günlük Sıra Numarası)

The LSN acts as the atomic physical clock across database engines:

1. 1Whenever a buffer page is mutated in shared memory, it receives an updated Page LSN header marking the exact journal transaction.
2. 2Under the Write-Ahead Log (WAL) Flush Rule, a dirty memory buffer can never be written to persistent storage until its corresponding journal record has been flushed and fsynced to disk up to that specific LSN.
3. 3In physical replication topologies, replication lag between primary and standby replicas is quantified with byte-level precision by calculating: pg_wal_lsn_diff(write_lsn, replay_lsn).