WAL & durabilitysenior🔒 Pro concept

WAL full-page images (FPI) and checkpoints

Simple terms

The first time a page is modified after a checkpoint, Postgres doesn't log the change. It logs the whole page. Every change after that logs only the difference. This looks wasteful right up until you picture a crash mid-write, leaving a page half old and half new. A small diff can't repair that, it assumes the page underneath is intact. A full copy can just overwrite it. It is also exactly why WAL volume spikes right after every checkpoint and then settles down again.

You might be asked

What is a WAL full-page image, when exactly does PostgreSQL write one, why does it exist, and how would you prove and measure it?

TopicWrite-ahead log / crash recovery
PostgreSQL15, 16, 17, 18 (pg_walinspect is PG15+; the FPI mechanism itself is all versions)
Tools usedpg_walinspect (pg_get_wal_records_info), pg_current_wal_lsn + psql \gset, CHECKPOINT, full_page_writes/wal_compression GUCs, source navigation @REL_17_10
Last reviewed2026-06-19

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