Also called: FPI, full-page image, full_page_writes
In plain English
A full-page write is a copy of an entire 8KB data page written into the WAL the first time that page is modified after a checkpoint — instead of just the small row-level change. It exists to protect against torn pages: if a crash interrupts an 8KB write so the page is half-old and half-new, recovery restores the whole page from the FPI and replays forward.
Why it matters
Full-page images are the main reason WAL volume spikes right after a checkpoint and then settles. Spacing checkpoints out (larger max_wal_size) reduces how often pages take a fresh FPI, and wal_compression shrinks the images — two of the most effective ways to cut WAL and replication traffic.