# pd_lsn vs record LSN Two LSNs that are easy to mix up, and the rule recovery uses to compare them. > Canonical URL: https://sudohulk.com/writing/pd-lsn-vs-record-lsn > Type: note ยท Stream: Tech > Maturity: budding. Checked once; the structure holds. > Published: 2026-09-21 > Tags: postgresql > Reading time: 1 min > Author: Vignesh Kumar A > Licence: CC BY 4.0. Quote freely, cite the canonical URL. --- Every WAL record has an address in the log stream: its **record LSN**. Every 8 KB data page also carries an LSN in its header, `pd_lsn`: the **page LSN**, which is the address just past the most recent WAL record already applied to that page. The rule that makes crash recovery idempotent: when replaying a record for a page, PostgreSQL applies it only if `pd_lsn` is older than the record's LSN. If the page already shows a newer LSN, that change reached disk before the crash and is skipped. :::gotcha An LSN is meaningful only inside one server. Two nodes' `1/2D3E` have nothing to do with each other. Never compare LSNs across machines. ::: Related: [[full-page-writes-and-torn-pages]] (once written).