pub type WalLsnFn = fn() -> u64;Expand description
v7.39 (round 476) — the WAL’s current byte position, as a PG LSN.
pg_current_wal_lsn() answered the literal 0/0 forever, so every
monitor watching WAL progress or replication lag saw an instance that
had never written anything. SPG’s WAL is a file and its length IS an
LSN in every sense a monitor uses one: monotonic, byte-denominated, and
comparable — pg_wal_lsn_diff over two samples gives real bytes.
None (embedded, or a server started without a WAL) keeps 0/0, which
is the honest answer there: nothing is being written.