Specifies how often the WAL writer flushes WAL, in volume terms.
At a glance
| Property | Value |
|---|---|
| Parameter | wal_writer_flush_after |
| Category | Write Ahead Log |
| Default | 1MB |
| Value type | integer |
| Change scope | Reload (postgresql.conf, SIGHUP) |
| Available in | PostgreSQL 12, 13, 14, 15, 16, 17, 18, 19 (added in 12) |
What it does
Specifies how often the WAL writer flushes WAL, in volume terms. If the last flush happened less than wal_writer_delay ago and less than wal_writer_flush_after worth of WAL has been produced since, then WAL is only written to the operating system, not flushed to disk. If wal_writer_flush_after is set to 0 then WAL data is always flushed immediately. If this value is specified without units, it is taken as WAL blocks, that is XLOG_BLCKSZ bytes, typically 8kB. The default is 1MB. This parameter can only be set in the postgresql.conf file or on the server command line.
(Description quoted from the official PostgreSQL documentation.)
How to apply a change
Set it in postgresql.conf (or with ALTER SYSTEM) and reload with SELECT pg_reload_conf(); or pg_ctl reload — no restart needed.
Inspect the current value and source with SHOW wal_writer_flush_after; or SELECT name, setting, unit, context, source FROM pg_settings WHERE name = 'wal_writer_flush_after';.
Tuning guidance
Tune the amount of WAL the writer buffers before forcing a flush; larger values batch more (better throughput, weaker durability window under async commit). Leave default unless chasing a specific WAL I/O pattern.