wal_decode_buffer_size

wal_decode_buffer_size sets the buffer size for reading ahead in the WAL during recovery. It is a postmaster setting present in PG15–18; the latest recorded boot default is 512 KiB.

Note

Fact — official short description: “Buffer size for reading ahead in the WAL during recovery.”

Identity

Type , Valueinteger: Upstream pg_settings type
Context , Valuepostmaster: Requires a server restart
Unit , ValueB: Raw unit
Range , Value65536 – 1073741823: Raw limits in the last observed version
Enum values , Value—: — for non-enum types
Category , ValueWrite-Ahead Log / Recovery: Upstream classification
Latest boot value , Value524288: 512 KiB

Lifecycle

Fact	Value
First observed	PG15
Present in	PG15–19 Beta 3
Removed in	No
Introduction commit	`1d257577e08d` — Optionally prefetch referenced data in recovery.
Commit date	2021-04-08
Discussion	thread 1

Default history

Measured PG9.0–19 Beta 3 boot defaults
Versions	Raw `boot_val`	Unit	Human value
PG15–19 Beta 3	`524288`	`B`	512 KiB

How it works

Buffer size for reading ahead in the WAL during recovery. The value is fixed when the server starts, so changing it requires a restart.

Recovery scans this far ahead in decoded WAL to discover future block references for prefetch. It is a restart-time memory/lead limit, not a logical-decoding output buffer, and only helps when recovery_prefetch and the storage workload benefit.

Monitor and change wal_decode_buffer_size together with recovery_prefetch, maintenance_io_concurrency, effective_io_concurrency. Validate on the relevant server role and real workload, then use its postmaster context to choose session change, reload, or restart; a historical boot default is not the current effective value.

Tuning advice

Tip

Advice. These are workload-specific starting points and must be validated with measurements.

Workload	Guidance
OLTP	Tune wal_decode_buffer_size with recovery/standby catch-up benchmarks on production-like storage while observing replay I/O wait, cache behavior, and maintenance_io_concurrency. Keep the default without measured benefit.
OLAP	Large data with slow random reads may benefit more, but excess prefetch competes with query I/O and cache. Test crash recovery and a query-serving standby separately.
Small nodes	The default is usually sufficient. Do not spend scarce memory or I/O queue depth on a larger look-ahead window unless a recovery-time objective is demonstrably missed.

Pigsty

Values use the fixed 8-vCPU, 32-GiB, 100-GiB SSD fixture and render the current Pigsty templates for PG19 Beta 3; this does not assert current Pigsty support for that historical or beta release.

Template	Effective value	Versus upstream boot	Source expression
OLTP	Unmodified	—	—
OLAP	Unmodified	—	—
CRIT	Unmodified	—	—
TINY	Unmodified	—	—

Caution

Advice — pending human review. Fact from the current Pigsty template projection: OLTP: PG15–19 Beta 3 unmodified; OLAP: PG15–19 Beta 3 unmodified; CRIT: PG15–19 Beta 3 unmodified; TINY: PG15–19 Beta 3 unmodified. No Pigsty-specific rationale is inferred from an absent override.

Common pitfalls

Confusing pg_settings base units with human-readable configuration units.
Benchmarking throughput without a crash-recovery and archive-restore test.
Ignoring interactions with checkpoints, replication slots, or archive failure.
Reloading a restart-context value and assuming it became active.

recovery_prefetch · maintenance_io_concurrency · effective_io_concurrency · shared_buffers · archive_cleanup_command · archive_command