summarize_wal

Definition PG 18 manual

Enables the WAL summarizer process. Note that WAL summarization can be enabled either on a primary or on a standby. This parameter can only be set in the postgresql.conf file or on the server command line. The default is off.

The server cannot be started with summarize_wal=on if wal_level is set to minimal. If summarize_wal=on is configured after server startup while wal_level=minimal, the summarizer will run but refuse to generate summary files for any WAL generated with wal_level=minimal.

How it works

Starts the WAL summarizer process to enable incremental backup. A configuration reload applies a new value; existing work already in flight is not retroactively changed.

The WAL summarizer records which blocks changed over WAL ranges so pg_basebackup can construct incremental backups. It can run on a primary or standby, but cannot summarize WAL generated at wal_level=minimal.

Monitor and change summarize_wal together with wal_summary_keep_time, wal_level, archive_mode. Validate on the relevant server role and real workload, then use its sighup context to choose session change, reload, or restart; a historical boot default is not the current effective value.

Operational considerations

Enabling it with wal_level=minimal and expecting usable summary files.

Treating WAL summaries as a WAL archive or a backup by themselves.

Turning the summarizer off and expecting wal_summary_keep_time cleanup to continue.

Taking an incremental backup without checking summarizer progress and summary coverage.

Workload guidance

OLAP: Bulk writes increase summarizer work. Capacity-test backup interval, WAL peak, and backup window together; a summary gap makes an incremental backup fail.

OLTP: Enable or tune summarize_wal only when using PostgreSQL incremental backup. Monitor summarizer progress and retain summaries across the complete WAL range between every dependent backup pair.

SMALL: Keep summarize_wal off when incremental backup is not used. If enabled, derive retention from the real backup chain and alert on summary-directory capacity.