autovacuum_multixact_freeze_max_age

Definition PG 18 manual

Specifies the maximum age (in multixacts) that a table's pg_class.relminmxid field can attain before a VACUUM operation is forced to prevent multixact ID wraparound within the table. Note that the system will launch autovacuum processes to prevent wraparound even when autovacuum is otherwise disabled.

Vacuuming multixacts also allows removal of old files from the pg_multixact/members and pg_multixact/offsets subdirectories, which is why the default is a relatively low 400 million multixacts. This parameter can only be set at server start, but the setting can be reduced for individual tables by changing table storage parameters. For more information see Section 24.1.5.1.

How it works

Multixact age at which to autovacuum a table to prevent multixact wraparound. The value is fixed when the server starts, so changing it requires a restart.

Reaching the max age forces an aggressive autovacuum even if ordinary autovacuum is disabled; a per-table override can only lower the global ceiling. Safety must be assessed across every database/table age together with long transactions and replication slots.

Monitor and change autovacuum_multixact_freeze_max_age together with autovacuum_freeze_max_age, vacuum_freeze_min_age, vacuum_freeze_table_age. 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.

Operational considerations

Ignoring MultiXact member storage pressure, which can force work earlier.

Assuming XID-age monitoring also covers MXID age.

Raising the ceiling while long-lived row locks keep generating old multixacts.

Changing the global value while a table storage parameter overrides it.

Treating reltuples and cumulative change statistics as exact real-time counts.

Workload guidance

OLAP: Proactively VACUUM (FREEZE) newly loaded or static partitions in batch windows and reserve I/O time for full scans. Convert age budgets using peak transaction rate, not a wall-clock guess.

OLTP: Calibrate autovacuum_multixact_freeze_max_age against the oldest XID/MXID age in every database and measured vacuum completion rate. Remove long transactions, stale slots, and blocked workers; never raise ages merely to hide a backlog.

SMALL: Upstream defaults are usually safest. A small system still needs anti-wraparound maintenance; monitor every database, not only the application database.