vacuum_multixact_failsafe_age — PostgreSQL configuration parameter

Specifies the maximum age (in multixacts) that a table’s pg_class.relminmxid field can attain before VACUUM takes extraordinary measures to avoid system-wide multixact ID wraparound failure.

At a glance

What it does

Specifies the maximum age (in multixacts) that a table’s pg_class.relminmxid field can attain before VACUUM takes extraordinary measures to avoid system-wide multixact ID wraparound failure. This is VACUUM’s strategy of last resort. The failsafe typically triggers when an autovacuum to prevent transaction ID wraparound has already been running for some time, though it’s possible for the failsafe to trigger during any VACUUM.

When the failsafe is triggered, any cost-based delay that is in effect will no longer be applied, and further non-essential maintenance tasks (such as index vacuuming) are bypassed.

(Description quoted from the official PostgreSQL documentation.)

How to apply a change

Can be set per session with SET, per role/database with ALTER ROLE/DATABASE ... SET, or globally in postgresql.conf.

Inspect the current value and source with SHOW vacuum_multixact_failsafe_age; or SELECT name, setting, unit, context, source FROM pg_settings WHERE name = 'vacuum_multixact_failsafe_age';.

Tuning guidance

The multixact emergency-vacuum threshold; same idea as vacuum_failsafe_age but for multixact wraparound. Leave near default unless you routinely approach multixact limits.

Reference

PostgreSQL documentation — vacuum_multixact_failsafe_age.