max_replication_slots

For DBAs and operators

The limit counts all slots in pg_replication_slots: physical, logical, temporary, and synced failover slots on a standby. Creating one more fails with:

ERROR: all replication slots are in use
HINT: Free one or increase "max_replication_slots".

Each logical subscription uses one slot on the publisher, plus additional ones during initial table synchronization (up to max_sync_workers_per_subscription extra per subscription), so size it generously on publishers. On a subscriber, PostgreSQL 17 and earlier also used this value to size replication origins; since 18 that is max_active_replication_origins.

Watch for unused slots

SELECT slot_name, slot_type, active, wal_status, inactive_since,
       pg_size_pretty(pg_wal_lsn_diff(pg_current_wal_lsn(), restart_lsn)) AS retained
  FROM pg_replication_slots;

An inactive slot retains WAL without limit unless max_slot_wal_keep_size or idle_replication_slot_timeout is set. A logical slot also holds back catalog_xmin, which blocks vacuum of system catalogs. Drop slots you no longer need with pg_drop_replication_slot().

How to change it

Change in postgresql.conf or ALTER SYSTEM, then restart the server.