Troubleshooting·PostgreSQL
PostgreSQL Replication Slots and WAL Retention
A slot guarantees a standby can always catch up. The bill for that guarantee arrives on the primary's disk - here 30 MB held for a standby that was never coming back, and nothing raises an error while it grows.
Replication Guide 29 of 47 Advanced
- OSUbuntu 26.04 LTS (resolute)
- PostgreSQL18.6-0ubuntu0.26.04.1
- ReplicationPhysical streaming, async, with slots
- TimeAbout 16 min
- Reviewed27 August 2026
Written against the versions above. `max_slot_wal_keep_size` caps how much a slot may retain and is unlimited by default. Setting it converts an unbounded disk risk into a broken replica, which is usually the better failure.
| Server Name | IP Address | OS | Roles | CPU | RAM | HDD |
|---|---|---|---|---|---|---|
| db-a01 | 192.168.0.81 | Ubuntu 26.04 LTS | Primary / Source / Replica Set Member 1 | 2 Core | 4 GB | 50 GB |
| db-b01 | 192.168.0.82 | Ubuntu 26.04 LTS | Standby / Replica / Replica Set Member 2 | 2 Core | 4 GB | 50 GB |
| db-c01 | 192.168.0.83 | Ubuntu 26.04 LTS | Replica Set Member 3 / Cascading and Delayed Replica | 2 Core | 4 GB | 50 GB |
Before you start
- A primary with at least one replication slot.
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Stop a standby and watch the slot hold on
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Measure what an orphaned slot is costing
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Find and drop the orphan
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Confirm the surviving standby is unaffected