Subject: performance impact on read replicas during heavy vacuums on primary

From: Dirk Krautschick <[email-masked]>
Date: Tue, 3 Feb 2026 02:02:52 +0100

Is there a scenario, where a very heavy vacuum on the primary side is 
affecting significantly
the read replica(s) in matter of performance (not the overall apply lag, 
which is expected) of
selects? I do not get this reproduced ...only the lag is clear but even 
hammering down the
primary with vaccum keeps my standby answering fine.

Cheers

---

From: Greg Sabino Mullane <[email-masked]>
Date: Wed, 4 Feb 2026 11:29:07 -0500

Is this a theoretical question, or have you seen performance issues? Yes,
it is certainly possible for vacuum to affect the replica performance. More
vacuum activity means more WAL means more WAL replay on the replica, using
up CPU and I/O. In most real-world cases, this will not be a problem, as
the replica should be able to keep up quite well even for near-constant
vacuum activity. You would need a very high WAL volume and a very
underprovisioned replica before seeing any ill effects. There are plenty of
other performance bottlenecks that will likely hit you before this one.

Cheers,
Greg