[ClusterLabs] [EXT] [EXT] Can stalled libqb SHM I/O delay Pacemaker's operation timer?

[Windl, Ulrich]

Hi!

A non-pacemaker answer: I can imagine two solutions for your problem:

  1.  Limit the reconstruction rate of the RAID
  2.  Limit the amount of (dirty) filesystem cache (we had read stalls when someone backed up the database and most of the RAM had been filled with dirty buffers to write out)

Kind regards,
Ulrich Windl

From: Users <users-bounces at clusterlabs.org> On Behalf Of Brian Cosgrove
Sent: Monday, August 10, 2026 10:03 PM
To: users at clusterlabs.org
Subject: [EXT] [EXT] [ClusterLabs] Can stalled libqb SHM I/O delay Pacemaker's operation timer?

Could Pacemaker’s controller block on a stalled block-device read while faulting in a swapped-
out libqb SHM page, before it receives the executor reply and starts the operation timer?

Would that behavior be expected?

I'm investigating a lockup where the PostgreSQL primary and Pacemaker DC were on the same node when a degraded RAID array caused reads to appear unbounded without returning errors. Pacemaker did not stop the PostgreSQL resource or promote a standby. crm_mon showed the DC as standby (with active resources).

Testing that simulates the incident reproduces that production Pacemaker behavior. In that setup a libqb SHM page is swapped out and controller blocks in shmem_fault() before the executor reply and operation timer. The surviving production logs (such as the crm_mon output) match. Corosync membership remained intact in both the incident and the test.


--
Brian Cosgrove
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