[ClusterLabs] PAF fails to promote slave: Can not get current node LSN location

Tiemen Ruiten t.ruiten at tech-lab.io
Mon Jul 8 13:27:00 EDT 2019

On Mon, Jul 8, 2019 at 4:59 PM Jehan-Guillaume de Rorthais <jgdr at dalibo.com>

> On Mon, 8 Jul 2019 13:56:49 +0200
> Tiemen Ruiten <t.ruiten at tech-lab.io> wrote:
> > Thank you for the clear explanation and advice.
> >
> > Hardware is adequate: 8x SSD and 20 cores per node, but I should note
> that
> > the filesystem is ZFS (stripe of mirrors) and there seems to be evidence
> > that the way the WAL writer allocates space and ZFS' Copy-on-Write nature
> > don't play nice. A patch that adds several GUCs to improve the situation
> Wait, how better performances on WAL writes will help you there?
> Checkpoints
> does not writes to WAL, it actually sync data from shared buffers to data
> files (heap, toast, index, internal stuffs, etc). Write performances to
> WAL is
> related to the number of xact you can achieve per seconds (if you have
> synchronous_commit >= local), not your checkpoint writes.

Wow, I completely misunderstood how that works then. This makes much more
sense (obviously..).

> > (at least it's worth trying, there was some disagreement on the
> > pgsql-general list over whether it would be helpful in my situation)
> Do you have a link to this thread ?


I managed to improve the average time checkpoints are taking already from
what I mentioned in that thread, mainly by decreasing checkpoint_timeout
and setting full_page_writes = off; ostensibly not necessary on ZFS.

> > has recently been merged: https://postgrespro.com/list/thread-id/2393057
> but
> > it won't be available in the 11.x release . So while I'm waiting until I
> can
> > upgrade to PostgreSQL 12, I'll increase the notify timeout.
> Do not hold your breath until you upgrade to 12, I'm not convinced (but I
> might miss something) this patch is useful to you.

Yes, since I have synchronous_commit = off; everywhere, I also don't see
anymore how it could help me.

> > A larger RTO is much more preferrable over manual actions in the middle
> of
> > the night!
> sure...but it depend on the usecase :)
> > Thanks again!
> You're welcome and good luck!
> --
> Jehan-Guillaume de Rorthais
> Dalibo
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