Re: Very slow checkpoints

Hi Steven,

On Wed, Mar 18, 2015 at 12:21 PM, Steven Jones
<steven(dot)jones1201(at)outlook(dot)com> wrote:
> - system is HP blade; 128Gb RAM, 2x 8-core, 12x 10k RPM RAID1+0 (database)

Have you BBU on your controller? And how your controller configured, I
mean cache mode, io mode, disk write cache mode. You have 15K SAS
(which form factor?) under WAL and 10K SAS under database, am I
correct?

> Full iostat/iotop, configuration, checkpoint stats, etc. are pasted below
> for completeness. Highlights are:
> checkpoint_segments=512
> shared_buffers=16GB
> checkpoint_timeout=15min
> checkpoint_completion_target=0.1

It looks like your checkpoint settings are a bit strange besides of
everything else. If you chose high value for checkpoint_segments, your
aim is to avoid checkpoints by timeout (or vice verse). If you have
checkpoint_segments=512, your checkpoint_timeout should be about
60min. And anyway - checkpoint_completion_target=0.9 or 0.7 in order
to spread disk load between checkpoints.

> ---
> sysctl settings for dirty pages
>
> vm.dirty_background_bytes = 0
> vm.dirty_background_ratio = 5
> vm.dirty_bytes = 0
> vm.dirty_expire_centisecs = 3000
> vm.dirty_ratio = 10
> vm.dirty_writeback_centisecs = 500

Values for this settings are really dependent of RAID (and BBU size).

And about further problem description: have you any graphical
representation of your % disc utilization?

Best regards,
Ilya

--
Ilya Kosmodemiansky,

PostgreSQL-Consulting.com
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