Re: Increasing Shared_buffers = slow commits?

["Merlin Moncure" <mmoncure(at)gmail(dot)com>]

On 5/21/07, Chris Hoover <revoohc(at)gmail(dot)com> wrote:
> Hi everyone,
>
> I am testing my shared_buffers pool and am running into a problem with slow
> inserts and commits.  I was reading in several places that in the 8.X
> PostgreSQL engines should set the shared_buffers closer to 25% of the
> systems memory.  On me development system, I have done that.  We have 9GB of
> memory on the machine and I set my shared_buffers = 292188 (~25% of total
> memory).
>
> When my users logged in today, they are noticing the system is much slower.
> Tracing my log files, I am seeing that most of the commits are taking over
> 1sec.  I am seeing a range of 1-5 seconds per commit.
>
>  What is the correlation here between the shared_buffers and the disk
> activity?  This is not something I would have expected at all.

have you overcommited your memory?  maybe you are thrashing a
bit...long commit times are usually symptom of high iowait.  can you
pop up top and monitor iowait for a bit?

can you lower shared buffers again and confirm that performance
increases?  how about doing some iostat/vmstat runs and looking for
values that are significantly different depending on the shared
buffers setting.

merlin