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Re: io storm on checkpoints, postgresql 8.2.4, linux

From: "Dmitry Potapov" <fortune(dot)fish(at)gmail(dot)com>
To: "Greg Smith" <gsmith(at)gregsmith(dot)com>
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: io storm on checkpoints, postgresql 8.2.4, linux
Date: 2007-08-23 14:37:39
Message-ID: 878c83960708230737j6103e177h8f23d5bd4676828c@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-performance
2007/8/23, Greg Smith <gsmith(at)gregsmith(dot)com>:
>
> On Wed, 22 Aug 2007, Dmitry Potapov wrote:
>
> If you do end up following up with this via the Linux kernel mailing list,
> please pass that link along.  I've been meaning to submit it to them and
> wait for the flood of e-mail telling me what I screwed up, that will go
> better if you tell them about it instead of me.


I'm planning to do so, but before I need to take a look at postgresql source
and dev documentation to find how exactly IO is done, to be able to explain
the issue to linux kernel people.  That will take some time, I'll post a
link here when I'm done.


> > looks to me as an elegant solution. Is there some other way to fix this
> > issue without disabling pagecache and the IO smoothing it was designed
> > to perform?
>
> I spent a couple of months trying and decided it was impossible.  Your
> analysis of the issue is completely accurate; lowering
> dirty_background_ratio to 0 makes the system much less efficient, but it's
> the only way to make the stalls go completely away.


By the way, does postgresql has a similar stall problem on freebsd/other
OS'es? It would be interesting to study their approach to io smoothing if it
doesn't.

I contributed some help toward fixing the issue in the upcoming 8.3
> instead; there's a new checkpoint writing process aimed to ease the exact
> problem you're running into there, see the new
> checkpoint_completion_target tunable at
> http://developer.postgresql.org/pgdocs/postgres/wal-configuration.html
>
> If you could figure out how to run some tests to see if the problem clears
> up for you using the new technique, that would be valuable feedback for
> the development team for the upcoming 8.3 beta.  Probably more productive
> use of your time than going crazy trying to fix the issue in 8.2.4.

We have a tool here to record and replay the exact workload we have on a
real production system, the only problem is getting a spare 16Gb box. I can
get a server with 8Gb ram and nearly same storage setup for testing
purposes. I hope it will be able to carry the production load, so I can
compare 8.2.4 and 8.3devel on the same box, in the same situation. Is there
any other changes in 8.3devel that can affect the results of such test? I
didn't really follow 8.3 development process :(

-- 
Regards,
            Dmitry

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