From: Greg Smith <greg@2ndQuadrant.com>
To: pgsql-performance@postgresql.org
Subject: Re: Benchmarking a large server
Date: Tue, 10 May 2011 00:44:33 -0400
Message-ID: <4DC8C2B1.1020004@2ndQuadrant.com> (raw)
In-Reply-To: <0683F5F5A5C7FE419A752A034B4A0B9702B61997@sswchi5pmbx2.peak6.net>
References: <BANLkTikONPZ_7zQNfbyyN4MGBnn5PXHrAg@mail.gmail.com>
<BANLkTikecJCGWH0WyKFiSnC08kmcwS7FqA@mail.gmail.com>
<4DC88793.4000708@emolecules.com>
<4DC88905.8030200@boreham.org>
<0683F5F5A5C7FE419A752A034B4A0B9702B61997@sswchi5pmbx2.peak6.net>
On 05/09/2011 11:13 PM, Shaun Thomas wrote:
> Take a look at /proc/sys/vm/dirty_ratio and
> /proc/sys/vm/dirty_background_ratio if you have an older Linux system,
> or /proc/sys/vm/dirty_bytes, and /proc/sys/vm/dirty_background_bytes
> with a newer one.
> On older systems for instance, those are set to 40 and 20 respectively (recent kernels cut these in half).
1/4 actually; 10% and 5% starting in kernel 2.6.22. The main sources of
this on otherwise new servers I see are RedHat Linux RHEL5 systems
running 2.6.18. But as you say, even the lower defaults of the newer
kernels can be way too much on a system with lots of RAM.
The main downside I've seen of addressing this by using a kernel with
dirty_bytes and dirty_background_bytes is that VACUUM can slow down
considerably. It really relies on the filesystem having a lot of write
cache to perform well. In many cases people are happy with VACUUM
throttling if it means nasty I/O spikes go away, but the trade-offs here
are still painful at times.
--
Greg Smith 2ndQuadrant US greg@2ndQuadrant.com Baltimore, MD
PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us
"PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books
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To: pgsql-performance@postgresql.org
Cc: greg@2ndQuadrant.com
Subject: Re: Benchmarking a large server
In-Reply-To: <4DC8C2B1.1020004@2ndQuadrant.com>
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