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Re: PG 8.3 and large shared buffer settings

From: Josh Berkus <josh(at)agliodbs(dot)com>
To: pgsql-performance(at)postgresql(dot)org
Subject: Re: PG 8.3 and large shared buffer settings
Date: 2009-09-28 17:36:04
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-performance
On 9/26/09 8:19 AM, Greg Smith wrote:
> This means that the question you want an answer to is "if the OS cache
> isn't really available, where does giving memory to shared_buffers
> becomes less efficient than not caching things at all?"  My guess is
> that this number is much larger than 10GB, but I don't think anyone has
> done any tests to try to quantify exactly where it is.  Typically when
> people are talking about systems as large as yours, they're dedicated
> database servers at that point, so the OS cache gets considered at the
> same time. If it's effectively out of the picture, the spot where
> caching still helps even when it's somewhat inefficient due to buffer
> contention isn't well explored.

It also depends on the filesystem.  In testing at Sun and on this list,
people have found that very large s_b (60% of RAM) plus directIO was
actually a win on Solaris UFS, partly because UFS isn't very agressive
or smart about readahead and caching.  On Linux/Ext3, however, it was
never a win.

I don't know what AIX's filesystems are like.

Josh Berkus
PostgreSQL Experts Inc.

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