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Re: shared_buffers advice

From: Konrad Garus <konrad(dot)garus(at)gmail(dot)com>
To:
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: shared_buffers advice
Date: 2010-05-24 11:25:57
Message-ID: AANLkTimrAAndq1reQSjXeYIX3Ztzt0tEWbSkliVTjkYl@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-performance
2010/3/11 Paul McGarry <paul(at)paulmcgarry(dot)com>:

> I'm basically wondering how the postgresql cache (ie shared_buffers)
> and the OS page_cache interact. The general advice seems to be to
> assign 1/4 of RAM to shared buffers.
>
> I don't have a good knowledge of the internals but I'm wondering if
> this will effectively mean that roughly the same amount of RAM being
> used for the OS page cache will be used for redundantly caching
> something the Postgres is caching as well?

I have a similar problem but I can't see an answer in this thread.

Our dedicated server has 16 GB RAM. Among other settings
shared_buffers is 2 GB, effective_cache_size is 12 GB.

Do shared_buffers duplicate contents of OS page cache? If so, how do I
know if 25% RAM is the right value for me? Actually it would not seem
to be true - the less redundancy the better.

Another question - is there a tool or built-in statistic that tells
when/how often/how much a table is read from disk? I mean physical
read, not poll from OS cache to shared_buffers.

-- 
Konrad Garus

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