Fwd: shared_buffers advice

See below:

On Wed, Mar 10, 2010 at 9:28 PM, Paul McGarry <paul(at)paulmcgarry(dot)com> wrote:

> Hi there,
>
> I'm after a little bit of advice on the shared_buffers setting (I have
> read the various docs on/linked from the performance tuning wiki page,
> some very helpful stuff there so thanks to those people).
>
> I am setting up a 64bit Linux server running Postgresql 8.3, the
> server has 64gigs of memory and Postgres is the only major application
> running on it. (This server is to go alongside some existing 8.3
> servers, we will look at 8.4/9 migration later)
>
> 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?
>
> IE when Postgres reads something from disk it will go into both the OS
> page cache and the Postgresql shared_buffers and the OS page cache
> copy is unlikely to be useful for anything.
>
> If that is the case what are the downsides to having less overlap
> between the caches, IE heavily favouring one or the other, such as
> allocating shared_buffers to a much larger percentage (such as 90-95%
> of expected 'free' memory).
>
> Pg apparently does not have an option of using direct IO with reads which
some other databases do (the O_DIRECT mode). Therefore, double-buffering
with read operations seems unavoidable. Counterintuitively, it may be a
good idea to just rely on OS buffering and keep shared_buffers rather small,
say, 512MB.

VJ

> Paul
>
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