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In-Reply-To: <04C98397-261D-4A50-99CB-61FA3E4A974B@richyen.com>
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Date: Tue, 30 Mar 2010 10:57:44 -0400
Message-ID: <603c8f071003300757s39a9096eyc6ce6af4ecb396ff@mail.gmail.com>
Subject: Re: why does swap not recover?
From: Robert Haas <robertmhaas@gmail.com>
To: Richard Yen <dba@richyen.com>
Cc: pgsql-performance@postgresql.org
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On Fri, Mar 26, 2010 at 7:57 PM, Richard Yen <dba@richyen.com> wrote:
> Note that it is constantly paging in, but never paging out. =A0This would=
 indicate that it's constantly reading from swap, but never writing out to =
it. =A0Why would postgres do this? (postgres is pretty much the only thing =
running on this machine).
>
> I'm planning on lowering the shared_buffers to a more sane value, like 25=
GB (pgtune recommends this for a Mixed-purpose machine) or less (pgtune rec=
ommends 14GB for an OLTP machine). =A0However, before I do this (and possib=
ly resolve the issue), I was hoping to see if anyone would have an explanat=
ion for the constant reading from swap, but never writing back.

Reading a page in from swap still leaves that data on the disk.  So it
may be that you're reading in pages from disk, not modifying them,
discarding them (without any need to write them out since they're
still on disk), and then reading them in again when they're accessed
again.

...Robert