To: Erik Jones <erik@myemma.com>
cc: PostgreSQL Performance <pgsql-performance@postgresql.org>
Subject: Re: Shared buffers, db transactions commited,
 and write IO on Solaris 
In-reply-to: <9B7F4B34-7E96-4501-BCE0-D0BED739530A@myemma.com> 
References: <9B7F4B34-7E96-4501-BCE0-D0BED739530A@myemma.com>
Comments: In-reply-to Erik Jones <erik@myemma.com>
	message dated "Wed, 28 Mar 2007 14:56:47 -0500"
Date: Thu, 29 Mar 2007 12:16:36 -0400
Message-ID: <23286.1175184996@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>

Erik Jones <erik@myemma.com> writes:
> We've recently made a couple changes to our system that have resulted  
> in a drastic increase in performance as well as some very confusing  
> changes to the database statistics, specifically  
> pg_stat_database.xact_commit.  Here's the details:

I'm kinda boggled too.  I can see how increasing shared buffers could
result in a drastic reduction in write rate, if the working set of your
queries fits in the new space but didn't fit in the old.  I have no idea
how that leads to a drop in number of transactions committed though.
It doesn't make sense that autovac would run less frequently, because
it's driven by number of tuples changed not number of disk writes; and
that could hardly account for a 10x drop anyway.

Did you by any chance take note of exactly which processes were
generating all the I/O or the CPU load?

			regards, tom lane