From: | Erik Jones <erik(at)myemma(dot)com> |
---|---|
To: | Josh Berkus <josh(at)agliodbs(dot)com> |
Cc: | pgsql-performance(at)postgresql(dot)org, Greg Smith <gsmith(at)gregsmith(dot)com>, Peter Schuller <peter(dot)schuller(at)infidyne(dot)com> |
Subject: | Re: shared_buffers in 8.3 w/ lots of RAM on dedicated PG machine |
Date: | 2008-02-15 18:37:10 |
Message-ID: | 72BE31DF-ABAF-459B-99A3-4E575EC3E08A@myemma.com |
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Thread: | |
Lists: | pgsql-performance |
On Feb 15, 2008, at 12:06 PM, Josh Berkus wrote:
> On Friday 15 February 2008 06:29, Greg Smith wrote:
>> PostgreSQL only uses direct I/O for writing to the WAL; everything
>> else
>> goes through the regular OS buffer cache unless you force it to do
>> otherwise at the OS level (like some Solaris setups do with
>> forcedirectio).
>
> Also, note that even when direct I/O is available, most users and
> benchmark
> tests have reported that having PostgreSQL "take over" the entire
> cache is
> not a net performance gain. I believe this is mostly because our I/
> O and
> caching code aren't designed for this kind of operation.
>
> I believe that MyEmma had a different experience on their workload,
> though.
Actually, while we did have shared_buffers set to 4G on an 8G system
when we were running with forcedirectio, the decision to even run
with forcedirectio was a temporary until we were able (welll, forced
to) migrate to a new system with a sane drive configuration. The old
set up was done horribly by a sysadmin who's no longer with us who
set us up with a RAID5 array with both the data and xlogs both
mirrored across all of the disks with no spares. So, I wouldn't
consider the numbers I was seeing then a reliable expectation as that
system was nowhere close to ideal. We've seen much more sane and
consistent numbers on a more normal setup, i.e. without forcedirectio
and with <= 25% system memory.
Erik Jones
DBA | Emma®
erik(at)myemma(dot)com
800.595.4401 or 615.292.5888
615.292.0777 (fax)
Emma helps organizations everywhere communicate & market in style.
Visit us online at http://www.myemma.com
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