From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Greaves Subject: Re: [PERFORM] Postgres on RAID5 (possible sync blocking read type issue on 2.6.11) Date: Mon, 14 Mar 2005 07:44:53 +0000 Message-ID: <423540F5.8090609@dgreaves.com> References: <4231F5F2.6000509@m-cam.com> <87hdje7sjm.fsf@stark.xeocode.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit In-Reply-To: <87hdje7sjm.fsf@stark.xeocode.com> Sender: linux-raid-owner@vger.kernel.org To: Greg Stark Cc: Arshavir Grigorian , linux-raid@vger.kernel.org, pgsql-performance@postgresql.org List-Id: linux-raid.ids Greg Stark wrote: >Arshavir Grigorian writes: > > > >>Hi, >> >>I have a RAID5 array (mdadm) with 14 disks + 1 spare. This partition has an >>Ext3 filesystem which is used by Postgres. >> >> > >People are going to suggest moving to RAID1+0. I'm unconvinced that RAID5 >across 14 drivers shouldn't be able to keep up with RAID1 across 7 drives >though. It would be interesting to see empirical data. > >One thing that does scare me is the Postgres transaction log and the ext3 >journal both sharing these disks with the data. Ideally both of these things >should get (mirrored) disks of their own separate from the data files. > >But 2-3s pauses seem disturbing. I wonder whether ext3 is issuing a cache >flush on every fsync to get the journal pushed out. This is a new linux >feature that's necessary with ide but shouldn't be necessary with scsi. > >It would be interesting to know whether postgres performs differently with >fsync=off. This would even be a reasonable mode to run under for initial >database loads. It shouldn't make much of a difference with hardware like this >though. And you should be aware that running under this mode in production >would put your data at risk. > Hi I'm coming in from the raid list so I didn't get the full story. May I ask what kernel? I only ask because I upgraded to 2.6.11.2 and happened to be watching xosview on my (probably) completely different setup (1Tb xfs/lvm2/raid5 served by nfs to a remote sustained read/write app), when I saw all read activity cease for 2/3 seconds whilst the disk wrote, then disk read resumed. This occured repeatedly during a read/edit/write of a 3Gb file. Performance not critical here so on the "hmm, that's odd" todo list :) David