Greg Stark wrote:
Arshavir Grigorian <ag@xxxxxxxxx> 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
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