Journal config for SAN array

[Andreas Dilger]

On Nov 23, 2008  17:34 +0100, Jure Pečar wrote:
> On Sun, 23 Nov 2008 07:06:28 -0800 (PST)
> KenShelby <kshelby at optonline.net> wrote:
> > I'm not looking for a single "magic bullet" answer.  Are there any
> > guidelines at all, any best practices that have been found over time to
> > be a good place to start?  What I seek is either informed opinion or
> > information - if it exists - about how to make choices on EXT3 journal
> > configuration for a SAN installation.
> 
> Common sense and knowlege of your application are a good start.
> 
> You said that you have lots of random io, so I would be more concerned over
> latency than bandwith. If your SAN provides you with good enough latency
> then I wouldn't bother much about journal configs and go with defaults, but
> if your SAN sits on something like iSCSI with noticeable latency then I'd
> test a setup with journal on local disk, possibly even in full data
> journaling mode. If I had the money I'd look into SSDs for that purpose,
> either in form of SSD sata/sas drive or in form of PCIe card (fusion-io for
> example).

One important consideration is if you have hardware RAID-5/6, then it is
a bad idea to put the journal on the same device as the filesystem.  That
causes a lot of read-modify-write of RAID stripes, and bad performance.
Much better to put the journal on a RAID-1 device.

Cheers, Andreas
--
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.