Re: Poor performance -- poor config?

[David Chinner <dgc@sgi.com>]

On Thu, Jun 21, 2007 at 08:37:36AM +0200, Sebastian Brings wrote:
> Not sure if it makes much sense to set stripe unit and width for a Raid
> which appears as a single device. 

Certainly it does.

That way you get stripe aligned allocation and therfore you are
much more likely to get full-stripe width writes instead of unaligned
writes that force RMW cycles on the RAID controller for parity calculations.

> As you state, the "width" of your DS lun is 4 x 512K == 2MB. In case you
> don't have write cache enabled each of your 1MB writes will cause the DS
> to write to two out of four disks only, causing heavy overhead to create
> parity. 

You're assuming stripe aligned I/O there. That 1MB could hit 3 of the 4
data disks - if you don't have a stripe unit set then that will
be the common case. i.e. its worse than you think :/

Cheers,

Dave.
-- 
Dave Chinner
Principal Engineer
SGI Australian Software Group