[CentOS] 3Ware 9550SX and latency/system responsiveness

[Ross S. W. Walker]

Pasi Kärkkäinen wrote:
> 
> On Tue, Oct 02, 2007 at 09:39:09AM -0400, Ross S. W. Walker wrote:
> > Simon Banton wrote:
> > > 
> > > At 12:30 +0200 2/10/07, matthias platzer wrote:
> > > >
> > > >What I did to work around them was basically switching 
> to XFS for 
> > > >everything except / (3ware say their cards are fast, but only on 
> > > >XFS) AND using very low nr_requests for every blockdev 
> on the 3ware 
> > > >card.
> > > 
> > > Hi Matthias,
> > > 
> > > Thanks for this. In my CentOS 5 tests the nr_requests 
> turned out by 
> > > default to be 128, rather than the 8192 of CentOS 4.5. 
> I'll have a go 
> > > at reducing it still further.
> > 
> > Yes, the nr_requests should be a realistic reflection of what the
> > card itself can handle. If too high you will see io_waits stack up
> > high.
> > 
> > 64 or 128 are good numbers, rarely have I seen a card that 
> can handle
> > a depth larger then 128 (some older scsi cards did 256 I think).
> > 
> 
> Hmm.. let's say you have a linux software md-raid array made of sata
> drives.. what kind of nr_request values you should use for 
> that for optimal
> performance? 
> 
> Thanks!

Pasi,

Good to hear from you again.

I haven't done much testing with software RAID, but after googling
around I have found that there truly is no 1 nr_requests setting
that fits all pictures.

The nr_requests is the maximum number of requests that can be queued
before the queue is unplugged and the perfect # of requests queued
is a reflection of the workload and the hardware together. Also most
system func unplug after each request, so it isn't such a big issue
unless the system is under high load.

If the default 128 isn't working I would explore hardware or RAID
configuration problems first before trying to tweak this setting.

The old nr_requests = 8192 was definitely too high.

-Ross

______________________________________________________________________
This e-mail, and any attachments thereto, is intended only for use by
the addressee(s) named herein and may contain legally privileged
and/or confidential information. If you are not the intended recipient
of this e-mail, you are hereby notified that any dissemination,
distribution or copying of this e-mail, and any attachments thereto,
is strictly prohibited. If you have received this e-mail in error,
please immediately notify the sender and permanently delete the
original and any copy or printout thereof.