Slow RAID IO on PE2970

Jason Ede J.Ede at birchenallhowden.co.uk
Thu Mar 19 14:47:13 CDT 2009


> -----Original Message-----
> From: linux-poweredge-bounces at dell.com [mailto:linux-poweredge-
> bounces at dell.com] On Behalf Of John LLOYD
> Sent: 13 March 2009 17:27
> To: linux-poweredge at dell.com
> Subject: RE: Slow RAID IO on PE2970
> 
> > > I would recommend you reconsider using RAID5 and 3 disks to
> > obtain the
> > > capacity of 2 disks -- RAID1 is a better choice here, and leaves
> you
> > > one
> > > disk spare.
> > Many thanks for that. It makes a good deal of sense.
> >
> > We have a 4th drive waiting to go into that box so would a
> > RAID 10 be a better answer in terms of performance? Are we
> > close to the limit of SATA drives irrespective of the array
> > type we use?
> 
> Yes, RAID10 is almost always better than RAID5 for performance, and
> certainly this is true during a RAID rebuild.  It is not really
> expensive as disks are cheap.  As the arithmetic shows, you have a lot
> of available  capability in the SATA disks if you can avoid the RAID5
> IO
> overhead.
> 
> >
> > How much performance would we get by moving to SAS drives and
> > could the Perc5i have 2 arrays of 1 SATA and 1 SAS (all
> > internal drives) as we're unsure if can have mixed array types?
> >
> 
> Yes we have successfully mixed SAS and SATA on the same PERC5 (and
> PERC6).  It is supported, although you cannot order a server that way
> from Dell you can add disks later.
> 
> For us the 15k rpm SAS disks are noticeably faster.  But you can't get
> 1TB SAS disks, so eventually you have to decide if you need speed over
> capacity.
> 
> 
> --John


We've done some more work on this box moving data off it again so we can look at redoing the RAID structure...

When we've a single IO stream we can easily read 20MB/s off the array and iostat still shows the array as not stressed at all. However, when its in production (its an openfiler box with ESXi hosts connecting to it via NFS for the virtuals) then the IO drops to what we saw earlier which is a few 10's or 100's of KB/s and 100% utilisation.

It all seems a bit odd... Also when copying the data off the drives every 5 mins or so the IO data rate suddenly drops to zero briefly. I think we'll run a full test on the RAID controller and the disks as the IO still seems poor and RAID 5 on those disks should be able to handle a better rate than we were seeing.

Jason



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