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xen-users
Re: [Xen-users] Xen and I/O Intensive Loads
On Thu, Aug 27, 2009 at 07:46:46AM -0600, Nick Couchman wrote:
> So here are some details on the SAN LUN...the SAN is a Compellent SAN
> attached to my FC Switch (McData Sphereon 4700, now the Brocade M4700)
> with 4 x 2Gb FC connections. The dom0 uses the QLE2462 adapter, with a
> single 4Gb connection hooked up. I did find that there is a later
> driver available - I'll try to switch to that when I get a chance. One
> interesting thing that I found is that it the adapter appears to be in a
> 4x PCIe slot, which means the max bandwidth for the card is 2.5Gbps.
> I'm not sure if this is a QLogic issue or if I need to move the card to
> a different slot in my Dell PowerEdge R610 chassis, but it looks like
> I'm being limited to 2/3 or so the speed of the FC connection by my PCIe
> bus. It's using a 4Gbps Point-to-Point connection, with a frame size of
> 2048. Any hints on whether any of that needs tuning would be great.
>
OK. I don't think the pci-e slot is your problem.
> I'm not really sure that bandwidth is an issue - perhaps latency more
> than that. I don't think the amount of data is what's causing the
> problem; rather the number of transactions that the e-mail system is
> trying to do on the volume. The file sizes are actually pretty small -
> 1 to 4 Kb on average, so I think it's the large number of these files
> that it has to try to read rather than streaming a large amount of data.
> Both the SAN and the iostat output on both dom0 and domU indicate
> somewhere between 5000 and 20000 kB/s read rates - that's somewhere
> around 40Mb/s to 160Mb/s, which is well within the capability of the FC
> connection. The SAN is indicating I/O operations between 500 and 1500
> I/O requests per second, which I assume is what's causing the problem.
>
What's the size of those requests? 4 kB? 1500 IOPS * 4kB/IO == 6000 kB/sec (6
MB/sec).
What kind of disk drives are you using on the Compellent storage array,
on the RAID set for this LUN?
1500 random IOPS requires at least 10x 7200 SATA disks (if using SATA).
each SATA 7200 rpm sata disk can do max around 150 random IOPS.
each 15k rpm SAS disk can do max ~300 random IOPS.
It's easy maths. Big write-back cache in the storage array will help
though.
-- Pasi
> Again, any tips on what to look at next would be greatly appreciated!
> Thanks for all the advice so far!
>
> -Nick
>
> >>> On 2009/08/27 at 03:00, Pasi Kärkkäinen<pasik@xxxxxx> wrote:
>
> On Wed, Aug 26, 2009 at 12:07:55PM -0600, Nick Couchman wrote:
> >
> > Doesn't really seem to make a difference which way I do it...I still
> see pretty intense disk I/O.
> >
> > Here is some sample output from iostat in the domU:
> >
> > Device: rrqm/s wrqm/s r/s w/s rkB/s wkB/s
> avgrq-sz avgqu-sz await svctm %util
> > xvdb 12.20 0.00 1217.40 26.20 9197.60 530.80
> 15.65 29.66 23.47 0.80 100.00
> >
> > Device: rrqm/s wrqm/s r/s w/s rkB/s wkB/s
> avgrq-sz avgqu-sz await svctm %util
> > xvdb 18.40 0.00 1121.20 19.60 8737.60 691.50
> 16.53 32.97 29.13 0.88 100.00
> >
> > Device: rrqm/s wrqm/s r/s w/s rkB/s wkB/s
> avgrq-sz avgqu-sz await svctm %util
> > xvdb 27.80 0.00 1241.40 29.20 8158.40 377.90
> 13.44 42.59 33.73 0.79 100.00
> >
> > Device: rrqm/s wrqm/s r/s w/s rkB/s wkB/s
> avgrq-sz avgqu-sz await svctm %util
> > xvdb 31.60 0.00 1256.60 35.00 9426.40 424.00
> 15.25 42.06 32.44 0.77 100.00
> >
> > Device: rrqm/s wrqm/s r/s w/s rkB/s wkB/s
> avgrq-sz avgqu-sz await svctm %util
> > xvdb 57.68 0.00 1250.50 17.76 8588.42 352.99
> 14.10 51.36 40.60 0.79 99.80
> >
> > the avgqu-sz is anywhere from 11 to 75, and the await is anywhere
> from 20 to 50. %util is always around 100.
> >
>
> Well.. it seems your SAN LUN is the problem. Have you checked the load
> from the FC Storage array?
>
> Or then the problem is in your FC HBA. Have you verified the FC link is
> at full speed?
>
> Are the FC switches OK?
>
> Do you have up-to-date HBA driver in dom0? Are the HBA/Switch/Storage
> firmwares up-to-date?
>
> -- Pasi
>
>
>
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