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Re: [Xen-users] Xen and I/O Intensive Loads

I think that my previous native system from which I migrated this VM was handling the load *better* than this one is, but, unfortunately, it's not a one-to-one comparison.  The previous system was a PowerEdge 2650 with 2 x CPUs and 4GB of RAM, and a single 4Gb FC connection to the SAN.  This system was seeing some I/O issues, but was also seeing some pretty severe CPU load.  Now I've eliminated the CPU bottleneck by moving it into a VM that has 4 x CPUs and 4GB of RAM, but seem to be up against the I/O bottleneck, now.  Due to the complexity of the software installation, switching the load over to the dom0 on the box really isn't an option for testing.  But, it seems that you're probably right, since iostat in both dom0 and domU show very similar statistics.
My real question is, what can I do to alleviate it?  Is it really a SAN issue?  Will tuning the filesystem (even if that means recreating the filesystem) help reduce the number of I/O operations per second?  I guess I have a few things to investigate, and I may file a support case with the SAN vendor and request some assistance from them.

>>> On 2009/08/27 at 03:36, "Fajar A. Nugraha" <fajar@xxxxxxxxx> wrote:
On Wed, Aug 26, 2009 at 11:01 PM, Nick Couchman<Nick.Couchman@xxxxxxxxx> wrote:
> Hi, folks,
> I'm attempting to run an e-mail server on Xen.  The e-mail system is Novell
> GroupWise, and it serves about 250 users.  The disk volume for the e-mail is
> on my SAN, and I've attached the FC LUN to my Xen host, then used the
> "phy:/dev..." method to forward the disk through to the domU.  I'm running
> into an issue with high I/O wait on the box (~250%) and large load averages
> (20-40 for the 1/5/15 minute average).

Just to be clear : can a native system handle your load?
Try iostat on both dom0 and domU. My guess is that you're I/O bound,
and even moving to it a native physical server won't help, since the
bottleneck is in the disk.

> I was wondering if anyone has ideas
> on tuning the domU to handle this - is there a better way to forward the
> disk device through, should I try using an iSCSI software initiator in the
> domU,

Some past threads on this list suggest otherwise. iSCSI in domU
provides worse performance compared to (for example) iscsi in dom0 and
passing the disk using phy:/.

> or is it just a bad idea to put an I/O load like this in a domU?

If it works on native system it should work on a domU.


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