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

You might have seen my other replies, but here you go...30 disks total active in the SAN, with 2 hot spares.  FCATA drives, 7200 RPM.  I can't really modify the SAN controllers much - they have 4GB read and write caching - I suppose I can ask the vendor if they go any higher on that, but I think that's about the highest it goes.  I may be able to purchase some SSD, but that gets real $$$$ real fast, and I'd rather try out some faster drives - maybe real FC at 10K or 15K RPM.


>>> On 2009/08/27 at 08:18, "Fajar A. Nugraha" <fajar@xxxxxxxxx> wrote:

On Thu, Aug 27, 2009 at 8:54 PM, Nick Couchman<Nick.Couchman@xxxxxxxxx> wrote:
> 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?

If dom0 iostat says near 100% usage, then yes, most probably it's a
storage 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.

How many disks you have in your SAN? How many disks are in use
exclusively by this system? A typical SATA disk handles < 100 random
IOPS, so that might be the issue, and increasing the number of disks
(and configure them to be used evenly) seems to be the solution.

As to how to reduce number of IOPS, well, I'm not really sure there's
a way to do it that doesn't involve changing your application.

Some things to try :
- if the load is bursty then usually adding more writeback memory
cache in SAN/SCSI controller helps.
- If it's mostily temporary files then using something like ext4 which
has delayed allocation should help.
- Another method would be switching to zfs and adding some SSD for
ZIL, but this belongs in a different list :P


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