Microsoft Virtualization Discussions

LUN/VM Misalignment


Here is something interesting.  We created a new Hyper-V host a few days
ago.  Lun type is Hyper-V created in system manager.  I had no VM’s on it and
checked Perf advisor…it showed nothing for aligned or unaligned WAFL ops (makes
sense no vm’s running).  I then created a
VM which is a 2k8 R2 sp1 VM which if I understand properly is already aligned.  (I ran msinfo32 and the number was divisible
by 4096 which I read is correct).  I
wanted to see what perf advisor would then show somewhat expecting the Wafl
Unaligned to be 0%. But, it isn’t.  Is
this indicative of a problem?  Is the
unaligned WAFL Ops Graph representative of misalignment?  I thought I read it is a good indicator, but,
maybe I am misjudging the counter altogether.
If not, then I have no idea why this is misaligned at all since the Lun
type is right and the VM is correct also.



As long as you didn't do something funny to misalign those W28 images you should be OK. I have seen some provisioning tools mess that up. However the Unaligned counter will likely never be 0% since windows allows Application to write on 512 byte sectors so some apps and some function naturally generate misaligned IO. A little misaligned IO however is not a problem, the system can easily handle it and will not impact the performance of the VM. However large amounts of misaligned IO is BAD. What sort of a percentage are you seeing?



You can run "stats show -i 2 lun:lunname:write_align_histo" on the console  for instance to see buckets-usage and compare it with your Graphs

Regars Bernd

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