topic LUN/VM Misalignment in Microsoft Virtualization Discussions

LUN/VM Misalignment

BOB_KOCHANSKI — Thu, 05 Jun 2025 06:30:52 GMT

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.

Re: LUN/VM Misalignment

bernd_wolters — Fri, 30 Mar 2012 13:52:59 GMT

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

Re: LUN/VM Misalignment

keitha — Fri, 30 Mar 2012 16:02:55 GMT

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?

Keith