LUN Disk Alignment on SAN and Guest OS

[einstein-a-go-g]

Due to another VMware/NetApp related issue, NetApp support have been looking over our perstat logs from our NetApp filers, the NetApp Engineer spotted and raised some health issues with our VMware LUNS, they are misaligned due to the high numbers of partial write activity, as well as partial reads.

My understanding of LUN/Disk alignment is that one the LUN must be aligned, and for a NetApp LUN the LUN type specified must be VMWARE (when using VMFS), but also the partition OS must be aligned in the virtual machine (vmdk).

I've read a recent document , which states

Note: Aligning the boot disk in the virtual machine is neither recommended nor required.

Align only the data disks in the virtual machine.

I assume neither recommended or requried means, it's difficult to do, because when we use P2V we have no control over where the OS partition is put on the vmdk, and when we've installed OSes for our templates, the partition is already created buy the install routine.

The only way for the boot partition to be correctly aligned, is it to be created manually before we install an OS, but this still leaves us with the P2V, unless we shutdown all our VMs, and use partition magic to move the partition 64 bytes!

All our data disks in the virtual machines are stored on LUNs via Snapdrive.

Any one like to comment, or have found a nice utility to move the partiton offset by 64 bytes, although VMware neither recommend or required.

[java_cat33]

I know that with ESX 3.5 that if you create your vmfs datastore via the VI client the lun is aligned. I assume it is the same for all of ESX 3.x

I've never done any manual alignments within a VM, however I do know there are some good guides for Exchange servers etc.

Here's a link to some discussions further on this...

Message was edited by: java_cat33

[dmaster]

hi einstein-a-go-go

i am wondering if restoring a partition magic image on a pre alligned c-drive is possible at all with preserving the disk allignment.

but i never tried it.. maybe this is indeed possible now with Partition Magic.

Until now the only working approach we found is with the use of diskpar or diskpart for the datadrives.

[einstein-a-go-g]

yes, read that, but it doesn't answer the question of how to align a boot drive?

or why should you align a boot drive, when VMware says don't bother!

I know if you are using ASIS wiht NetApp you could obtain savings if aligned, but if you cannot easily align a boot drive, and you can only align data drives, I cannot understand why NetApp engineers afre moaning about the health of our LUNS, because everybodys boot drives when using NetApp will be mis-aligned!

you cannot change it!

[einstein-a-go-g]

and if you LUNs are misaligned, is it really noticeable!

or is it like the which is faster RAID 1 or RAID 5?

[dmaster]

Theoritically you will get a maximum of 10% performance increase with disk io..

But, we never had any performance issues caused by not aligning the disks. (we have here around 1500 vm's mixed. prod,dev,qa)

But if you will get the absolute max from it, you need to re-allign.

about the comparison RAID1 or RAID5 mostly RAID1 is faster.

[einstein-a-go-g]

Umm, created a bootable aligned Win2k3 R2 Standard Edition template. Don't see any difference!

I think we may chance the existing templates, now that we know, but I don't think we will be changing any of the existing P2V or Production VMs

Thanks to all that replied.

[wpatton]

Aligning a boot lun is not required nor recommended because of many of the risks and just the pure deployment headaches to try to do it to a system disk, but really, the main issue is that little to none of your actual IO on any production system will be using the boot drive. Alignment only benefits high IO drives/LUNs.

Windows/Linux/OS will be the only thing running on the boot lun, and the little IO those require gain no benefits from alignment. As a VMWare rep once told me, "If you are running Exchange, which benefits greatly from alignment, from your boot drive; you have bigger problems than alignment can address."

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[einstein-a-go-g]

I sent the VMware document to the NetApp Engineer, and this was the response:-

"As for the VMware white paper, it is full of caveats and assumptions. I would take it as a general guide, not specific to any particular storage. Our particular storage is significantly impacted by partial writes."

The storage system we are discussing here is NetApp Filer 3040Cs.