The Real (and Virtual) Adventures of Nathan the IT Guy

Jul 11 2013   9:52PM GMT

VMware/Hyper-V and SAN Aligned Virtual Disks

Nathan Simon Nathan Simon Profile: Nathan Simon

If you are working with VMware of Hyper-V then you know what the heck I am talking about. Aligning the OS’ data blocks with the SAN.

There are many ways to do this, but a lot of people are unaware of easy ways to do so.

Some people know of the WMIC command, and some people know of Sysinternals DiskEXT. So how do you use these tools? Well let me show you.


type this at the command prompt : wmic partition get BlockSize, StartingOffset, Name, Index

You will get the following output:

 BlockSize Index Name StartingOffset

512 0 Disk #1, Partition #0 1048576

512 0 Disk #2, Partition #0 1048576

512 0 Disk #0, Partition #0 1505755136

So what do those numbers mean to you? well as long as the StartingOffset (1048576 or 1505755136 in this example) are divisible by 4096 you are aligned with your storage.

Sysinternals is a little bit of the same, use this is WMIC gives you an error message.


Disk Extent Dumper v1.1
Copyright (C) 2001-2007 Mark Russinovich
Sysinternals –

Volume: \\?\Volume{0ea1f149-17f2-11e2-be66-806e6f6e6963}\
Mounted at: <unmounted>
Extent [1]:
Disk:   0
Offset: 1048576
Length: 367001600
Volume: \\?\Volume{3053fd39-1885-11e2-be70-e006e6b16937}\
Mounted at: D:\
Extent [1]:
Disk:   1
Offset: 105906176
Length: 319965626368
Volume: \\?\Volume{739a5bc3-185d-11e2-be6e-e006e6b16937}\
Mounted at: X:\
Extent [1]:
Disk:   2
Offset: 4194304
Length: 16025387008
Volume: \\?\Volume{0ea1f14a-17f2-11e2-be66-806e6f6e6963}\
Mounted at: C:\
Extent [1]:
Disk:   0
Offset: 368050176
Length: 119664541696


As you can see, the disk are all aligned to the SAN.

If you are in a VMware specific environment and have miss-aligned VMs, the easiest way to re-align VMs is to V2V them. VMware Converter 5.x can create an optimized partition layout, if you are looking at cold cloning and hoping it will align blocks, it won’t. You need to warm clone the server. Believe in the software though, it works! Block level replication baby! 🙂

Oh and don’t forget to disable SSL encryption to increase your transfer rates by 150%! Go here to find out.

 Comment on this Post

There was an error processing your information. Please try again later.
Thanks. We'll let you know when a new response is added.
Send me notifications when other members comment.

Forgot Password

No problem! Submit your e-mail address below. We'll send you an e-mail containing your password.

Your password has been sent to:

Share this item with your network: