VMware Solutions Discussions

mbralign on boot drive?


Should mbralign be run against the boot drive? I have read elsewhere that aligning the boot drive is not recommended and the only thing that should be aligned are data drives. Most of my VMs only have one drive the boot drive and on a few of them it is quite active so I would like to align them. The articles I have read talk about aligning the boot drive somehow screws up vss service.



Hi hendersond,

Indeed VMware states the following in its document "Recommendations for Aligning VMFS Partitions" (http://www.vmware.com/pdf/esx3_partition_align.pdf😞

Note: Aligning the boot disk in the virtual machine is neither recommended nor required.
Align only the data disks in the virtual machine.

However no reason is given why you shouldn't. I'd say test it and see if you notice performance improvements with aligned boot drives.


Last January, when we were having IO performance issues, it was strongly recommended by the virutualization gurus inside NetApp to align the disks.  VMware may not care, but NetApp does.


My understanding from conversations with NetApp people is that VMware is correct in their recommendation....but from a storage system perspective if you have a lot of unaligned boot vmdk's it just adds up (possibly very noticeably as the environment grows) so is absolutely worth having aligned vmdk's in a template and possibly to likely worth aligning after the fact.


Hello, I'm cross posting the answers to some similar questions posted in the comments of this doc.

>Should mbralign be run against the boot drive?

Absolutely!  The mbralign tool was originally written to deal specifically with boot drives.  It has shown to be useful on data drives as well, however data drives can be migrated using guest based tools as well.

>I have read elsewhere that aligning the boot drive is not recommended...

IMHO, this is a poor (perhaps dated) recommendation.  The boot drive tends to have the guest virtual memory files (swap/pagefile.sys), temp directories, binaries and common libraries.  The result is that the boot drive is very likely to do enough IO (even if you split out the guest virtual memory files) to warrant alignment.

>The articles I have read talk about aligning the boot drive somehow screws up vss service.

I haven't come across anything that documents this, nor can I think of a reason the filesystem starting offset would effect vss usage.  You do want to release any snapshots (VM or VSS) before aligning, perhaps thats the issue?

Hope this helps!


Also and for completion sake, Windows 2008 and Windows 7 are aligned by default so if your VM's use any of these, there's no need for mbralign.



Although its an old post but still wanted to update....

Yes.. they are already aligned BUT if you have performed an upgrade from Windows XP or Windows 2003.. they will be unaligned.




I believe you're right. The Netapp documentation is leaning towards aligning boot disks, at least for new VM's. From the TR3747 document (http://media.netapp.com/documents/tr-3747.pdf😞

Aligning the boot disk in existing virtual machines is not required but will improve performance under some circumstances. If you have diagnosed performance issue due to misalignment, it may be necessary to align boot disks as well as data disks. In any case, it is essential to align templates and "gold image" VMs in order to prevent proliferation of misaligned VMs, and to align any new VMs using procedures in this document.

To get back to VMware's statement. I don't know why it wouldn't be recommended.


I have ran mbralign on the boot drive of many of my VMs and have never had a problem.

NetApp on Discord Image

We're on Discord, are you?

Live Chat, Watch Parties, and More!

Explore Banner

Meet Explore, NetApp’s digital sales platform

Engage digitally throughout the sales process, from product discovery to configuration, and handle all your post-purchase needs.

NetApp Insights to Action
I2A Banner