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VM OS disk alignment, should it be 32 or 64

What's the correct disk alignment setting for a guest machine running Windows 2003.  TR-3749 version 2 page 95 states "create partition primary align=32, but MS states that it should be 64 unless otherwise.  So can someone confirm the correct alignment settings for a partition that's used in Windows 2003 (200GB+ partition).  Should it be 32 or 64?



Re: VM OS disk alignment, should it be 32 or 64

i also found another link that talks about 64 alignments.

Re: VM OS disk alignment, should it be 32 or 64

did a little more digging around the net and found a post on OS disk alignment on Netapp here.  per some of the comments it appears that it's up to you if you wanted to use either 32 or 64 alignments.  If this is the case then what is mbralign configured to use when you run it against a vmdk file?

Re: VM OS disk alignment, should it be 32 or 64

I would say that either would work just fine however lets consider the standardization of alignment for the cloud.  Some vendors recommend different sizes based on their own filesystem and what is best for them.  It is the opinion of many that making the starting offset 1mb as that is compatible with all storage vendors and allows for maximum portability should a migration be required from one vendor to another.  MS has elected to use the 1mb offset for Window 2k8 and Windows 7 and this is a move in the right direction.

Please have a look at Vaughn Stewarts blogs which he details alignement and provides links to others documentation.


Chris Gebhardt

Re: VM OS disk alignment, should it be 32 or 64

If i'm interpretting everything correctly, 64 should be used even though 32 or 64 would work.  Thanks.

Re: VM OS disk alignment, should it be 32 or 64

OK some background first.

NetApp: The logical block size of NetApp storage is: 4KB

EMC: Logical block size is 64KB

So.....In my experience,

  • Windows 2008:
    • You do not need to align the disk as windows will altomatically do this for you. By default, for any disk larger than 4gb in size the partition alignment is 1mb. This is same in Vista and windows 7 too I beleieve. As this is properly divisible by 4KB and 64KB and all of their (when I say all, I mean most of its practically used) permutations, we are all good and no manual alignments are necessary.
  • Windows 2003:
    • Since windows 2003 and most other old OS's start their first logical partition at sector (logical block) 63,you need to manually align the disks and the starting offset should be something appropriately divisible by 4KB in NetApps case (so both 32 and 64KB would do) and 64KB in EMC's case (so 64KB for EMC)

Refer to the below document for a good understanding.