Hyper-V server HDD allocation unit

Hello experts,

A server will be running Hyper-V soon. All virtual machines and their virtual HDDs will be stored on D driver of the physical machine.

What is the best allocation unit size for formatting the physical D drive? Shall it be the biggest available option (64k) for getting better performance?
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Svet PaperovConnect With a Mentor IT ManagerCommented:
Just giving out my experience… I cannot say with certainty what the allocation unit size should be.

I had the same question and I did a lot of testing without having conclusive results.

My setup was with a Dell MD3000i iSCSI external disk array. The virtual disks (on the MD3000i) were configured as for database usage; the other option was as for file server usage, so the Dell engineer advised me to use the database option for the volumes that will host VHD files to achieve better performance (I wasn’t sure but I followed his advice).

I created virtual servers with dynamic and fixed size VHD files (as with Hyper-V R2 there were no differences in the performance between both) on physical volumes with 4, 32 and 64kB allocation size (physical should be in “” because there were actually RAID 10 virtual volumes of 6 15k rpm SCSI disks on MD 3000i)

I used SQLIO test tool to run multiple different read/write tests.

The conclusion was that there was almost no difference between the allocation sizes.

So, I played safe and I went with 32kB allocation unit size.

Hope this helps... still looking for the write answer.
MuhajreenAuthor Commented:
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