Hyper-V 2TB limit: A bug turned into a feature for the next version

A few weeks ago I decided to upgrade the storage system in my server. Besides the disks for the OS and VHD files, I have a RAID5 volume of four disks for data storage. As 1 TB disks are becoming cheaper with the day, I decided to replace the four 500GB disks with four 1 TB disk to upgrade the data store from 1.5 TB to about 3 TB. This was the end of a dream.

After replacing the disks and creating the new 3 TB RAID volume, I noticed that MBR disks are no longer an option for disks larger than 2 TB. Fortunately this is no problem when you run Vista or Windows Server 2008, because those OS-es support GPT (GUID Partition Table) disks which can be larger than 2 TB. In my server setup, one of the VMs uses the data store as a physically attached disk. Ben Armstrong has a nice description how to do this at the Virtual PC Guy’s Weblog.

I was just fine with my new 3 TB volume. I partitioned and formatted the volume from the parent partition (host OS). Set the disk in the offline state. Attached the volume to the file server VM and booted the VM. That’s when things went down hill. From the VM there was no way to access the newly created volume. No matter what I did, all I got were messages about disks being write protected or device I/O errors. Fortunately my post in the Technet Virtualization forum proved I was not getting crazy at all.

The current version of Hyper-V will not support physically attached disks larger than 2 TB. This will be a feature of an upcoming version. I’m actually quite sad that in a time that prices of 1 TB disk are only 120 euros, the coming RTM release of Microsoft’s server virtualization technology will not support file servers with volumes larger than 2 TB.

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