Q: My Hyper-V Virtual Hard Disks are stored on a SAN, and I want to use thin provisioning on the SAN: What special steps are required to format the file system within the Virtual Hard Disks?

A: There are different types of thin provisioning; however, they all fundamentally work by not allocating the full disk space of a disk at creation time but rather allocate space as data is written or blocks accessed.

The thin provisioning capability exists at a software level with the dynamic Virtual Hard Disk (VHD) type of disk. It expands an initially very small VHD file as data is written to it, up to the maximum size configured for the VHD.

This can also be achieved at a hardware storage level by using a SAN. Even though the SAN is told to create a large file, for example a 80GB fixed size VHD, it allocates only a very small amount of disk space, then allocates additional real storage as data is written to the VHD.

When you format the file system for the VHD within the virtual machine (VM), make sure a quick format is performed. It doesn’t check for bad sectors, so it doesn’t try to access all the blocks of the VHD, which would cause space to be provisioned on the SAN and defeat the purpose of thin provisioning. Using the Quick Format option (see screen shot below) applies to storage used for regular volumes and for volumes that are part of Cluster Shared Volumes.


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