Windows Server 2008’s Hyper-V has been in public beta for a while now and lots of people have been experimenting with it. One aspect that I am focusing on is storage for those virtualized environments and more specifically the options related to SAN storage.
Virtualization terminology: Before we start, I wanted to define some terms commonly used in virtualization. We refer to the physical computer running the Hyper-V software as host, as opposed to guest, which is the term used for virtual machine. You can, say, for instance, that the host must support hardware-assisted virtualization or that you can now run a 64-bit OS in the guest.
The other term used with Hyper-V is Integration Components. This is the additional software you run on the guest to better support Hyper-V. Windows Server 2008 already ships with Hyper-V Integration Components, but older operating systems will need to install them separately. In Virtual Server or Virtual PC, these were called “additions”.
Exposing storage to the host: A Hyper-V host is a server running Windows Server 2008 and it will support the many different storage options of that OS. This includes directly-attached storage (SATA, SAS) or SAN storage (FC, iSCSI). Once you expose the disks to the host, you can expose it to the guest in many different ways.
VHD or offline disk on the host: As with Virtual Server and Virtual PC, you can create a VHD file in one of the host’s volume and expose that as a virtual hard disk to the guest. This VHD functions simply as a set of blocks, stored as a regular file using the host OS file system (typically NTFS). There are a few different types of VHD, like fixed size or dynamically expanding. This hasn’t changed from previous versions. The maximum size of a VHD continues to be 2040 GB (8 GB short of 2 TB).
You can now expose a host disk to the guest without even putting a volume on it. Hyper-V will let you “bypass” the host’s file system and access a disk directly. This raw disk, which is not limited to 2040 GB in size, can be a physical HD on the host or a logical unit on a SAN. To make sure the host and the guest are not trying to use the disk at the same time, Hyper-V requires the disk to be in the offline state on the host. This is sometimes referred to as “LUN pass trough”, if the disk being exposed to the guest is a LUN on a SAN from the host perspective.
IDE or SCSI on the guest: When you configure the guest’s virtual machine settings, you need to choose how to show the host disk (be it VHD file or offline disk) to the guest. The guest can see that disk either as a virtual ATA device on a virtual IDE controller or as a virtual SCSI disk device on a virtual SCSI controller. Note that you do not have to expose the device to the guest in the same way it is exposed to the host. For instance, a VHD file on a physical IDE disk on the host can be exposed as a virtual SCSI disk on the guest. A physical SAS disk on the host can be exposed as a virtual IDE disk on the guest.
Microsoft, WS2008, Windows Server 2008, Hyper-V, Storage, iSCSI