Tuesday, March 3, 2009

Virtual Machine Disk Options

A review of the virtual machine disk options that are available on Microsoft Windows Server 2008 Hyper-V.

In order to make virtual machine storage portable while also delivering performance, Microsoft developed the Virtual Hard Disk (VHD) Image Format Specification which includes details of how to read and modify data contained in a VHD file. Microsoft provides the VHD file format specification to third-party developers under a royalty-free license, and many vendors, including Citrix, have adopted and use the format for their virtualization products. If you are interested in obtaining more details on the VHD file format, you can download the specification file from the Microsoft website.

Microsoft Windows Server 2008 Hyper-V uses the VHD format to encapsulate virtual machine data (operating system, application, and data files) into one or more files that are equivalent to physical drives associated with a traditional server. Therefore, if you browse the virtual machine folders on a Hyper-V server, a virtual hard disk is simply stored as a file with a .vhd extension on an attached physical disk. Virtual machines connect to a virtual hard disk through a virtualized IDE or SCSI adapter, and Hyper-V provides the mapping between the virtual hard disk and the .vhd file on the physical disk. A VHD can be stored on any IDE, SCSI, iSCSI, SAN, or NAS storage system supported by the Windows Server 2008 operating system.

You can use the Hyper-V Manager MMC or the WMI API to create new virtual hard disks. A virtual machine running on Hyper-V can support a maximum of 260 virtual hard disks through a combination of 4 IDE and 256 SCSI-connected VHDs. The bus type (IDE or SCSI) used to attach a VHD to a virtual machine imposes a size limitation on virtual hard disks. Specifically, IDE-attached VHDs cannot exceed 127 GB, while SCSI-attached VHDs cannot exceed 2 TB.

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