A virtual hard drive (VHD) can be created on the parent partition’s volume with access granted to the child partition. The VHD operates as a set of blocks, stored as a regular file using the host OS file system (which is NTFS).

Within Hyper-V there are different types of VHDs, including fixed size, dynamically expanding, and “differencing” disks:

• Dynamically expanding. This type of virtual hard drive starts small but automatically expands to react to need. It will expand up to the maximum size indicated when the virtual hard drive is created. “Dynamic,” in this context, is sort of a misnomer. Dynamic implies that the size of the virtual drive changes up and down based on need. But in actuality, the hard drive will keep expanding until it reaches the maximum limit. If you remove content from the virtual hard drive, it will not shrink to meet the new, smaller capacity.

• Fixed. This type of virtual hard drive is created at a certain size and it remains at that size. While fixed virtual hard drives are created to be a certain size, their size can be adjusted—just not automatically. To change the size of the virtual hard drive, you use the Hyper-V Manager Edit Disk action to make changes.

• Differencing. The third type of virtual hard drive is the differencing drive. In this scheme, a parent disk and a child disk are used. In this configuration, the parent disk contains a baseline hard disk image with the operating system (and maybe some applications and data). When this parent drive is configured, a differencing disk is allocated to a child. When the virtual machine to which the child virtual disk is assigned makes changes, those changes are made to the child differencing disk and the parent disk is left alone. Several child disks may be assigned to a parent disk, thus allowing multiple virtual machines to share the same parent disk. Differencing disks can also be chained together. The changes on one differencing disk can be merged to the parent disk. On the other hand, a new virtual hard disk can be created via the merger of a child and parent disk. This leaves the parent disk unaltered. When a differencing disk is created, a parent disk needs to be specified. The size of the differencing disk grows as space is needed.

In Hyper-V, you can expose a host disk to the guest without putting a volume on it by using a pass-through disk. Hyper-V allows you to bypass the host’s file system and access the disk directly. This disk is not limited to 2,040 GB and can be a physical hard drive on the host or a logical one on a SAN. Hyper-V ensures that the host and guest are not trying to use the disk at the same time by setting the drive to be in the offline state for the host. Pass-through disks have their downsides. You lose some VHD-related features, like VHD snapshots and dynamically expanding VHDs.

Source of Information : Microsoft Virtualization with Hyper V


Subscribe to Developer Techno ?
Enter your email address:

Delivered by FeedBurner