fragmented virtual hard disk.
Differencing A differencing hard disk is linked to a parent virtual hard disk and
stores only the changes from the parent hard disk. As writes are made, the
differencing (child) disk will have the writes committed, while read operations will
be sourced from the parent VHD unless an update was made to the original data or
its new data, in which case the data will be read from the differencing disk. Once a
VHD becomes a parent disk, it becomes read-only. A differencing disk has the name
of AVHD instead of VHD and will grow as data is written, behaving similarly to a
dynamic disk.Figure 2.18 gives an overview of the types of VHD and how they function.
Figure 2.18 The key types of VHD disks
Although there is little performance difference between a dynamic and fixed VHD in
Windows Server 2008 R2, the recommendation for production environments is to use
a fixed VHD. The primary reason is that when dynamic VHDs are used, there is always
the possibility that the underlying storage runs out of space, so as the dynamic VHD
tries to grow, it will fail, causing unpredictable results. If systems have well-defined
processes to monitor disk space usage and alert as required, then the use of dynamic
VHDs in production may be possible. VHD is a published standard by Microsoft, and it
is used by other vendors, like Citrix. The specification can be found on the Microsoft
Download Center website; just search for “VHD specification” at
http://www.microsoft.com/download.
Virtual machines can have numerous VHDs attached to them, but a single VHD
cannot be used by multiple virtual machines at the same time. Hyper-V supports both
an IDE bus and a SCSI bus to connect VHDs to virtual machines. While the IDE bus
must be used for DVD drives and the disk from which the virtual machine will boot,
for all other VHDs, the SCSI bus is recommended for best performance and maximum
flexibility.
It is possible to perform conversions between dynamic and fixed VHDs using Hyper-V
Manager and command-line tools. The conversion process creates a new VHD and