Implement Dynamic Hard Disk Image

[jhermsmeier]

A dynamic hard disk image is a file that at any given time is as large as the actual
data written to it plus the size of the header and footer. Allocation is done in blocks.

As more data is written, the file dynamically increases in size by allocating more blocks. For example, the size of file backing a virtual 2-GB hard disk is initially
around 2 MB on the host file system. As data is written to this image, it grows with
a maximum size of 2 GB.

Dynamic hard disks store metadata that is used in accessing the user data stored
on the hard disk. The maximum size of a dynamic hard disk is 2040 GB. The actual
size is restricted by the underlying disk hardware protocol. For example, ATA hard
disks have a 127-GB limit.

VHD Functions:

• Create
  (Creates a VHD image file, either using default parameters or using an existing virtual disk or physical disk)
• Compact
  (Reduces the size of a VHD backing store file)
• Expand
  (Increases the size of a fixed or dynamically expandable VHD)
• Mirror
  (Initiates a mirror operation for a virtual disk)
• Resize
  (Resizes a virtual disk)