Q. Why does a fixed-size virtual disk perform better than a dynamic virtual disk?

A. When you create a fixed-size virtual disk, the virtual hard disk (VHD) file that’s created is the size of the VHD. So, for example, if you create a 40GB fixed VHD, a 40GB VHD file will be created in the file system.

A dynamic VDH consumes space in the file system only as content is written to the VHD, so that if you create a 40GB dynamic VHD, a file of only a few hundred kilobytes is created. (The size of the initial file is based on the size specified for the dynamic disk. The initial file stores only the lookup table that will be used to map the virtual disk to the physical drive. A larger VHD needs a larger lookup table, so the initial VHD file size will be larger. The lookup table is about 0.0008 percent of the size of the actual VHD.)

When data is written to a dynamic VHD, the OS has to first find space on the physical drive holding the VHD and update the lookup table; only then can the data be written. This multistep process delays the write operation.

Because a fixed VHD allocates all the space in advance, a fixed-size VHD will perform better than a dynamic VHD. A fixed virtual disk actually performs about as well as pass-through disk access (in which a virtual machine directly accesses a physical drive or LUN) and offers the portability and backup benefits of a VHD.