Virtual disk formats

Windows Server 2012 R2 Hyper-V supports the original VHD disk image file and the new VHDX format. The original VHD format was created by a company called Connectix for its Virtual PC product. Microsoft later acquired the product and used the VHD format for all its subsequent virtualization products, including Hyper-V. There are three types of VHD files, as follows:

- Fixed hard disk image An image file of a specified size in which all the disk space required to create the image is allocated during its creation. Fixed disk images can be wasteful in terms of storage because they can contain large amounts of empty space, but they are also efficient from a performance standpoint because there is no overhead due to dynamic expansion.
- Dynamic hard disk image An image file with a specified maximum size, which starts small and expands as needed to accommodate the data the system writes to it. This option conserves disk space but can negatively affect performance.
- Differencing hard disk image A child image file associated with a specific parent image. The system writes all changes made to the data on the parent image file to the child image, to manage disk space or to facilitate a rollback at a later time.
VHD images are limited to maximum size of 2 TB and are compatible with all versions of Hyper-V and Microsoft Type II hypervisor products, such as Virtual Server and Virtual PC.
Windows Server 2012 introduced an updated version of the format, which uses a VHDX filename extension.
VHDX image files can be as large as 64 TB, and they also support 4-KB logical sector sizes to provide compatibility with new 4-KB native drives. VHDX files can also use larger block sizes (up to 256 MB), which enable administrators to fine-tune the performance level of a virtual storage subsystem to accommodate specific applications and data file types. However, VHDX files are not backward compatible and can only be read by Windows Server 2012, Windows Server 2012 R2, Windows 8, and Windows 8.1 Hyper-V servers. If migrating your VMs from Windows Server 2012 R2 to an older version of Hyper-V is even a remote possibility, you should continue using the VHD file format.