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VHD and VHDX File

VHD and VHDX file extensions are used by files in Virtual Hard Drive file format. This file format was introduced by Microsoft and Connectix to store virtual hard drive representation in a file. VHD is an older version of the format which supported up to 2TB of storage while VHDX is a newer format with support of up to 64TB of storage. VHDX also has built-in protection against power failures and prevents performance degradation on large sector count physical disks.

Following disk formats are supported by VHD and VHDX:

  • Fixed disk image. VHD or VHDX file is allocated with fixed size and information is stored in a raw form (matches raw hard drive representation). 512 byte footer with disk metadata is stored at the end of the file.
  • Dynamically allocated disk image. In this case size of VHD or VHDX disk image can change depending on space allocated by files inside. The disk image will grow when more files are copied to it.
  • Differencing hard drive image. In this scenario VHD or VHDX file will store the original state of the disk and all incremental changes which happened to it. For hard drive user disk content will look exactly the same as it would be on a normal hard drive but due to incremental storage many advanced features are possible. This way of storage is similar to how transactional database stores tables log in order to be able to rollback transaction to a certain point. VHD and VHDX in differencing mode provide similar functionality by allowing a roll back to any point in hard drive image history. It is possible to convert VHD or VHDX file from differencing mode to a fixed or dynamic hard drive image by mergin all changes into it.
  • VHD or VHDX file is linked to an actual physical hard drive.

Both VHD and VHDX formats are supported by Hyper-V – Microsoft’s native hypervisor. Oracle VirtualBox, Red Gate Software and Windocks also support VHD hard drive images for SQL database cloning.

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