The old and original VHD (Virtual Hard Disk) format was first introduced by Connectix which was later bought by Microsoft and turned into Microsoft Virtual PC.  It contains everything a physical HDD (Hard-Disk Drive) would contain including disk partition and a file system (NTFS, FAT32, etc.)  It is the storage foundation for VMs (virtual machines).

VHDX is the new file format bringing several improvements to match Hyper-V 3.0 in Windows Server 2012’s increased processing and memory capacity.

Bigger, Better & Faster

  • Size – Format supports up to 64 TB which is 32 times more than the 2 TB limit found in VHDs
  • Protection Against Disk Corruption – provides corruption protection during power failures for VHDX files by a logging mechanism for the updates of the VHDX metadata structures.
  • Support to run on large sector disks without a performance hit.
  • Larger block sizes up to 256 MB for dynamic and differencing disks resulting in better performance with workloads that allocate in larger chunks such as RAW images and videos.
  • Stores custom metadata in the VHDX file including OS version, patches applied or whatever you can think of up to 1024 entries of 1 MB.
  • Takes advantage of Offline Data Transfer (ODX).


  • Speed improvements of creating VHDX files with the ability to offload to a SAN if the storage vendor supports ODX.
  • Merge and mirror operations are also offloaded to hardware which is specifically for merging snapshots or live storage migration.


According to documentation we get “efficiency in representing data (also known as “trim”), which results in smaller file size and allows the underlying physical storage device to reclaim unused space. TRIM requires physical disks directly attached to a virtual machine or SCSI disks in the VM, and TRIM-compatible hardware as well as Windows Server 2012 on hosts & guests.