VHD and VHDX Virtual Disk Formats
Categories: VHD, VHDX, Restore, Hyper-V 2008 – 2012, 2008R2, Restore, Replication, Alike v3, Alike v4
Starting in Hyper-V 2012, Microsoft allows the use of either VHD or VHDX file formats for virtual disk images. Alike fully supports backup and restore of both VHD and VHDX formats, and can interoperate between the two.
The VHD format is an old format dating back to its initial release in 1997. It supports dynamic, fixed, and differencing disk types, and is widely adopted in virtualization applications. Both Citrix XenServer and Microsoft Hyper-V support VHD format and allow import and export of these files.
VHDX has technical similarities to VHD but is a completely different file format. VHDX files are not supported by Hyper-V 2008R2 or by Citrix XenServer.
VHDX does not suffer from VHD’s 2TB size cap. This resolves a longstanding annoyance with the VHD format. VHDX also has explicit crash-consistency due to its log replay mechanisms, making it a harder format to corrupt. The VHDX format itself is slightly less I/O intensive when reading bitmap information, which can make its performance on long chains of disk images significantly faster.
It also has a data block size that can be configured on a per-disk basis, making it potentially more scalable than VHD, especially when large data sizes and network access factor in.
Alike Support In Detail
Alike can back up both VHD and VHDX disks. When restoring a VM, Alike will create VHDX disks if you are restoring to Hyper-V 20012 or later, even if the source was originally VHD. Similarly, if you are restoring to Hyper-V 2008R2, Alike will always create VHD disks, even if the source was originally VHDX.
Alike replication works the same way. If you are replicating to 2008R2, Alike will write to VHD; if you are replicating to any other Hyper-V environment, Alike will write to VHDX.
You can also restore to either format directly by using Alike’s “Restore to Filesystem” option. This will build one VHD or VHDX file per disk on your VM and write it to the directory of your choosing. Note this restore path supersedes Alike’s OVA support starting in Alike 3.5.0.