copies over the content from the source VHD to the target.
VHDX
The VHDX format was introduced in Windows Server 2012 to address some of the
scalability and, to a lesser extent, performance challenges of the VHD implementation.
VHD can still be used in Windows Server 2012 Hyper-V, but the recommendation is
always to use VHDX unless backward compatibility is required with Windows Server
2008 R2 Hyper-V or, at the time of this writing, Windows Azure IaaS (which currently
supports only VHD).
The same capabilities for VHD apply to VHDX, such as Boot from VHDX, the three
types of VHDX (dynamic, fixed, and differencing), and native mounting and use
within Windows Server 2012. VHDX builds on VHD, and it provides new scalability
and options.
VHDX supports a maximum virtual hard disk size of 64TB, 32 times that of the VHD
format. In all my years of consulting with the largest companies in the world, I have
never seen an NTFS volume bigger than 14TB. There is no workload that could not be
contained within the new VHDX format, removing the few cases where pass-through
storage was previously required because of size limitations with VHD. While it is true
that the new ChkDsk features introduced in Windows Server 2012 will allow larger
NTFS volumes because volumes can now have any corruption fixed with only seconds
of downtime instead of hours or days, I still don’t believe it will be common for any
organization to require single volumes bigger than 64TB.
VHDX also leverages an improved logging mechanism for updates to the VHDX
Metadata, which protects against corruption in the case of unplanned events such as
power loss. In addition, VHDX features the ability to have custom Metadata stored,
which can be useful to store user notes about a virtual hard disk. The TRIM function
is supported for VHDX files, which allows space to be reclaimed, provided the
hardware is TRIM-compatible.
VHDX files automatically align with the underlying physical structure of the disk,
giving the most optimal performance, and also leverage larger block sizes for dynamic
and differencing disk, giving better performance. When you’re using dynamic VHDX
files, the performance difference is even more negligible than with VHD, making
dynamic the default choice when provisioning new VHDX files. However, the same
guidance with VHD does apply if you don’t have a good monitoring solution in place;
to ensure that you don’t run out of physical disk space, you would still use fixed
VHDX format.
When VHDX is combined with the SCSI controller in Windows Server 2012 R2, the
VHDX can be dynamically resized while being used by a running virtual machine,
allowing the disk to be expanded and even shrunk, provided there is sufficient
unpartitioned space in the volume.
VHDX files can also be shared by multiple virtual machines in Windows Server 2012