Q. What are accelerated VHDX operations in Windows Server 2016 with ReFS?
A. Traditionally operations with dynamic VHDX files that involve growing the file incur performance penalty as the blocks have to be zeroed before used by the file. With the new Windows Server 2016 accelerated VHDX operations feature which applies to both VHD and VHDX files when a file is extended instead of zeroing out the actual blocks on disk ReFS updates the metadata to respond back that the content is zero without incurring the performance penalty of actually zeroing them out. This means new VHD/X creations, extend VHD/X operations and growing dynamic VHD/X files will all be near instantaneous.
The same updating of metadata will also be used when merging checkpoints which traditionally resulted in blocks being copied from a differencing disk to its parent but with the ReFS accelerated operations the metadata is just updated to point to the data already on disk that is now needed and data not needed deleted.
These improvements are important as checkpoints (production) are now used as part of backup operations.