Dynamically Expanding VHD not Compacting in Hyper-V

OK, I feel duty bound to report this as I only accidentally found an answer, so hopefully this’ll help someone else.

Long story short – Hyper-V compact doesn’t work, but “Diskpart Compact vDisk” command does!

So, you have a massive Hyper-V VHD or VHDX file and you need to shrink it down. So you open the Hyper-V toold, go to “Edit Disk” compact, wait the 4 billion years or so that it takes to finish and look at your VHD size aaaannddd… No change! Hyper-V is not compacting your VHD file at all, it’s still the same size, or in my case, slightly larger!

In trying to find a solution a lot of pages at this point suggest setting up a new VHD and copying the contents across before deleting the old one – in my case I couldn’t do this because the VHD was well over half of my available disk space, and it was big – over 1TB so it’s not trivial to be able to chuck a bit more disk space in to accomodate it.

So, anyway turns out that you can compact a VHD or VHDX using the Diskpart command. Now, I’m not much of a command line guy but I have to say I’ve grown quite fond of diskpart, it’s a really well written and versatile tool. So it’ll crunch your VHD’s where Hyper-V fails, and the kicker is that it took about 1/8th the time the proper Hyper-V tool took.

The procedure is like this:

Run an administrative command prompt
Type “Diskpart” (No Quotes)
Type the following commands:

select vdisk File=”I:\path\to\your.vhd”
attach vdisk readonly
compact vdisk
detach vdisk

That’s it. My particular VHD had been set to be a “dynamic volume” in it’s Virtual Machine’s disk management (It was a dynamically expanding VHD, but also, in disk management of its VM it was further configured as a “Dynamic Volume”) Perhaps that had something to do with it, but in all honesty I don’t know why this works and Hyper-v fails to compact, and I don’t know if it’ll work in your case, but it got me out of a hole.

Good Luck!