What are the possible reasons why Hyper-V can't compact a VHD?

Question

I am hosting several virtual machines on a host running Hyper-V Server 2008 R2. The disks were originally provisioned on a different server (2008, not R2) about 16 months ago, and were migrated onto this server one year ago. Most guests are running Windows 7.

Recently I was comparing the VHD size to used space reported by the guest, and noticed that several machines had VHD files that were 50-100% larger than actually needed. So I decided to try compacting the VHD files to free up space on the host. I followed instructions I found on the internet, roughly as follows:

Run CHKDSK in the guest machine
Run a defrag in the guest machine (Windows built-in defrag)
Shut down the guest
Open "Edit Disk..." tool in Hyper-V Manager
Select the VHD of interest and choose to compact it

The first time I tried this, it worked flawlessly and reduced the size of my VHD by about 50%. But for the disks of every other guest, I get this error message:

The server encountered an error trying to edit the virtual disk.

'The system failed to compact 'C:\example.vhd'. Error Code: The requested
operation could not be completed due to a file system limitation

No guests have NTFS compression enabled. There are no snapshots of these disks. What else could be causing this error?

Edit: Bonus points for anybody who describes a solution to keep dynamic VHD sizes in check automatically.

Ah, yes. That was the problem. But not why post that as an answer? — Nic, Feb 10 '12 at 03:08
Also the only way to keep dynamic disks from screwing things up is to not allow dynamic disks. — tony roth, Feb 11 '12 at 01:05

score 8 · Answer 1 · answered Nov 23 '15 at 14:18

8

In the interest of Internet search consolidation: Eventually found this link thas an approach that has worked when HyperV compact did not: https://fiddley.wordpress.com/2014/01/27/dynamically-expanding-vhd-not-compacting-in-hyper-v/

In Administrative command prompt, run DISKPART

At the DISKPART command prompt

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

answered Nov 23 '15 at 14:18

Stan

191
1
3

1

This series of commands worked for me, maybe because the guests are all linux. It's important to at least "Save" your VM before you try these, otherwise it will fail. And don't forgot to detach before you try starting your machine again :) – gcode Dec 30 '20 at 04:10

score 7 · Accepted Answer · edited Mar 24 '17 at 23:54

7

Remove the shadow copies from the guest. That will do it!

vssadmin delete shadows /all

edited Mar 24 '17 at 23:54

Oliver Salzburg

4,505
16
53
80

answered Feb 10 '12 at 21:08

tony roth

3,844
17
14

1

If the above command complains about undeleteable shadows, try using `diskshadow` and issue `delete shadows all` in it. – Oliver Salzburg Mar 25 '17 at 01:06

score 1 · Answer 3 · answered Nov 02 '19 at 17:03

Apart from the suggestion of deleting the shadow copies you should use SDelete for zeroing the empty space before compaction, since sometimes it allows compacting much more unused space. For example, for zeroing the empty space in C: you should run this from inside the virtual machine:

sdelete -z c:

In my case I had a virtual disk file with a total size of 80 GB, the first compaction reduced it only to 78 GB, deleting the shadows copies and compacting again reduced it to 72 GB, but after running sdelete the next compaction reduced the file size to 56 GB.

What are the possible reasons why Hyper-V can't compact a VHD?

3 Answers3

Linked