Compact Linux guest virtual hard disk

Dan Craciun
Dan Craciun used Ask the Experts™
on
Hi,

The situation: Windows Server 2012 Hyper-V host, with mixed CentOS 6.4 and Server 2012 guests.

What I'm trying to do: compact the virtual image of a CentOS VM, named XXX, with a vhdx of 43GB.

What I did:
Shut down the VM, booted SystemRescueCD 3.8.1.
Used pvscan to get logical volume information.
Changed /dev/mapper/vg_XXX-lv-root to active:
vgchange /dev/mapper/vg_XXX-lv-root -a y

Ran zerofill -v /dev/mapper/vg_XXX-lv-root
Ran zerofill -v /dev/mapper/vg_XXX-lv-home

Both successful.

Shut down the VM, then tried to compact it using Hyper-V Manager. No effect whatsoever. The vhdx is still at 43GB.

Any ideas what I did wrong and/or how I can successfully compact that VM?

Thanks.

Below you have the disk space usage, as reported by df -h. As you can see, the real space usage is at around 11GB.
Actual disk usage
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Can you try the following method for zeroing out inside the guest?

cat /dev/zero > /tmp/zero.out
sync && rm /tmp/zero.out

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Author

Commented:
Thank you for your reply.

Tried your commands. Still no effect after compact.

Which is strange. Creating the zero.out file should have increased the vhdx size, but that did not happen...
Commented:
OK, after about 8 hours of searching and testing, I found a solution.

Looks like the compact function in Hyper-V manager has problems with Linux guests.

I managed to compact the vhdx from Powershell, using:

Mount-VHD -Path X:\path\to\file.vhdx -ReadOnly
Optimize-VHD -Path X:\path\to\file.vhdx -Mode Full
Dismount-VHD -Path X:\path\to\file.vhdx

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Took about 3 minutes and reduced the size from 43 GB to 18GB.

Tried -Mode Quick too, but with no results.

I posted this so the next person that has the same problem can save him/her self some time.
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Good to know! Are you running 2012 R2? May be this is the reason I couldn't reproduce this in my lab.

Author

Commented:
Nope. Plain 2012. Will upgrade probably in March-June. Don't like being an early adopter\beta tester on production servers.

A tricky one to be sure. Still don't know why -Mode Quick did not work.

I even spent about 2 hours in a hex viewer, mapping the vhdx. I could see where the partitions ended, huge 0 filled blocks and some 11 GB of pure 0's at the end of the file, and still compact did not work.

Weird, but at least I found a solution.
You have done a great job on narrowing this down.

R2 is typically a stable release. The current build number is 9600 and they will continue to be updated. There are updates made in VHDX resizing in Windows 2012 R2, I would suggest testing in R2 and see if this issue has been resolved.

Author

Commented:
Had to do a lot of research to get around what I can only call "a bug" in Hyper-V compacting routines.

Thanks Mazdajai. Will try in R2 to see if the problem is solved.

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