How to resize the qcow2 image without impact the application

Question

My question is about to resize the qcow2 VM image. I have a image with packages and applications built in. For example when creating the image, the size is 40G, but for now it in reality use about 5G disk space. So i would like to shrink it.

My method is like below:

qemu-img convert -O raw VM1.qcow2 VM1.raw
qemu-img resize VM1.raw -20G
qemu-img convert -c -O qcow2 VM1.raw VM1.qcow2

But after i did that, although the size is shrunk a lot, the VM could not be able to boot correctly. I am using linux with KVM/libvirt.

Answer 1

If your guest OS is windows, you can expand the partition from within the guest, while it is booted. First copy your original image (for safety) and use qemu-img resize to add space to the disk image:

cp small_image.qcow2 large_image.qcow2
qemu-img resize large_image.qcow2 +100G

Then boot the windows VM in large_image.qcow2. Open the "Disk Management" utility. Right click on C:, and select either "Extend Volume" or "Shrink Volume"

Your VM guest will now have access to the space added by qemu-img resize.

Answer 2

To shrink a disk, you must do some work on the Guest VM.

An excellent description: http://www.jamescoyle.net/how-to/323-reclaim-disk-space-from-a-sparse-image-file-qcow2-vmdk. I've added defrag, and expanding a disk.

From a windows guest:

• De-fragment your disk so that all the files are moved toward the beginning of the disk.

• Download: http://technet.microsoft.com/en-gb/sysinternals/bb897443.aspx

Fill all free space with 0's.

c:\sdelete.exe -z c:

From a Linux guest, fill all free space with 0's:

dd if=/dev/zero of=~/mytempfile
rm -f ~/mytempfile

On the libvirt host:

mv original_image.qcow2 original_image.qcow2_backup
qemu-img convert -O qcow2 original_image.qcow2_backup original_image.qcow2

To expand a disk, it can all be done on the libvirt host.

mv original_image.qcow2 original_image.qcow2_backup
truncate -s <desired number of Gigabytes>G original_image.qcow2
qemu-img convert -O qcow2 original_image.qcow2_backup original_image.qcow2

In BOTH cases, check your work.

qemu-img info original_image.qcow2