32

Is there any way to shrink qcow2 image without converting it raw?

I cannot convert it to raw because I don't have disk space enough for raw image.

5 Answers 5

37

Noop conversion (qcow2-to-qcow2) removes sparse space:

qemu-img convert -O qcow2 source.qcow2 shrunk.qcow2

You can also try add compression (-c) to the output image:

qemu-img convert -c -O qcow2 source.qcow2 shrunk.qcow2

Before running above command be sure to shutdown the VM. After you are satisfied with the result, you can rename shrunk.qcow2 to original name your VM config wants.

4
  • 4
    the -c flag in qemu-img convert -c -O is for compression, not shrink... and it can decrease the disk speed... Jul 4, 2014 at 14:41
  • 1
    This creates a second file, though, so you need twice as much free space as the image? Is there a way to shrink it in-place?
    – endolith
    Feb 13, 2019 at 16:37
  • I doubt this is going to shrink much if you do not zero-out the free disk space first. zero-out the free disk space takes very long time and host disk space.
    – Wang
    Aug 17, 2020 at 13:30
  • @Wang if you enable TRIM support first then it should be able to compress better. pve.proxmox.com/wiki/Shrink_Qcow2_Disk_Files
    – Wes
    Dec 15, 2020 at 16:14
15

Try virt-sparsify - it will zerofill the unused blocks in the image and then deduplicate the zeroes.

3
  • Thanks for you answer, but virt-sparsify said it cannot resize disk images and I found virt-resize from that, but looks like I still need to allocate spaces to expand (that I don't have enough spaces on my harddisk). Anyway, Thank you very much for you info.
    – YOU
    Sep 27, 2012 at 8:17
  • 3
    @YOU Current versions of virt-sparsify can work in-place without allocating space for a complete copy of the disk image. Feb 20, 2018 at 5:07
  • From man: Since virt-sparsify ≥ 1.26, you can now sparsify a disk image in place by doing: virt-sparsify --in-place disk.img Feb 14 at 15:18
7

I use virt-sparsify:

virt-sparsify /path/to/source.qcow2 --compress /path/to/output.qcow2
4

I have been successfully using this procedure many times now. In short, first in the virtual guest fill all free space on all partitions with zeroes:

dd if=/dev/zero of=zerofile bs=1M

When done, empty the disk cache with sync, remove the zerofile(s) and shut down the guest. On the host side convert the image (raw to qcow2 in this example):

qemu-img convert -f raw -O qcow2 guest.img guest-copy.qcow2

This will automatically sparsify the image. Also remember to update the vm definition file if the image file suffix is changed.

A benefit of this method over virt-sparsify is that in this method the guest can be up while the zeros are written, and hence the downtime is shorter. If you can afford a longer downtime, then use virt-sparsify, as it takes care of the whole process with just one command, and also gives useful feedback during the process.

2
  • What exactly do you mean by "remove the zerofile(s)" ?
    – Rino Bino
    Sep 20 at 23:29
  • The zerofile is produced by the dd command at the beginning of the recipe. The filename in this case is zerofile, so you must do rm zerofile. Sep 22 at 5:12
0

First, shrink the partition & filesystem from inside the VM. Windows guests can do this from Disk Management. Linux can do this with resize2fs and fdisk. If you are not comfortable with this process use a bootable ISO like gparted instead.

When you are done there should be unused space AT THE END of the disk. For example, in Windows this is what is looks like to have 11MB of free space at the end of the disk (in a real example you should have more than 11MB): Disk Management showing 11mb of unallocated space at the end of the disk

Now that we have moved all your data to the front of the disk we are going to chop off the end. You should be 100% sure you are not about to chop off real data. Some tools count GB/GiB differently so you should probably give yourself some padding (ex shrink partition by 60GB then chop off 50GB). You can always re-grow the partition after the chop.

This command will resize myhd.qcow2 to 200GB, chopping off any data that comes after the first 200GB:

qemu-img resize --shrink myhd.qcow2 200G

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.