Do I need to zero-fill a qcow2 image before compacting it?
I want to recreate a dynamically allocated qcow2 image in order to shrink it. Is it sufficient that all unnecessary files have been deleted, or do I also need to fill the space formerly occupied by those files with zeros? In other words, is qemu-img filesystem-aware?
Solution 1:
Yes, you do need to zero-fill the filesystem if you want to recover the space used by deleted files. And no, qemu-img isn't fs-aware.
I forgot to do this for one VM image I created today (a minimal Debian Sid image for my openstack cloud at work) and it ended up being almost 900MB, even with "-c" for qcow2 compression.
I recreated it after running "dd if=/dev/zero of=/root/zero ; rm -f /root/zero ; shutdown -h now", and the image size shrunk down to about 335MB. That's a lot less (worthless) data to copy around whenever I start up a new instance.
there were a lot of deleted files, because the VM started out as debian squeeze and was apt-get upgraded to sid.
Solution 2:
See also: virt-sparsify
, an utility which can zero-fill filesystems inside disk images (supporting various formats):
http://libguestfs.org/virt-sparsify.1.html
Solution 3:
I´m using zerofree ( apt-get install zerofree
) for this task:
Zerofree finds the unallocated blocks with non-zero value content in an ext2, ext3 or ext4 file-system and fills them with zeroes
after that you can shrink your image:kvm-img convert -O qcow2 original_image.qcow2 deduplicated_image.qcow2