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?
4 Answers
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.
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See also: virt-sparsify
, an utility which can zero-fill filesystems inside disk images (supporting various formats):
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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
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Personally, I think it works better to clone the disk using Clonezilla or Symantec Ghost. It's a lot quicker than filling up the drive with zeros. Also it avoids the growing the image even more.
I have done this with Ghost and Win guests countless times. It's actually quicker if "used space" is smaller than those to be zeroed. Also you can use qemu-nbd
to mount the images and run Clonezilla from host, avoiding the hassle of Clonezilla-within-guest. Either way it's always much quicker than sdelete
/dd
in my experience. (Also I often end up with no space available on host for a full zero-out-guest-disk operation, so filling up available space in guest in seldom feasible to me.)
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Running Clonezilla or Ghost inside the VM ? Seems like a lot of unnecessary work. – Apr 03 '15 at 15:09
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1@AndréDaniel I have done this with Ghost and Win guests countless times. It's actually quicker if "used space" is smaller than those to be zeroed. Also you can use `qemu-nbd` to mount the images and run Clonezilla from host. Either way it's always much quicker than `sdelete`/`dd` to me. (Also I often end up with no space available on host for a full zero-out-guest-disk operation. – phoeagon Apr 03 '15 at 16:12
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Please add your comment to the answer to make it more substantive. – Deer Hunter Apr 03 '15 at 18:10