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Created August 14, 2014 07:51
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shrink + compress virtual disk image in Qemu/KVM
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Qemu-KVM-shrink+compress.MD

shrink + compress virtual disk image in Qemu/KVM

originally from Martin Kopta's blog

Tue Apr 22 10:41:10 UTC 2014

In order to shrink virtual disk of your virtual machine (process called disk space reclamation) in Qemu/KVM, you can go with following. In your running virtual machine, fill all remaining empty space with zeroes. Then, shut down virtual machine and convert the disk image from original format to the same format (esentially doing nothing). In the process of conversion, Qemu will skip the zeroes and save you some space. You can even add some compression to save even more space.

root@vm# dd if=/dev/zero of=/file
root@vm# rm /file
root@vm# poweroff

user@host$ mv image.qcow2 image.qcow2.bk
user@host$ qemu-img convert -p -c -f qcow2 image.qcow2.bk image.qcow2

Option -p will display progress bar. See qemu-img(1) for more information.

@tomer
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tomer commented Feb 4, 2018

You should add -O qcow2 to qeum-img or else the generated image might be formatted as RAW.

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