==== Shrink a qcow2 image file using qemu-img tool ====

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In case you **did allocate more than you needed**, and want to **get some disk space back**, you can use this command on a **qcow2 image**, to shrink it.

<code bash>
# using a low priority is recommended, otherwise your host machine becomes unusable
nice -n 19 qemu-img convert -O qcow2 ubuntu.qcow2 ubuntu.new.qcow2

# search the process on another terminal, 
# and also apply a low io priority using ionice
ionice -n 7 -p `pgrep qemu-img | head -n1`
</code>