Compress qcow2 ? how ?

On kvm documentation you have comands for how move and copy sparse files , perfect , but what is the
command for compress it again in .xz ? for example i made copy the updated and upgrated kvm images in var\lib\libvirt\images to another external usb disk for having backup the whonix , but there are to big -they need 90 GB on disk! * if the compression take to much time , e.g. more than 20 minutes , i thing will be unusable , will better buy a larger disk. Thank you.

run:

tar -Sczvf whonix.tar.gz [multiple file names separated by spaces]

i am sorry i make a mistake , the images not need 90 GB but 22 GB , so there is not need to compress.
As for the compression , take to much time , even for 20 GB need to much time , is unpractical , better to store on a disk as they are (not compressed)

There is a problem in the documentation on the KVM page with the new
text “To re-compress files use:”. The placement/context implies, that
one could boot compressed images. It belongs elsewhere. Under optional
configurations perhaps.

done