qemu-img create with preallocation=full and compression_type=zlib

Hi! When running this command on Fedora 35 I get: [#] qemu-img create -o preallocation=full -f qcow2 /var/vm_mail/mytest.qcow2 10GB => Formatting '/var/vm_mail//mytest.qcow2', fmt=qcow2 . . . preallocation=full compression_type=zlib . . . ." Question: When using 'preallocation=full’ I was surprised that the resulting qcow2 image includes the 'compression_type=zlib’ part. Why/when is this compression used when the qcow2 image is fully preallocated? Thanks for input on this! :-) Best regards Richard Taubo