On Mon, Mar 14, 2022 at 01:14:27PM -0400, Lonnie Cumberland wrote: Sure ... $ qemu-img create -f qcow2 test.qcow2 1T Formatting 'test.qcow2', fmt=qcow2 cluster_size=65536 extended_l2=off compression_type=zlib size=1099511627776 lazy_refcounts=off refcount_bits=16 Real size about 200K: $ ls -lh test.qcow2 -rw-r--r--. 1 rjones rjones 208K Mar 14 17:38 test.qcow2 Virtual size 1TB: $ qemu-img info test.qcow2 image: test.qcow2 file format: qcow2 virtual size: 1 TiB (1099511627776 bytes) <--- disk size: 1 MiB There's no magic here, the qcow2 format is simply able to ignore all the blank sectors in the image, and even if the image contained some data it would map the sectors efficiently into a smaller file. https://web.archive.org/web/20100221075914/https://people.gnome.org/~mark... libguestfs has several APIs to get the virtual size. guestfs_disk_virtual_size is just a wrapper around qemu-img info: $ guestfish --ro disk-virtual-size test.qcow2 1099511627776 guestfs_blockdev_getsize64 works from inside the appliance and can be used to get the size of whole disks, partitions, etc within the image: $ guestfish --ro -a test.qcow2 run : blockdev-getsize64 /dev/sda 1099511627776 We don't have an API to get the real size because you can just read it using normal libc APIs like stat(2). I think we need a bit of clarity about what exactly you're trying to achieve here. What does "compress ... these images but based upon the [real] size and not the [virtual] size" mean? nbdkit xz-filter + file plugin can do _something_, whether it's what you need or not I'm not sure: https://libguestfs.org/nbdkit-xz-filter.1.html Rich. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Read my programming and virtualization blog: http://rwmj.wordpress.com libguestfs lets you edit virtual machines. Supports shell scripting, bindings from many languages. http://libguestfs.org