[Libguestfs] virt-resize for shrinking

[Continuing a discussion from IRC so we have a permanent record] We want to shrink a disk image using virt-resize. Shrinking is harder and not really documented. It requires some manual calculations too. Here is the original: $ virt-df -a 209e6911-fe48-4321-900f-928b3400df88 Filesystem 1K-blocks Used Available Use% 209e6911-fe48-4321-900f-928b3400df88:/dev/sda1 51564664 8000404 40943880 16% $ virt-filesystems -a 209e6911-fe48-4321-900f-928b3400df88 --all --long Name Type VFS Label MBR Size Parent /dev/sda1 filesystem ext4 cloudimg-rootfs - 53678177280 - /dev/sda1 partition - - 83 53678177280 /dev/sda /dev/sda device - - - 53687091200 The first question was: Why doesn't the filesystem Size in virt-filesystems match the 1K-blocks field in virt-df? That's because the 1K-blocks field in virt-df doesn't include filesystem metadata overhead (more accurately: it depends on the filesystem and different fses will report slightly different things for this field). The virt-filesystems Size field is reporting the entire, true size of the partition. My observations are: (1) There's plenty of scope for shrinking. Note that Use% is just 16%. (2) There's only one filesystem, no swap or boot partitions, and that makes the calculations easy. To shrink down to, say, 1/2 of the original size, total device size of 53687091200 / 2 == 26843545600 bytes (25 GB). As there is always some filesystem overhead, you'll need to shrink the filesystem by a bit more than 1/2. I would shrink it down to 20 GB (which is still well over 16% of the original). Start the guest up in virt-rescue: virt-rescue -a 209e6911-fe48-4321-900f-928b3400df88 [... boot messages ...] Now after that, it should be possible to run virt-resize: rm -f output.img truncate -s 25G output.img virt-resize 209e6911-fe48-4321-900f-928b3400df88 output.img \ --shrink /dev/sda1 Note (1) The shrink path of virt-resize isn't well tested at all, so you might hit bugs. If you do, run it again with --debug and file a bugzilla. Now (you can check with virt-filesystems) you should have 'output.img' which has one partition that is just under 25GB, but contains a 20GB filesystem. You can now *expand* the filesystem on /dev/sda1 to occupy the full space: virt-rescue -a output.img Note (2) The virt-rescue steps could also be done using guestfish (or the API) which is especially useful if you wish to automate any of this. Note (3) There's an entirely different way to think about this, which is also much easier. Make the file sparse (or use qcow2 or LVM thinp), using virt-sparsify. It'll save MORE disk space ... Rich. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones virt-top is 'top' for virtual machines. Tiny program with many powerful monitoring features, net stats, disk stats, logging, etc. http://people.redhat.com/~rjones/virt-top