Note that disk images can only be compacted if they are not in a snapshot chain and do not use disk encryption!
libVirt
virt-sparsify --compress /path/to/image.qcow2 /path/to/new-image.qcow2
VirtualBox
Windows
Disable the paging file (Settings → System → About → Advanced system settings → Advanced → Performance → Settings… → Advanced → Virtual memory → Change…), clean all temporary files (C: → Properties → Disk Cleanup), defrag (defrag C: /FreespaceConsolidate /Verbose
), and then zero out free space with SDelete (sdelete -z C:
).
Then, compact the disk image on the host:
vboxmanage modifymedium /path/to/image.vdi --compact
Remember to turn the paging file back on after compacting the drive!
Linux
Zero out free disk space on the guest:
sudo dd if=/dev/zero of=/temp.zeros bs=4096k
sudo rm -f /temp.zeros
Compact the disk image on the host:
vboxmanage modifymedium /path/to/image.vdi --compact