The libvirt driver uses common backing files for ephemeral and swap disks. These are generated on the local compute host by running mkfs or mkswap as appropriate. The output of these files for a particular size and format is stored in the image cache on the compute host which ran it.
When all things are equal, 2 runs of mkfs or mkswap are guaranteed never to produce identical output, because at the very least they have different uuids. When you also consider the potential for different patch levels on different compute hosts, the potential for other differences is also significant.
When block migrating an ephemeral disk, the libvirt driver copies the 'overlay' qcow2 from source to dest. Assuming that some other instance on dest also has a similar ephemeral disk, the backing file will already exist on dest. However, it is guaranteed not to be the same as the disk's original backing file for the reasons above. If this works currently, it is either by luck, or because the tiny amount of metadata originally written by mkfs or mkswap is likely to have been overwritten if it has been in use for any amount of time.
The libvirt driver should not cache the output of mkfs and mkswap. The space and performance benefits are negligible, but it introduces the potential for data corruption.
Related fix proposed to branch: master
Review: https:/