Re: [libvirt] Patch to enable transient disks in libvirt for qemu

On Sat, May 14, 2016 at 09:10:12 -0400, Richard Harman wrote: You are right. There wasn't anyone motivated enough to add this feature properly. Please post patches against the current git master tree. libvirt-1.2.18 is 9 months old already. Problem with snapshot=on is that qemu places the snapshot file into the temp file path as you've pointed out. Few reasons why this is not a good idea: 1) the temp directory isn't a good place to store huge files The file containing the disk overlay may grow up to the total size of the disk visible in the guest if every block is overwritten. The /tmp directory usually isn't meant to handle huge files for long time as it's usually either placed on the root filesystem or backed by tmpfs or governed by quotas. Exceeding the empty space on the storage will then make the VM fail to write any new sectors. 2) This wouldn't work well with migrations with shared storage. As the file is created in a different path than the original storage volume and the temp directory is not shared among hosts where the VM can be migrated the overlay file won't be accessible after migration. 3) Break of storage and host boundaries In some configurations the storage for the VMs is separated from the hosts providing the hypervisor. For network bakcked storage as gluster or ceph this would create the overlay on the local storage rather than the storage technology itself. Adding the code you are suggesting would add a half-baked feature that would work only in certain cases. The correct approach to handle this would be to pre-create a qcow2 storage file in libvirt in the correct storage path and make it refer to the original volume. The file then needs to be discarded after use. For apps that use libvirt this is trivial to work around, since they can pre-create the storage file in any path they wish and use it as source. Peter