Re: [libvirt] RFC: using a network device as a destination for a disk snapshot

Right now, when creating an external snapshot, the snapshot xml requires that the destination be in the local file system: http://libvirt.org/formatsnapshot.html <domainsnapshot> ... <disks> <disk name='vda' snapshot='external'> <driver type='qcow2'/> <source file='/path/to/new'/> </disk> ...

But libvirt already allows for a network device with qcow2 contents, provided that there is no backing chain: # qemu-img info gluster://red/vol1/img2 image: gluster://red/vol1/img2 file format: qcow2 virtual size: 10M (10485760 bytes) disk size: 193K cluster_size: 65536 # virsh dumpxml dom <domain type='kvm'> ... <disk type='network' device='disk'> <driver name='qemu' type='qcow2'/> <source protocol='gluster' name='vol1/img2'> <host name='red'/> </source> <target dev='vdc' bus='virtio'/> <address type='pci' domain='0x0000' bus='0x00' slot='0x09' function='0x0'/> </disk> ... I'm working on patches to the backing chain code to allow a gluster (or other network device) with qcow2 contents to have a backing file, but for the code to be useful, we also need to patch <domainsnapshot> xml to allow the destination file to be a gluster or other network device, rather than forcing it to be a local file name. Here's the XML I think we need to add to domainsnapshot: <domainsnapshot> ... <disks> <disk name='vda' snapshot='external' type='network'> <driver type='qcow2'/> <source protocol='gluster' name='vol1/img2'> <host name='red'/> </source> </disk> that is, add an optional /disk@type attribute (if absent, it defaults to type='file'), and where if present, the <source> subelement then takes on alternate forms in the same manner in which //domain/devices/disk handles alternates (here, allowing a protocol, name, and host specification).

[Ultimately, we need to fix //domain/devices/disk to specify a full backing chain, but one step at a time...]