Re: [libvirt] [PATCH] snapshot: improve qemu handling of reused snapshot targets

The oVirt developers have stated that the real reasons they want to have qemu reuse existing volumes when creating a snapshot are: 1. the management framework is set up so that creation has to be done from a central node for proper resource tracking, and having libvirt and/or qemu create things violates the framework, and 2. qemu defaults to creating snapshots with an absolute path to the backing file, but oVirt wants to manage a backing chain that uses just relative names, to allow for easier migration of a chain across storage locations. When 0.9.10 added VIR_DOMAIN_SNAPSHOT_CREATE_REUSE_EXT (commit 4e9953a4), it only addressed point 1, but libvirt was still using O_TRUNC which violates point 2. Meanwhile, the new qemu 'transaction' monitor command includes a new optional mode argument that will force qemu to reuse the metadata of the file it just opened (with the burden on the caller to have valid metadata there in the first place). So, this tweaks the meaning of the flag to cover both points as intended for use by oVirt. It is not strictly backward-compatible to 0.9.10 behavior, but it can be argued that the O_TRUNC of 0.9.10 was a bug.

Note that this flag is all-or-nothing, and only selects between 'existing' and the default 'absolute-paths'. A more flexible approach that would allow per-disk selections, as well as adding support for the 'no-backing-file' mode, would be possible by extending the <domainsnapshot> xml to have a per-disk mode, but until we have a management application expressing a need for that additional complexity, it is not worth doing.