Re: [libvirt] [RFC PATCH] Add new migration flag VIR_MIGRATE_DRY_RUN

A dry run can be used as a best-effort check that a migration command will succeed. The destination host will be checked to see if it can accommodate the resources required by the domain. DRY_RUN will fail if the destination host is not capable of running the domain. Although a subsequent migration will likely succeed, the success of DRY_RUN does not ensure a future migration will succeed. Resources on the destination host could become unavailable between a DRY_RUN and actual migration.

Signed-off-by: Jim Fehlig <jfehlig(a)suse.com&gt; --- If it is agreed this is useful, my thought was to use the begin and prepare phases of migration to implement it. qemuMigrationDstPrepareAny() already does a lot of the heavy lifting wrt checking the host can accommodate the domain. Some of it, and the remaining migration phases, can be short-circuited in the case of dry run. One interesting wrinkle I've observed is the check for cpu compatibility. AFAICT qemu is actually invoked on the dst, "filtered-features" of the cpu are requested via qmp, and results are checked against cpu in domain config. If cpu on dst is insufficient, migration fails in the prepare phase with something like "guest CPU doesn't match specification: missing features: z y z". I was hoping to avoid launching qemu in the case of dry run, but that may be unavoidable if we'd like a dependable dry run result.