On Fri, Nov 02, 2018 at 04:34:02PM -0600, Jim Fehlig wrote: > 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. I'm not really convinced this is a particularly useful concept, as it is only going to catch a very small number of the reasons why migration can fail. So you still have to expect the real migration invokation to have a strong chance of failing. > > Signed-off-by: Jim Fehlig <jfehlig@xxxxxxxx> > --- > > 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. Even launching QEMU isn't good enough - it has to actually process the migration data stream for devices to get a good indication of success, at which point you're basically doing a real migration. Regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :| -- libvir-list mailing list libvir-list@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/libvir-list
