On 04/23/2013 10:06 AM, Eric Blake wrote: >> >> we can change "drive_mirror" to use a new command to see if there >> are the new features. > > drive-mirror changed in 1.4 to add optional buf-size parameter; right > now, libvirt is forced to limit itself to 1.3 interface (no buf-size or > granularity) because there is no introspection and no query-* command > that witnesses that the feature is present. Idea was that we need to > add a new query-drive-mirror-capabilities (name subject to bikeshedding) > command into 1.5 that would let libvirt know that buf-size/granularity > is usable (done right, it would also prevent the situation of buf-size > being a write-only interface where it is set when starting the mirror > but can not be queried later to see what size is in use). > > Unclear whether anyone was signing up to tackle the addition of a query > command counterpart for drive-mirror in time for 1.5. Further discussion on this topic: Luiz proposed such a command: https://lists.gnu.org/archive/html/qemu-devel/2013-04/msg04937.html in the meantime, Paolo and I discussed on IRC, and realized that: 1. Current libvirt does not expose buf-size or granularity to the end user. Until a future libvirt release actually wants to use these options, we are in no rush to get it into qemu 1.5; it's okay to leave things in the same state they were in for 1.4, and have qemu 1.6 be the first release that coordinates with a new libvirt release actually wanting to use the options. 2. At least with drive-mirror, the "try-and-fail" approach generates a reasonable-enough error message. Having a capability query may allow libvirt to save time and/or give a better quality error message, but this is one case where not knowing whether the parameter exists is not a fatal flaw to the algorithm, since libvirt can still gracefully recover from attempting to use the parameter (only when the user requested a non-default value). Distros that want to prove their value-added quality-of-implementation can easily backport whatever solution goes into qemu 1.6 for nicer detection into the distro stable build, even if the distro is based on 1.5; libvirt will use the nicer detection when available but should have no problems using the try-and-fail approach otherwise. So at this point, I'm comfortable with not trying to add anything into 1.5 dealing with drive-mirror; it feels like too much of a new feature past soft freeze with no known current client, where we would be better off waiting to 1.6 to get the interface right. -- Eric Blake eblake redhat com +1-919-301-3266 Libvirt virtualization library http://libvirt.org
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