[Qemu-devel] block-stream/drive-mirror and default bandwidth

[Eric Blake]

Libvirt found out that we have a pretty nasty data race between
block-stream and block-job-set-speed.  If a user wants to throttle a
job, they cannot request the throttling until after the job has been
started (thanks to the DeviceNotActive error of block-job-set-speed),
but by the time you actually get a chance to throttle the job, you have
no idea how much data transpired between the job start and the
throttling request.  migrate_set_speed does not have this issue, since
it can be issued _prior_ to the migration.

Is there any way we can add an optional parameter to 'block-stream' (in
the tree now) and 'drive-mirror' (in Paolo's pending patch series) to
set the speed as part of starting the job, or to atomically fail with
NotSupported if throttling cannot be managed, rather than having a race
window before getting a chance to call block-job-set-speed?  Now's the
time to do it, since 'block-stream' is new to qemu 1.1.

-- 
Eric Blake   address@hidden    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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