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

[Paolo Bonzini]

Il 17/04/2012 04:50, Eric Blake ha scritto:
> 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.

The NotSupported error from block-job-set-speed is really an
out-of-range error, and we should report it as such...

I think that rather than adding lots of arguments, two better choices
are: 1) cancel the job if set-speed fails (for some definition of
better); 2) let block-job-set-speed work before starting a job, like you
said migration does.

Paolo