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


From: Eric Blake
Subject: Re: [Qemu-devel] block-stream/drive-mirror and default bandwidth
Date: Tue, 17 Apr 2012 06:14:52 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:11.0) Gecko/20120329 Thunderbird/11.0.1

On 04/17/2012 03:20 AM, Stefan Hajnoczi wrote:
> I think it's cleanest to support block-job-set-speed even when no job
> is running.  The speed will be used as the default value when a job is
> started.  This poses the question of what happens if the job does not
> do throttling or cannot support the value for some reason - does
> creation fail until block-job-set-speed is set to 0 or a valid value
> again, or do we allow it but silently perform no throttling?

I'd prefer failure for any request for an out-of-range speed, and I like
the idea of always letting the user set the speed, even when a job is
not already running.  Am I correct that only one job can run at a time,
and therefore, the speed can be a property associated with the
BlockDevice as a whole, rather than only a property associated with each
individual job?

Libvirt would then implement virDomainBlockPull(dom, disk, bandwidth,
flags) as a call to block-job-set-speed(bandwidth) followed by
block-stream, rather than it's current implementation of block-stream
followed by block-job-set-speed.  And for the RHEL 6.2 implementation
where things were spelled block_job_set_speed, libvirt will know that
the old spelling implies that the speed has to be set second and live
with the race that it implies.

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

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