[libvirt] RFC New virDomainBlockPull API family to libvirt

Stefan Hajnoczi stefanha at gmail.com
Wed Jul 20 13:00:19 UTC 2011

On Mon, Jul 18, 2011 at 9:10 PM, Adam Litke <agl at us.ibm.com> wrote:
> On 07/18/2011 09:35 AM, Stefan Hajnoczi wrote:
>>>>>>   On the other hand I suspect that we are missing the mechanism to
>>>>>> control the rate of the transfer in the new API, which is something
>>>>>> which could be implemented in the old incremental mechanism, but not
>>>>>> anymore. So I wonder if the virDomainBlockPull() shouldn't get an
>>>>>> "unsigned long bandwidth" (to stay coherent with migration APIs)
>>>>>> before the flags. I don't know if the support is ready in QEmu but
>>>>>> I suspect that will be an important point, so if not present will
>>>>>> be added :-)
>>>>> Hopefully Stefan can comment on any throttling mechanisms that might be
>>>>> added to the qemu implementation.  It would be nice to have this feature
>>>>> built into the API to match the migration APIs, but if that is not
>>>>> feasible then we could always add it later.
>>>> If we follow the live migration API then a block_stream_set_speed
>>>> command would be used.  libvirt would issue it as a separate command
>>>> immediately after starting the streaming operation.  There is the
>>>> window of time after streaming has started but before
>>>> block_stream_set_speed has been called where no throttling takes
>>>> place, but I don't think this is a practical problem.
>>>> It also opens the possibility of allowing the user to change the rate
>>>> limit value while the block streaming operation is active.
>>> In light of our decision to provide a generic BlockJobAbort/BlockJobInfo
>>> interface, should SetSpeed be generic too?
>>> virDomainBlockJobSetSpeed() or virDomainBlockPullSetSpeed() ?
>> The block_stream_set_speed semantics allow the command to be used when
>> no streaming operation is active.  This can be used to set the speed
>> without a window of time after creation and before set_speed is
>> called.
>> If we switch to block_job_set_speed then it is cleaner to restrict the
>> command to work on active jobs only.  This is because there is only
>> one "speed" variable kept but there might be multiple job types
>> (streaming, compaction, etc) which would need different settings.
>> What do you think about this?
> I think the block_job_set_speed semantics seem reasonable to me. What is
> the method for querying the current speed setting?  I would suggest
> extending the query-block-job command to include this information.  In
> this case, I would extend virDomainBlockJobInfo to contain:
>        /* The maximum allowable bandwidth for this job (MB/s) */
>        unsigned long bandwidth;

Working QEMU code is pushed to:

I am updating the QEMU wiki page with the latest changes.


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