Re: [PATCH v5 11/12] migration/dirtyrate: Implement qmp_cal_dirty_rate()/qmp_get_dirty_rate() function

[David Edmondson <dme@dme.org>]

On Thursday, 2020-08-27 at 17:34:13 +08, Zheng Chuan wrote:

>>> +    /*
>>> +     * Only support query once for each calculation,
>>> +     * reset as DIRTY_RATE_STATUS_UNSTARTED after query
>>> +     */
>>> +    (void)dirtyrate_set_state(&CalculatingState, CalculatingState,
>>> +                              DIRTY_RATE_STATUS_UNSTARTED);
>> 
>> Is there a reason for this restriction? Removing it would require
>> clarifying the state model, I suppose.
>> 
> We only support query once for each calculation.
> Otherwise, it could always query dirtyrate, but maybe the dirtyrate is calculated
> long time ago.

There's nothing in the current interface that prevents this from being
the case already - the caller could initiate a 1 second sample, then
wait 24 hours to query the result.

Obviously this would generally be regarded as "d'oh - don't do that",
but the same argument would apply if the caller is allowed to query the
results multiple times.

Perhaps a complete solution would be to include information about the
sample period with the result. The caller could then determine whether
the sample is of adequate quality (sufficiently recent, taken over a
sufficiently long time period) for its' intended use.

dme.
-- 
I walk like a building, I never get wet.