Re: [Qemu-devel] [PATCH v12 6/6] migration: add postcopy total blocktime into query-migrate

[Eric Blake]

On 01/05/2018 06:15 AM, Alexey Perevalov wrote:
> On 01/03/2018 12:26 AM, Eric Blake wrote:
>> On 10/30/2017 08:16 AM, Alexey Perevalov wrote:
>>> Postcopy total blocktime is available on destination side only.
>>> But query-migrate was possible only for source. This patch
>>> adds ability to call query-migrate on destination.
>>> To be able to see postcopy blocktime, need to request postcopy-blocktime
>>> capability.
>> Why not display the stats unconditionally when they are available,
>> instead of having to set a capability knob to request them?
> That knob necessary to avoid regression if this information
> is not necessary, we decided during so long discussion in previous
> version of the patch set - it's not necessary always.
> But if user requested blocktime and host can't calculate it,
> e.g. due to appropriate feature isn't supported in host kernel,
> yes, the value will be 0.
> 

>>>   #
>>> +# @postcopy-blocktime: total time when all vCPU were blocked during
>>> postcopy
>>> +#           live migration (Since 2.11)
>> 2.12 now.
>>
>> Should this mention the capability knob needed to enable this stat (or
>> else get rid of the capability knob and always expose this when
>> possible)?
>>

Okay, so you've explained that the knob is necessary because there is a
noticeable performance difference for users that don't care about the
statistic; in which case, we DO need a followup patch (or a v2 of Juan's
pull request) that documents that this statistic is useless unless you
set the migration capability.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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