Re: [Qemu-devel] [RFC] dirty bitmap state uncertainty under certain conditions

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

22.11.2016 19:07, John Snow wrote:
>
>
> On 11/22/2016 07:01 AM, Nikolay Shirokovskiy wrote:
>> Hi, everyone.
>>
>>   There is a problem with current incremental backups. Imagine I ask 
>> qemu to
>> make an incremental backup then go away and return back when backup
>> job is finished. Qemu process dismisses the job completely and I missed
>> all the events so I don't know the result of the operation and what is
>> most important I don't know the base for dirty bitmap now. In case of 
>> failure
>> it is previous backup and in case of success it is the last backup. 
>> Qemu does
>> not track dirty bitmap base for me so I have no choice other then clear
>> dirty bitmap and make full backup which would be rather unexpected 
>> from user
>> POV (The situation of going away/coming back is libvirt crash/restart 
>> of course.)
>>
>
> Why was the completion/failure event missed? Is there some reason why 
> you cannot guarantee that you will observe the completion?
>
>>   I guess problem has wider scope. In case I miss successfull 
>> completion of full
>> backup my only option is to drop backup file and redo the backup 
>> completely
>> which is rather wasteful. AFAIU I can not query backup completion 
>> result from
>> backup file itself. I guess there can be similar issues for other 
>> qemu jobs.
>>
>> Nikolay
>>
>
> I would personally advocate for a job-neutral solution where jobs can 
> be given a parameter such that the job persists in memory in a new 
> "completed" state until such time that it is queried explicitly, then 
> it can be dropped.
>
> I am not sure if we can make this the default behavior, as it might 
> confuse libvirt to occasionally see jobs that have already completed.
>
> Talking to Kevin off-list, he suggested that we might be able to make 
> this the default behavior if we pivot to the new jobs API that I have 
> been proposing, accompanied by a new explicit command to put a command 
> to rest.
>
> I can work on this for 2.9; though we may still need a "temporary" 
> solution for the old jobs API until we're ready to officially 
> deprecate the older interface.
>

Jobs is completed state sounds good for me, I've thought about it too.


-- 
Best regards,
Vladimir