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

[John Snow]

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.