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

From: Vladimir Sementsov-Ogievskiy
Subject: Re: [Qemu-devel] [RFC] dirty bitmap state uncertainty under certain conditions
Date: Tue, 22 Nov 2016 19:16:22 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.4.0

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.


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,

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