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From: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-ID: <9f94317f-a5c7-1fe5-8c6e-cc7336477ed0@virtuozzo.com>
Date: Tue, 22 Nov 2016 15:34:02 +0300
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Subject: Re: [Qemu-devel] [RFC] dirty bitmap state uncertainty under certain
 conditions
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To: Nikolay Shirokovskiy <nshirokovskiy@virtuozzo.com>, qemu-devel@nongnu.org, qemu block <qemu-block@nongnu.org>
Cc: Denis Lunev <den@virtuozzo.com>, Maxim Nestratov <mnestratov@virtuozzo.com>, John Snow <jsnow@redhat.com>, Eric Blake <eblake@redhat.com>, Fam Zheng <famz@redhat.com>

22.11.2016 15:01, 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.)
>
>    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 suggest an additional int field for BdrvDirtyBitmap, which will count 
number of bdrv_dirty_bitmap_abdicate() calls and will be available to 
the user through qmp query-block. So, for the user, <counter 
incremented> <=> <backup was successful and bitmap updated>.


-- 
Best regards,
Vladimir