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

From: John Snow
Subject: Re: [Qemu-devel] [RFC] dirty bitmap state uncertainty under certain conditions
Date: Tue, 22 Nov 2016 12:26:34 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.4.0

On 11/22/2016 11:16 AM, Eric Blake wrote:
On 11/22/2016 10:07 AM, 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
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 think the intent of some of the on-error parameters is to make it so
that the job can't go away on error, only on success.  Admittedly,
libvirt isn't using those policies as well as it could.

  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
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


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.

Yeah, revisiting the overall job API will require some overhaul in
libvirt as well, but it is probably worth it.

I wonder if I should try to rectify this temporarily for 2.9, or just jump straight into a new interface.

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.


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