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Re: [Qemu-block] backup bug or question

From: Vladimir Sementsov-Ogievskiy
Subject: Re: [Qemu-block] backup bug or question
Date: Mon, 12 Aug 2019 17:02:28 +0000

12.08.2019 19:49, Kevin Wolf wrote:
> Am 12.08.2019 um 18:09 hat Vladimir Sementsov-Ogievskiy geschrieben:
>> 12.08.2019 16:23, Kevin Wolf wrote:
>>> Am 09.08.2019 um 15:18 hat Vladimir Sementsov-Ogievskiy geschrieben:
>>>> Hi!
>>>> Hmm, hacking around backup I have a question:
>>>> What prevents guest write request after job_start but before setting
>>>> write notifier?
>>>> code path:
>>>> qmp_drive_backup or transaction with backup
>>>>       job_start
>>>>          aio_co_enter(job_co_entry) /* may only schedule execution, isn't 
>>>> it ? */
>>>> ....
>>>> job_co_entry
>>>>       job_pause_point() /* it definitely yields, isn't it bad? */
>>>>       job->driver->run() /* backup_run */
>>>> ----
>>>> backup_run()
>>>>       bdrv_add_before_write_notifier()
>>>> ...
>>>> And what guarantees we give to the user? Is it guaranteed that write 
>>>> notifier is
>>>> set when qmp command returns?
>>>> And I guess, if we start several backups in a transaction it should be 
>>>> guaranteed
>>>> that the set of backups is consistent and correspond to one point in 
>>>> time...
>>> Do the patches to switch backup to a filter node solve this
>>> automatically because that node would be inserted in
>>> backup_job_create()?
>> Hmm, great, looks like they should. At least it moves scope of the
>> problem to do_drive_backup and do_blockdev_backup functions..
>> Am I right that aio_context_acquire/aio_context_release guarantees no
>> new request created during the section? Or should we add
>> drained_begin/drained_end pair, or at least drain() at start of
>> qmp_blockdev_backup and qmp_drive_backup?
> Holding the AioContext lock should be enough for this.
> But note that it doesn't make a difference if new requests are actually
> incoming. The timing of the QMP command to start a backup job versus the
> timing of guest requests is essentially random. QEMU doesn't know what
> guest requests you mean to be included in the backup and which you don't
> unless you stop sending new requests well ahead of time.
> If you send a QMP request to start a backup, the backup will be
> consistent for some arbitrary point in time between the time that you
> sent the QMP request and the time that you received the reply to it.
> Draining in the QMP command handler wouldn't change any of this, because
> even the drain section starts at some arbitrary point in time.

Hmm and it don't guarantee even that requests started before qmp command are
taken into backup, as they may be started in guest point of view, but not yet
in QEMU..

>> Assume scenario like the this,
>> 1. fsfreeze
>> 2. qmp backup
>> 3. fsthaw
>> to make sure that backup starting point is consistent. So in our qmp command 
>> we should:
>> 1. complete all current requests to make drives corresponding to fsfreeze 
>> point
>> 2. initialize write-notifiers or filter before any new guest request, i.e. 
>> before fsthaw,
>> i.e. before qmp command return.
> If I understand correctly, fsfreeze only returns success after it has
> made sure that the guest has quiesced the device. So at any point
> between receiving the successful return of the fsfreeze and calling
> fsthaw, the state should be consistent.
>> Transactions should be OK, as they use drained_begin/drained_end
>> pairs, and additional aio_context_acquire/aio_context_release pairs.
> Here, draining is actually important because you don't synchronise
> against something external that you don't control anyway, but you just
> make sure that you start the backup of all disks at the same point in
> time (which is still an arbitrary point between the time that you send
> the transaction QMP command and the time that you receive success), even
> if no fsfreeze/fsthaw was used.
> Kevin

OK, thanks for explanation!

Best regards,

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