Re: [PATCH v3 0/8] blockdev: avoid acquiring AioContext lock twice at do_drive_backup and do_blockdev_backup

[Sergio Lopez <slp@redhat.com>]

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Kevin Wolf <kwolf@redhat.com> writes:

> Am 19.11.2019 um 11:54 hat Sergio Lopez geschrieben:
>> 
>> Max Reitz <mreitz@redhat.com> writes:
>> 
>> > On 13.11.19 14:24, Sergio Lopez wrote:
>> >> 
>> >> Sergio Lopez <slp@redhat.com> writes:
>> >> 
>> >>> no-reply@patchew.org writes:
>> >>>
>> >>>> Patchew URL: https://patchew.org/QEMU/20191112113012.71136-1-slp@redhat.com/
>> >>>>
>> >>>>
>> >>>>
>> >>>> Hi,
>> >>>>
>> >>>> This series failed the docker-quick@centos7 build test. Please find the testing commands and
>> >>>> their output below. If you have Docker installed, you can probably reproduce it
>> >>>> locally.
>> >>>>
>> >>>> === TEST SCRIPT BEGIN ===
>> >>>> #!/bin/bash
>> >>>> make docker-image-centos7 V=1 NETWORK=1
>> >>>> time make docker-test-quick@centos7 SHOW_ENV=1 J=14 NETWORK=1
>> >>>> === TEST SCRIPT END ===
>> >>>>
>> >>>>   TEST    iotest-qcow2: 268
>> >>>> Failures: 141
>> >>>
>> >>> Hm... 141 didn't fail in my test machine. I'm going to have a look.
>> >> 
>> >> So here's the output:
>> >> 
>> >> --- /root/qemu/tests/qemu-iotests/141.out	2019-11-12 04:43:27.651557587 -0500
>> >> +++ /root/qemu/build/tests/qemu-iotests/141.out.bad	2019-11-13 08:12:06.575967337 -0500
>> >> @@ -10,6 +10,8 @@
>> >>  Formatting 'TEST_DIR/o.IMGFMT', fmt=IMGFMT size=1048576 backing_file=TEST_DIR/t.IMGFMT backing_fmt=IMGFMT
>> >>  {"timestamp": {"seconds":  TIMESTAMP, "microseconds":  TIMESTAMP}, "event": "JOB_STATUS_CHANGE", "data": {"status": "created", "id": "job0"}}
>> >>  {"timestamp": {"seconds":  TIMESTAMP, "microseconds":  TIMESTAMP}, "event": "JOB_STATUS_CHANGE", "data": {"status": "running", "id": "job0"}}
>> >> +{"timestamp": {"seconds":  TIMESTAMP, "microseconds":  TIMESTAMP}, "event": "JOB_STATUS_CHANGE", "data": {"status": "paused", "id": "job0"}}
>> >> +{"timestamp": {"seconds":  TIMESTAMP, "microseconds":  TIMESTAMP}, "event": "JOB_STATUS_CHANGE", "data": {"status": "running", "id": "job0"}}
>> >>  {"error": {"class": "GenericError", "desc": "Node 'drv0' is busy: node is used as backing hd of 'NODE_NAME'"}}
>> >>  {"return": {}}
>> >>  {"timestamp": {"seconds":  TIMESTAMP, "microseconds":  TIMESTAMP}, "event": "JOB_STATUS_CHANGE", "data": {"status": "aborting", "id": "job0"}}
>> >> 
>> >> Those extra lines, the "paused" and "running", are a result of the job
>> >> being done in a transaction, within a drained section.
>> >> 
>> >> We can update 141.out, but now I'm wondering, was it safe creating the
>> >> job at do_drive_backup() outside of a drained section, as
>> >> qmp_drive_backup was doing?
>> >
>> > I think it is.  Someone needs to drain the source node before attaching
>> > the job filter (which intercepts writes), and bdrv_backup_top_append()
>> > does precisely this.
>> >
>> > If the source node is in an I/O thread, you could argue that the drain
>> > starts later than when the user has invoked the backup command, and so
>> > some writes might slip through.  That’s correct.  But at the same time,
>> > it’s impossible to drain it the instant the command is received.  So
>> > some writes might always slip through (and the drain will not stop them
>> > either, it will just let them happen).
>> >
>> > Therefore, I think it’s fine the way it is.
>> >
>> >> Do you think there may be any potential drawbacks as a result of always
>> >> doing it now inside a drained section?
>> >
>> > Well, one drawback is clearly visible.  The job goes to paused for no
>> > reason.
>> 
>> This is something that already happens when requesting the drive-backup
>> through a transaction:
>> 
>> {"execute":"transaction","arguments":{"actions":[{"type":"drive-backup","data":{"device":"drv0","target":"o.qcow2","sync":"full","format":"qcow2"}}]}}
>> 
>> I don't think it makes sense to have two different behaviors for the
>> same action. So we either accept the additional pause+resume iteration
>> for qmp_drive_backup, or we remove the drained section from the
>> transaction based one.
>> 
>> What do you think?
>
> Draining all involved nodes is necessary for transactions, because you
> want a consistent backup across all involved disks. That is, you want it
> to be a snapshot at the same point in time for all of them - no requests
> may happen between starting backup on the first and the second disk.
>
> For a single device operation, this requirement doesn't exist, because
> there is nothing else that must happen at the same point in time.

This poses a problem with the unification strategy you suggested for qmp
commands and transactions. I guess that, if we really want to preserve
the original behavior, we can extend DriveBackup to add a flag to
indicate whether the transaction should create a drained section or not.

Does this sound reasonable to you?

Thanks,
Sergio.

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