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Date: Sat, 23 May 2015 18:51:27 +0200
From: Max Reitz <mreitz@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH v6 01/13] block: Add op blocker type
 "device IO"
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To: Fam Zheng <famz@redhat.com>, Wen Congyang <wency@cn.fujitsu.com>
Cc: Kevin Wolf <kwolf@redhat.com>, qemu-block@nongnu.org, jcody@redhat.com, armbru@redhat.com, qemu-devel@nongnu.org, Stefan Hajnoczi <stefanha@redhat.com>, amit.shah@redhat.com, pbonzini@redhat.com

On 22.05.2015 06:54, Fam Zheng wrote:
> On Thu, 05/21 15:32, Fam Zheng wrote:
>> On Thu, 05/21 15:06, Wen Congyang wrote:
>>> On 05/21/2015 02:42 PM, Fam Zheng wrote:
>>>> It blocks device IO.
>>>>
>>>> All bdrv_op_block_all/blk_op_block_all callers are taken care of:
>>>>
>>>> - virtio_blk_data_plane_create
>>>> - virtio_scsi_hotplug
>>>>
>>>>    Device creation, unblock it.
>>>>
>>>> - bdrv_set_backing_hd
>>>>
>>>>    Backing hd is not used by device, so blocking is OK.
>>>>
>>>> - backup_start
>>>>
>>>>    Blocking target when backup is running, unblock it.
>>> Do you forget it?
>> Oh I think the commit log is wrong: the target image is only written to by
>> block job, there cannot be a device on it, so it it's similar to
>> bdrv_set_backing_hd.
> Correction: if it's blockdev-backup, the target could have a device, in that
> sense it should be unblocked like block_job_create(). I'll fix it.

Really? I think it makes sense not to allow I/O on a backup target. At 
least I can't imagine a use case where you'd want to do that... But that 
doesn't necessarily mean anything, of course.

Max