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Re: [Qemu-devel] [Qemu-block] [PATCH v10 08/10] Implement new driver for
From: |
Wen Congyang |
Subject: |
Re: [Qemu-devel] [Qemu-block] [PATCH v10 08/10] Implement new driver for block replication |
Date: |
Thu, 15 Oct 2015 10:19:17 +0800 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.2.0 |
On 10/14/2015 10:27 PM, Stefan Hajnoczi wrote:
> On Tue, Oct 13, 2015 at 05:08:17PM +0800, Wen Congyang wrote:
>> On 10/13/2015 12:27 AM, Stefan Hajnoczi wrote:
>>> On Fri, Sep 25, 2015 at 02:17:36PM +0800, Wen Congyang wrote:
>>>> + /* start backup job now */
>>>> + bdrv_op_unblock(s->hidden_disk, BLOCK_OP_TYPE_BACKUP_TARGET,
>>>> + s->active_disk->backing_blocker);
>>>> + bdrv_op_unblock(s->secondary_disk, BLOCK_OP_TYPE_BACKUP_SOURCE,
>>>> + s->hidden_disk->backing_blocker);
>>>
>>> Why is it safe to unblock these operations?
>>>
>>> Why do they have to be blocked for non-replication users?
>>
>> hidden_disk and secondary disk are opened as backing file, so it is blocked
>> for
>> non-replication users.
>> What can I do if I don't unblock it and want to do backup?
>
> CCing Jeff Cody, block jobs maintainer
>
> You need to explain why it is safe remove this protection. We can't
> merge code that may be unsafe.
>
> I think we can investigate further by asking: when does QEMU code assume
> the backing file is read-only?
The backing file is opened in read-only mode. I want to reopen it in read-write
mode here in the next version(So the patch 1 will be dropped)
>
> I haven't checked but these cases come to mind:
>
> Operations that move data between BDS in the backing chain (e.g. commit
> and stream block jobs) will lose or overwrite data if the backing file
> is being written to by another coroutine.
>
> We need to prevent users from running these operations at the same time.
Yes, but qemu doesn't provide such API.
>
> Also, accessing bs->backing_blocker is a layering violation. No one
> outside block.c:bdrv_set_backing_hd() is supposed to access this field.
I agree with it.
Thanks
Wen Congyang
>
> Let's figure out the safety concerns first and then the
> bs->backing_blocker access will probably be eliminated as part of the
> solution.
>
> Stefan
> .
>
- Re: [Qemu-devel] [Qemu-block] [PATCH v10 08/10] Implement new driver for block replication, Stefan Hajnoczi, 2015/10/12
- Re: [Qemu-devel] [Qemu-block] [PATCH v10 08/10] Implement new driver for block replication, Wen Congyang, 2015/10/15
- Re: [Qemu-devel] [Qemu-block] [PATCH v10 08/10] Implement new driver for block replication, Stefan Hajnoczi, 2015/10/15
- Re: [Qemu-devel] [Qemu-block] [PATCH v10 08/10] Implement new driver for block replication,
Wen Congyang <=
- Re: [Qemu-devel] [Qemu-block] [PATCH v10 08/10] Implement new driver for block replication, Stefan Hajnoczi, 2015/10/15
- Re: [Qemu-devel] [Qemu-block] [PATCH v10 08/10] Implement new driver for block replication, Wen Congyang, 2015/10/15
- Re: [Qemu-devel] [Qemu-block] [PATCH v10 08/10] Implement new driver for block replication, Wen Congyang, 2015/10/16
- Re: [Qemu-devel] [Qemu-block] [PATCH v10 08/10] Implement new driver for block replication, Stefan Hajnoczi, 2015/10/16
- Re: [Qemu-devel] [Qemu-block] [PATCH v10 08/10] Implement new driver for block replication, Wen Congyang, 2015/10/26
- Re: [Qemu-devel] [Qemu-block] [PATCH v10 08/10] Implement new driver for block replication, Stefan Hajnoczi, 2015/10/27
Re: [Qemu-devel] [Qemu-block] [PATCH v10 08/10] Implement new driver for block replication, Stefan Hajnoczi, 2015/10/15