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Re: [Qemu-block] [PATCH v2 2/3] block/mirror: Fix target backing BDS

From: Max Reitz
Subject: Re: [Qemu-block] [PATCH v2 2/3] block/mirror: Fix target backing BDS
Date: Wed, 8 Jun 2016 16:42:02 +0200
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On 08.06.2016 16:40, Max Reitz wrote:
> On 08.06.2016 13:28, Paolo Bonzini wrote:
>> ----- Original Message -----
>>> From: "Kevin Wolf" <address@hidden>
>>> To: "Max Reitz" <address@hidden>
>>> Cc: address@hidden, address@hidden, "Fam Zheng" <address@hidden>, 
>>> address@hidden,
>>> address@hidden, address@hidden
>>> Sent: Wednesday, June 8, 2016 11:32:29 AM
>>> Subject: Re: [PATCH v2 2/3] block/mirror: Fix target backing BDS
>>> Am 06.06.2016 um 16:42 hat Max Reitz geschrieben:
>>>> Currently, we are trying to move the backing BDS from the source to the
>>>> target in bdrv_replace_in_backing_chain() which is called from
>>>> mirror_exit(). However, mirror_complete() already tries to open the
>>>> target's backing chain with a call to bdrv_open_backing_file().
>>>> First, we should only set the target's backing BDS once. Second, the
>>>> mirroring block job has a better idea of what to set it to than the
>>>> generic code in bdrv_replace_in_backing_chain() (in fact, the latter's
>>>> conditions on when to move the backing BDS from source to target are not
>>>> really correct).
>>>> Therefore, remove that code from bdrv_replace_in_backing_chain() and
>>>> leave it to mirror_complete().
>>>> However, mirror_complete() in turn pursues a questionable strategy by
>>>> employing bdrv_open_backing_file(): On the one hand, because this may
>>>> open the wrong backing file with drive-mirror in "existing" mode, or
>>>> because it will not override a possibly wrong backing file in the
>>>> blockdev-mirror case.
>>> Careful, this "wrong" backing file might actually be intended!
>>> Consider a case where you want to move an image with its whole backing
>>> chain to different storage. In that case, you would copy all of the
>>> backing files (cp is good enough, they are read-only), create the
>>> destination image which already points at the copied backing chain, and
>>> then mirror in "existing" mode.
>>> The intention is obviously that after the job completion the new backing
>>> chain is used and not the old one.
>> Yes, this is the intention and it should not be changed.  In addition
>> to what Kevin said, you can use drive-mirror to collapse the image to a
>> single file; in this case, QEMU should not be using the backing files of
>> the source.
> That is an issue that we have right now. If you do drive-mirror in
> absolute-paths mode with sync=full, the target will have the backing
> chain of the source. This is something that this patch fixes.

As a clarification: I mean the backing chain inside QEMU (in the BDS
graph), not the on-disk backing chain, i.e. how the physical image files
link to each other.


> In fact, I think if you do drive-mirror in existing mode or
> blockdev-mirror and the target image does not have a backing file
> (whatever sync mode you have used), the same will happen.
> Max
>> bdrv_open_backing_file() is used because what we want to do is to
>> "undo" the BDRV_O_NO_BACKING flag used by qmp_drive_mirror.
>> If the contents change under the guest feet, it's the layers above
>> QEMU that have screwed up.
>> Paolo

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