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Re: [Qemu-devel] [PATCH 5/6] add-cow: support snapshot_blkdev

From: Stefan Hajnoczi
Subject: Re: [Qemu-devel] [PATCH 5/6] add-cow: support snapshot_blkdev
Date: Thu, 19 Jul 2012 10:57:37 +0100

On Thu, Jul 19, 2012 at 3:20 AM, Dong Xu Wang
<address@hidden> wrote:
> On Thu, Jun 14, 2012 at 7:33 PM, Kevin Wolf <address@hidden> wrote:
>> Am 14.06.2012 13:18, schrieb Paolo Bonzini:
>>> Il 14/06/2012 12:59, Kevin Wolf ha scritto:
>>>> Am 13.06.2012 16:36, schrieb Dong Xu Wang:
>>>>> add-cow will let raw file support snapshot_blkdev indirectly.
>>>>> Signed-off-by: Dong Xu Wang <address@hidden>
>>>> Paolo, what do you think about this magic?
>>> Besides the obvious layering violation (it would be better to add a new
>>> method bdrv_ext_snapshot_create perhaps) I don't see very much a problem
>>> in it.  Passing image creation options sounds like a good idea that we
>>> can add in the future as an extension.
>>> But honestly, I don't really see the point of add-cow in general...  The
>>> raw image is anyway not usable without a pass through qemu-io convert,
>>> and mirroring will also allow collapsing an image to raw (with the
>>> persistent dirty bitmap playing the role of the add-cow metadata).
>> Well, the idea was that you stream into add-cow and once the streaming
>> has completed, you can just drop the bitmap.
>> There might be some overlap with mirroring, though when we discussed
>> introducing add-cow, mirrors were not yet on the table. One difference
>> that remains is that with streaming the guest only writes to the target
>> image and can't have any problem with convergence.
>>>> I think I can see its use especially for HMP because it's quite
>>>> convenient, but on the other hand it assumes a fixed image path for the
>>>> new raw image. This isn't going to work for block devices, for example.
>>> True, but then probably you would use mode='existing', because you need
>>> to pre-create the logical volume.
>> I think it might be convenient to have the raw volume precreated (you
>> need to do that anyway), but create the COW bitmap only during the
>> snapshot command. But yeah, not really important.
>>>> If we don't do it this way, we need to allow passing image creation
>>>> options to the snapshotting command so that you can pass a precreated
>>>> image file.
>>> This sounds like a useful extension anyway, except that passing an
>>> unstructured string for image creation options is ugly...  Perhaps we
>>> can base a better implementation of options on Laszlo's QemuOpts visitor.
>> Yes, I wouldn't want to use a string here, we should use something
>> structured. Image creation still uses the old-style options instead of
>> QemuOpts, but maybe this is the opportunity to convert it.
> Kevin, do you mean I need to replace QEMUOptionParameter with QemuOpts?
> If true, other image formats should also be changed, I noticed Stefan
> has an un-comleted patch:
> http://repo.or.cz/w/qemu/stefanha.git/commitdiff/b49babb2c8b476a36357cfd7276ca45a11039ca5
> then I can work based on Stefan's patch.

It's been a while so I don't remember details, but if you want to
discuss more let me know and I'll refresh my memory.


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