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

From: Kevin Wolf
Subject: Re: [Qemu-devel] [PATCH 5/6] add-cow: support snapshot_blkdev
Date: Thu, 14 Jun 2012 13:33:58 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:12.0) Gecko/20120430 Thunderbird/12.0.1

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.


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