Re: [Qemu-devel] [PATCH v5 12/22] block: Allow recursive "file"s

[Eric Blake <eblake@redhat.com>]

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On 12/13/2013 01:19 PM, Kevin Wolf wrote:
> Am 13.12.2013 um 18:10 hat Max Reitz geschrieben:
>> It should be possible to use a format as a driver for a file which in
>> turn requires another file, i.e., nesting file formats.
>>
>> Signed-off-by: Max Reitz <mreitz@redhat.com>
> 
> Hm, does this do what I think it does?

That depends on what you think it's doing :)

> 
> $ ./qemu-img convert -O qcow2 /home/kwolf/images/hd.img /tmp/hd.qcow2
> $ ./qemu-img convert -f raw -O qcow2 /tmp/hd.qcow2 /tmp/hd.qcow2.qcow2
> $ x86_64-softmmu/qemu-system-x86_64 -drive driver=qcow2,file.driver=qcow2,file.file.driver=file,file.file.filename=/tmp/hd.qcow2.qcow2
> 
> I can't decide whether this is awesomeness or insanity, but in any case
> it works with this patch. :-)

So if I understood your example, you just created a qcow2 file, where
the non-metadata contents of that file are themselves a qcow2 data format.

Oh my.  This could get nasty if someone ever wanted to try to get
libvirt to feed the nested contents to a guest (right now, libvirt has a
hard-coded assumption that qcow2 files only ever wrap raw data in the
non-metadata portion; there's no easy way to represent in the libvirt
storage volume XML a file where you want the nested guest contents of
qcow2 data wrapped inside the non-metadata portion of an outermost qcow2
file).

I guess it's not much different from a raw block device that has a
partition table, then in the first partition, you recursively add
another partition table (which is what you get when you hand one
partition of a host block device to the guest, and the guest then
partitions what it thinks is a full disk).  And libguestfs is able to
see through nested partition tables, so on that front, the functionality
is similar.

Of course, the recursion has its use for blkverify and blkdebug, but
maybe (for our sanity) it would be worth preventing nesting across any
format that has metadata wrapping contents.

> 
> Worth a qemu-iotests case, I think.

That's for sure :)

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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