Re: [Qemu-devel] qemu-img behavior for locating backing files

[Eric Blake]

On 04/01/2015 10:16 AM, John Snow wrote:
> Kevin, what's the correct behavior for qemu-img and relative paths when
> creating a new qcow2 file?
> 
> Example:
> 
> (in e.g. /home/qemu/build/ or anywhere not /home: )
> qemu-img create -f qcow2 base.qcow2 32G

creates /home/qemu/build/base.qcow2

> qemu-img create -f qcow2 -F qcow2 -b base.qcow2 /home/overlay.qcow2

Tries to create /home/overlay.qcow2; requires /home/base.qcow2 to exist
for the creation to be well-formed.  (Any use of
/home/qemu/build/base.qcow2 should be wrong)

If you want, you could do:

qemu-img create -f qcow2 /home/overlay.qcow2 $size
qemu-img rebase -u -f qcow2 -F qcow2 -b base.qcow2 /home/overlay.qcow2

to create the file that would relatively point to /home/base.qcow2,
whether or not that file already exists; and it could be argued that we
may even want to support that via a single create command (that is,
'create an image with this string as the backing file, but without
actually chasing through that string')

> 
> In 1.7.0., this produces a warning that the base object cannot be found
> (because it does not exist at that location relative to overlay.qcow2),
> but qemu-img will create the qcow2 for you regardless.

Sounds almost ideal (or at least an argument for the 'create with an
unsafe string for backing) - but how did it pick the size for that image?

> 
> 2.0, 2.1 and 2.2 all will create the image successfully, with no warnings.

Oops.

> 
> 2.3-rc1/master as they exist now will emit an error message and create
> no image.

Sounds like a bug fix, not a regression.

> 
> Since this is a change in behavior for the pending release, is this the
> correct/desired behavior?

Yes, I think so.

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

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