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Re: [Libguestfs] [PATCH v8 2/4] v2v: add function qemu_img_version to utils



On Mon, 6 Feb 2017 15:25:53 +0000
"Richard W.M. Jones" <rjones redhat com> wrote:

> On Mon, Feb 06, 2017 at 11:06:46AM +0300, Roman Kagan wrote:
> > On Sat, Feb 04, 2017 at 03:10:44PM +0100, Tomáš Golembiovský wrote:  
> > > The function returns version of qemu-img as a tuple (major, minor), or
> > > the value (0,9) in case there was an error detecting the version.  
> > 
> > Depending on QEMU version is extremly unreliable.  Vendors often stick
> > with fairly old versions but backport important features (you at RedHat
> > know that, don't you ;)  So it's not unlikely that e.g. a number of
> > future RHEV/RHEL releases won't use this new feature even though the
> > included QEMU already supports it.
> > 
> > IMO a simpler and more reliable check is to construct a command line for
> > qemu-img that is lightweight to execute and succeeds if the option is
> > supported or fails otherwise.
> > 
> > [ sorry for being so late with this ]  
> 

Good point. I didn't think this through.

> Yes, I agree.  My quick testing shows that the following command
> should be sufficent to test for the "offset" parameter:
> 
> $ truncate -s1024 /tmp/test2
> $ qemu-img create /tmp/test -f qcow2 -b 'json:{"offset":512,"file":{"filename":"/tmp/test2"}}'
> 
> On RHEL 7.3 it fails with:
> 
> qemu-img: /tmp/test: Block format 'raw' used by device '' doesn't support the option 'offset'
> 
> and return code 1.
> 
> (Obviously we'd have to use real temporary files)

No we don't. Something like this also works:

    qemu-img info 'json:{ "offset": 0, "file": { "filename": "/dev/null" } }'


    Tomas


> I'll see if I can make a patch ...
> 
> Rich.
> 
> -- 
> Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
> Read my programming and virtualization blog: http://rwmj.wordpress.com
> libguestfs lets you edit virtual machines.  Supports shell scripting,
> bindings from many languages.  http://libguestfs.org


-- 
Tomáš Golembiovský <tgolembi redhat com>


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