[libvirt] Changing the default for qcow2 creation

[Richard W.M. Jones]

On Tue, Oct 26, 2010 at 02:11:22AM +1100, Justin Clift wrote:
> On 10/26/2010 01:06 AM, Richard W.M. Jones wrote:
> <snip>
> >> 4. A metadata preallocated qcow2 file:		 14.5 seconds
> >>     qemu-img create -f qcow2 -o preallocated=metadata test.img 300M
> 
> Just tested this on a F13 host, and it didn't like the the preallocated
> option:
> 
>   $ qemu-img create -f qcow2 -o preallocated=metadata testvm.qcow2 20G
>   Unknown option 'preallocated'
>   qemu-img: Invalid options for file format 'qcow2'.
>   $
> 
> Looks like we'd need to add some kind of testing for the capability
> first. :)

The option should be '-o preallocation=metadata'.  This works for me
on Fedora 13 too.

This option was introduced to qemu upstream in
commit a35e1c177debb01240243bd656caca302410d38c (Aug 17 2009).

You could actually detect this at runtime by running:

$ qemu-img create -f qcow2 -o preallocation=metadata /dev/null 1M
Formatting '/dev/null', fmt=qcow2 size=1048576 encryption=off cluster_size=0 preallocation='metadata' 
$ echo $?
0

$ qemu-img create -f qcow2 -o preallocation=foobar /dev/null 1M 
Formatting '/dev/null', fmt=qcow2 size=1048576 encryption=off cluster_size=0 preallocation='foobar' 
Invalid preallocation mode: 'foobar'
qemu-img: Error while formatting
$ echo $?
1

However the first command segfaults on Fedora 14 (so near and yet so
far!)  I'm going to file a bug about that.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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