Re: libvirt, libxl and QDISKs

[David Scott <dave.scott@eu.citrix.com>]

On 25/04/13 12:57, Ian Campbell wrote:
> On Thu, 2013-04-25 at 12:55 +0100, Stefano Stabellini wrote:
>
>> Right, but going back to the original question: if we have in our hands
>> something that is not vhd, raw or qcow and blktap2 is available,
>> should we really try to pass it to it?
>
> That's not quite the original question because in that case it was raw,
> at least as far as the libxl interface is concerned.
>
>> We do know that blktap2 only handles qcow, qcow2, raw and vhd (and the
>> implementation of the first two formats is too buggy to be used).
>>
>> Thus I think that the answer is pretty obvious here: we should try with
>> QEMU, whose supported format list is way wider.
>
> Which I think we do, we only try and use blktap for raw and vhd.

As well as the disk format dimension (vhd, qcow2 etc) there's also the 
network disk access protocol (iSCSI, ceph/RBD, sheepdog?). Although both 
blktap/tapdisk can handle raw, alas all the interesting (perhaps I'm 
showing my bias here ;-) disk access protocols are in qemu. So if we 
default to blktap/tapdisk we can only handle raw *files*, whereas if we 
default to qemu we can also do these new fancy things as well.

Cheers,
Dave