Re: [Xen-devel] libvirt, libxl and QDISKs

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Thu, 2013-04-25 at 14:22 +0100, Dave Scott wrote:
> On 25/04/13 13:43, Ian Campbell wrote:
> > On Thu, 2013-04-25 at 13:12 +0100, Dave Scott wrote:
> >> 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.
> >
> > Remind me why you can't specify backend=qdisk explicitly? Does libvirt
> > not propagate anything like that?
> 
> I'll give this a go tomorrow when I'm not at the mercy of dodgy wifi :-) 
>  From my reading of the libvirt code it does allow the selection of a 
> driverName from the set "tap" "tap2" "file" and "phy".

In libxl the choices are phy, tap and qdisk. There's no file nor any
distinction between tap and tap2. I expect the libxl libvirt driver is
trying to make things easier for people migrating from xend (which is
to be applauded).

> It looks like we could define a "qdisk" driverName.

Yes, I think this would work. WRT to your comment below it looks like
other libvirt drivers call this driver "qemu".

> Actually this reminds me of a general concern I had: I worry that people 
> who choose to use libvirt will likely use all the default options, and 
> not customise their code too much for a specific hypervisor. After all, 
> if they wanted to customise their code for xen they would just use libxl 
> and use the richer interface. I think we ought to make sure as much 
> stuff just works by default as possible. In my ideal world it would be 
> possible to take a simple libvirt domain configuration (perhaps one 
> which used to run on kvm) and run it on xen, transparently benefiting 
> from xen features such as driver domains (and automatic blktap/qemu 
> selection?). I think if the domain XML requires too much tweaking then 
> people will just not use the features or may give up altogether.

This would be ideal, I seem to remember hearing that disk configuration
was something which libvirt didn't abstract, tending to just expose
underlying configuration strings of the underlying toolstack. So this is
one area where you actually have to change things when you migrate
between hypervisors or they won't work at all.

I may be talking rubbish or be years out of date with that though.

Ian.


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