Re: [Xen-users] where does qemu fit in?

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Wed, 2013-01-02 at 17:30 +0000, Miles Fidelman wrote:
> Ian Campbell wrote:
> > On Wed, 2013-01-02 at 16:35 +0000, Miles Fidelman wrote:
> >
> >
> >> Can you elaborate on this just a bit?  It sounds like one can use qemu
> >> drivers from an HVM, to access the PV drivers.  But what about the other
> >> way around?  I'm running PV guests, so normally qemu doesn't apply,
> >> but... what I'm trying to figure out is whether there's some way to use
> >> the qemu drivers to access the sheepdog cluster file system - which only
> >> provides a qemu interface.
> > I'm not sure what you mean by "qemu drivers". Do you mean virtio?
> qemu-dm

I still don't really know what you mean. Apart from being a Xen specific
term qemu-dm provides emulation of physical devices and Xen PV devices.
Drivers are typically things in the guest which talk to those.

> > Looking (very briefly) at the sheepdog page it looks KVM specific to me,
> > I wouldn't expect it to Just Work on Xen -- someone would need to do a
> > porting job.
> 
> There's a good diagram on slide 5 of 
> http://www.osrg.net/sheepdog/_static/lca2010_miniconf.pdf
> 
> It looks like sheepdog's servers (which talk to disks) run directly on 
> linux, while VMs access virtual block devices via qemu. Performance may 
> be better w/ KVM, but sheepdog doesn't seem to depend on it.

> 
> There's been some discussion on the sheepdog development lists that 
> sheepdog vdi devices (virtual disk images) are compatible with qemu-dm - 
> which suggests that one could mount a sheepdog device from a HVM.  I'm 
> wondering if there's a way to use qemu-dm from a PVM.

So it looks like sheepdog can contain disk images which are exposed to
the guest as either an emulated IDE device or a virtio block device
(although those slides mention neither explicitly) and the clustered
filesystem aspect is not actually exposed to the guest.

I suppose it ought to be possible to have the qdisk backend in Qemu use
the sheepdog backend instead of a file or block device or whatever.

It would be worth trying a vbd configured with
"backendtype=qdisk,target=sheepdog:<foo>" since sheepdog:<foo> seems to
be the syntax (judging from the "Start the VM" section of
https://github.com/collie/sheepdog/wiki/Getting-Started).

I don't know if the Xen toolstack supports passing arbitrary disk
configurations to qemu though, someone (like you) would need to
investigate and possibly do some plumbing on the Xen toolstack side (I'd
be happy to advise on list if it comes to this).

The same sort of thing ought to apply to using the Ceph backend in qemu.
It would be worth googling to see if anyone has done this since I'd
expect the approach ought to carry across.

Ian.


_______________________________________________
Xen-users mailing list
Xen-users@xxxxxxxxxxxxx
http://lists.xen.org/xen-users