Re: [libvirt] rbd storage pool support for libvirt

["Daniel P. Berrange" <berrange@redhat.com>]

On Fri, Nov 19, 2010 at 09:27:40AM +0000, Stefan Hajnoczi wrote:
> On Thu, Nov 18, 2010 at 5:13 PM, Sage Weil <sage@newdream.net> wrote:
> > On Thu, 18 Nov 2010, Daniel P. Berrange wrote:
> >> On Wed, Nov 17, 2010 at 04:33:07PM -0800, Josh Durgin wrote:
> >> > Hi Daniel,
> >> >
> >> > On 11/08/2010 05:16 AM, Daniel P. Berrange wrote:
> >> > >>>>In any case, before someone goes off and implements something, does this
> >> > >>>>look like the right general approach to adding rbd support to libvirt?
> >> > >>>
> >> > >>>I think this looks reasonable. I'd be inclined to get the storage pool
> >> > >>>stuff working with the kernel RBD driver&  UDEV rules for stable path
> >> > >>>names, since that avoids needing to make any changes to guest XML
> >> > >>>format. Support for QEMU with the native librados CEPH driver could
> >> > >>>be added as a second patch.
> >> > >>
> >> > >>Okay, that sounds reasonable.  Supporting the QEMU librados driver is
> >> > >>definitely something we want to target, though, and seems to be route that
> >> > >>more users are interested in.  Is defining the XML syntax for a guest VM
> >> > >>something we can discuss now as well?
> >> > >>
> >> > >>(BTW this is biting NBD users too.  Presumably the guest VM XML should
> >> > >>look similar?
> >> > >
> >> > >And also Sheepdog storage volumes. To define a syntax for all these we need
> >> > >to determine what configuration metadata is required at a per-VM level for
> >> > >each of them. Then try and decide how to represent that in the guest XML.
> >> > >It looks like at a VM level we'd need a hostname, port number and a volume
> >> > >name (or path).
> >> >
> >> > It looks like that's what Sheepdog needs from the patch that was
> >> > submitted earlier today. For RBD, we would want to allow multiple hosts,
> >> > and specify the pool and image name when the QEMU librados driver is
> >> > used, e.g.:
> >> >
> >> >     <disk type="rbd" device="disk">
> >> >       <driver name="qemu" type="raw" />
> >> >       <source vdi="image_name" pool="pool_name">
> >> >         <host name="mon1.example.org" port="6000">
> >> >         <host name="mon2.example.org" port="6000">
> >> >         <host name="mon3.example.org" port="6000">
> >> >       </source>
> >> >       <target dev="vda" bus="virtio" />
> >> >     </disk>
> >> >
> >> > Does this seem like a reasonable format for the VM XML? Any suggestions?
> >>
> >> I'm basically wondering whether we should be going for separate types for
> >> each of NBD, RBD & Sheepdog, as per your proposal & the sheepdog one earlier
> >> today. Or type to merge them into one type 'nework' which covers any kind of
> >> network block device, and list a protocol on the  source element, eg
> >>
> >>      <disk type="network" device="disk">
> >>        <driver name="qemu" type="raw" />
> >>        <source protocol='rbd|sheepdog|nbd' name="...some image identifier...">
> >>          <host name="mon1.example.org" port="6000">
> >>          <host name="mon2.example.org" port="6000">
> >>          <host name="mon3.example.org" port="6000">
> >>        </source>
> >>        <target dev="vda" bus="virtio" />
> >>      </disk>
> >
> > That would work...
> >
> > One thing that I think should be considered, though, is that both RBD and
> > NBD can be used for non-qemu instances by mapping a regular block device
> > via the host's kernel.  And in that case, there's some sysfs-fu (at least
> > in the rbd case; I'm not familiar with how the nbd client works) required
> > to set up/tear down the block device.
> 
> An nbd block device is attached using the nbd-client(1) userspace tool:
> $ nbd-client my-server 1234 /dev/nbd0 # <host> <port> <nbd-device>
> 
> That program will open the socket, grab /dev/nbd0, and poke it with a
> few ioctls so the kernel has the socket and can take it from there.

We don't need to worry about this for libvirt/QEMU. Since QEMU has native
NBD client support there's no need to do anything with nbd client tools
to setup the device for use with a VM.

Regards,
Daniel
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