Re: [libvirt] rbd storage pool support for libvirt

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(a)newdream.net&gt; 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.