[libvirt] rbd storage pool support for libvirt

[Stefan Hajnoczi]

On Thu, Nov 18, 2010 at 5:13 PM, Sage Weil <sage at 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.

Stefan