[libvirt-users] couple of ceph/rbd questions

[Brian Kroth]

Hi, I've recently been working on setting up a set of libvirt compute 
nodes that will be using a ceph rbd pool for storing vm disk image 
files.  I've got a couple of issues I've run into.

First, per the standard ceph documentation examples [1], the way to add a 
disk is to create a block in the VM definition XML that looks something 
like this:

<disk type='network' device='disk'>
	<source protocol='rbd' name='libvirt-pool/new-libvirt-image'>
		<host name='{monitor-host-1}' port='6789'/>
		<host name='{monitor-host-2}' port='6789'/>
		<host name='{monitor-host-3}' port='6789'/>
	</source>
	<target dev='vda' bus='virtio'/>
	<auth username='libvirt'>
		<secret type='ceph' uuid='9ec59067-fdbc-a6c0-03ff-df165c0587b8'/>
	</auth>
</disk>

The trouble with this approach is that those ceph cluster details 
(secret uuid and monitor host lists), need to be stored separately in 
every single VM disk definition separately.  That makes for a lot of 
maintenance when those details need to change (eg: replace a monitor 
host (common), or change the auth details (less common)).

I'd prefer to be able to define a libvirt storage pool that contains 
those details, and then reference the disks within each VM as volumes, 
so that I only need to change the ceph monitor/auth details once per 
libvirt compute host, rather than for every single VM disk definition.

I've rebuilt my libvirt packages using --with-rbd-support so that I can 
successfully define a libvirt storage pool as follows:

<pool type='rbd'>
        <name>libvirt-rbd-pool</name>
        <source>
                <name>libvirt-pool</name>
		<host name='{monitor-host-1}' port='6789'/>
		<host name='{monitor-host-2}' port='6789'/>
		<host name='{monitor-host-3}' port='6789'/>
                <auth username='libvirt' type='ceph'>
			<secret uuid='9ec59067-fdbc-a6c0-03ff-df165c0587b8'/>
                </auth>
        </source>
</pool>

However, when I go to start a VM with a volume created in that pool as 
follows, I get an error:

<disk type='volume' device='disk'>
	<source pool='libvirt-rbd-pool' volume='{rbd-volume-name}'/>
	<driver name='qemu' type='raw' cache='writethrough'/>
	<target dev='vda' bus='virtio'/>
</disk>

"using 'rbd' pools for backing 'volume' disks isn't yet supported"

When I dug through the code, it appears that there's an explicit check 
for RBD type storage pools (VIR_STORAGE_POOL_RBD) that disables that 
(libvirt-1.2.13/src/storage/storage_driver.c:3159).

Is there a particular reason for that?  Has it just not been implemented 
yet, or am I specifying the disk definition in the wrong way?



Second, using the former disk definition method, I'm able to run VMs 
under qemu, *and* migrate them.  Very slick.  Nice work all.

However, I found that since by default virt-manager leaves the VM 
defined on both the source and destination, I'm actually able to start 
the VM in both places.  I didn't see an option to disable that, so I 
just wrote a simple wrapper script to do the right thing via virsh using 
--undefinesource, but I can't guarantee that some other admin might not 
skip that and just use the GUI.   It appears that libvirt (or is it 
qemu?) doesn't set rbd locks on the disk image files by default.

After running across [2], I had originally thought about writing some 
hooks to set and release locks on the VMs using the rbd cli, but after 
reading the docs on the migration process [3], I think that's probably 
not possible since the VM is started in both places temporarily.

I think my other option is to setup some shared fs (maybe cephfs) and 
point virtlockd at it so that all of the libvirt compute hosts register 
locks on VMs properly.  However, I thought I'd ask if anyone knows if 
there's some magic other parameter or setting I can use to have 
libvirt/qemu just use rbd locks natively.  Or, is that not implemented 
either?

Thanks for your help.

Cheers,
Brian

[1] <http://ceph.com/docs/master/rbd/libvirt/#configuring-the-vm>
[2] <https://www.redhat.com/archives/libvirt-users/2014-January/msg00058.html>
[3] <https://libvirt.org/hooks.html#qemu_migration>
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