On 03/31/2015 11:47 AM, Brian Kroth wrote:
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)).
You can use a hostname for the monitors here for a level of indirection.
Monitors in ceph require fixed IPs, so they shouldn't need to be added
or removed often in any case.
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?
Just needs implementing afaik. There were patches a while back:
https://www.redhat.com/archives/libvir-list/2014-March/msg00107.html
It'd be great to get those working again.
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.
In the upcoming hammer release of ceph, rbd images can have exclusive
locking built in to prevent concurrent access. If two vms end up running
against the same rbd image, they will trade the lock back and forth,
which is far from ideal, but at least will not corrupt the disk.
It seems like changing the GUI or other wrapper around libvirt to use
--undefinesource is a better solution than adding another layer of
locking.
Josh
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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