Setting up a Ceph cluster and exporting a RBD volume to a KVM guest

Posted: October 12th, 2011 | Filed under: Fedora, libvirt, Virt Tools | Tags: , , , , , , , | 5 Comments »

Yesterday I talked about setting up Sheepdog with KVM, so today is it is time to discuss use of Ceph and RBD with KVM.

Host Cluster Setup, the easy way

Fedora has included Ceph for a couple of releases, but since my hosts are on Fedora 14/15, I grabbed the latest ceph 0.3.1 sRPMs from Fedora 16 and rebuilt those to get something reasonably up2date. In the end I have the following packages installed, though to be honest I don’t really need anything except the base ‘ceph’ RPM:

# rpm -qa | grep ceph | sort

Installing the software is the easy bit, configuring the cluster is where the fun begins. I had three hosts available for testing all of which are virtualization hosts. Ceph has at least 3 daemons it needs to run, which should all be replicated across several hosts for redundancy. There’s no requirement to use the same hosts for each daemon, but for simplicity I decided to run every Ceph daemon on every virtualization host.

My hosts are called lettuce, avocado and mustard. Following the Ceph wiki instructions, I settled on a configuration file that looks like this:

    auth supported = cephx
    keyring = /etc/ceph/keyring.admin

    keyring = /etc/ceph/keyring.$name
    host = lettuce
    host = avocado
    host = mustard

    osd data = /srv/ceph/osd$id
    osd journal = /srv/ceph/osd$id/journal
    osd journal size = 512
    osd class dir = /usr/lib64/rados-classes
    keyring = /etc/ceph/keyring.$name
    host = lettuce
    host = avocado
    host = mustard

    mon data = /srv/ceph/mon$id
    host = lettuce
    mon addr =
    host = avocado
    mon addr =
    host = mustard
    mon addr =

The osd class dir bit should not actually be required, but the OSD code looks in the wrong place (/usr/lib instead of /usr/lib64) on x86_64 arches.

With the configuration file written, it is time to actually initialize the cluster filesystem / object store. This is the really fun bit. The Ceph wiki has a very basic page which talks about the mkcephfs tool, along with a scary warning about how it’ll ‘rm -rf’ all the data on the filesystem it is initializing. It turns out that it didn’t mean your entire host filesystem, AFAICT, it only the blows away the contents of the directory configured for ‘osd data‘ and ‘mon data‘, in my case both under /srv/ceph.

The recommended way is to let mkcephfs ssh into each of your hosts and run all the configuration tasks automatically. Having tried the non-recommended way and failed several times before finally getting it right, I can recommend following the recommended way :-P There are some caveats not mentioned in the wiki page though:

  • The configuration file above must be copied to /etc/ceph/ceph.conf on every node before attempting to run mkcephfs.
  • The configuration file on the host where you run mkcephfsmust be in /etc/ceph/ceph.conf or it will get rather confused about where it is in the other nodes.
  • The mkcephfscommand must be run as root since, it doesn’t specify ‘-l root’ to ssh, leading to an inability to setup the nodes.
  • The directories /srv/ceph/osd$i must be pre-created, since it is unable to do that itself, despite being able to creat the /srv/ceph/mon$idirectories.
  • The Fedora RPMs have also forgotten to create /etc/ceph

With that in mind, I ran the following commands from my laptop, as root

 # n=0
 # for host in lettuce avocado mustard ; \
   do \
       ssh root@$host mkdir -p /etc/ceph /srv/ceph/mon$n; \
       n=$(expr $n + 1; \
       scp /etc/ceph/ceph.conf root@$host:/etc/ceph/ceph.conf
 # mkcephfs -a -c /etc/ceph/ceph.conf -k /etc/ceph/keyring.bin

On the host where you ran mkcephfs there should now be a file /etc/ceph/keyring.admin. This will be needed for mounting filesystems. I copied it across to all my virtualization hosts

 # for host in lettuce avocado mustard ; \
   do \
       scp /etc/ceph/keyring.admin root@$host:/etc/ceph/keyring.admin; \

Host Cluster Usage

Assuming the setup phase all went to plan, the cluster can now be started. A word of warning though, Ceph really wants your clocks VERY well synchronized. If your NTP server is a long way away, the synchronization might not be good enough to stop Ceph complaining. You really want a NTP server on your local LAN for hosts to sync against. Sort this out before trying to start the cluster.

 # for host in lettuce avocado mustard ; \
   do \
       ssh root@$host service ceph start; \

The ceph tool can show the status of everything. The ‘mon’, ‘osd’ and ‘msd’ lines in the status ought to show all 3 host present & correct

# ceph -s
2011-10-12 14:49:39.085764    pg v235: 594 pgs: 594 active+clean; 24 KB data, 94212 MB used, 92036 MB / 191 GB avail
2011-10-12 14:49:39.086585   mds e6: 1/1/1 up {0=lettuce=up:active}, 2 up:standby
2011-10-12 14:49:39.086622   osd e5: 3 osds: 3 up, 3 in
2011-10-12 14:49:39.086908   log 2011-10-12 14:38:50.263058 osd1 197 : [INF] 2.1p1 scrub ok
2011-10-12 14:49:39.086977   mon e1: 3 mons at {0=,1=,2=}

The cluster configuration I chose has authentication enabled, so to actually mount the ceph filesystem requires a secret key. This key is stored in the /etc/ceph/keyring.admin file that was created earlier. To view the keyring contents, the cauthtool program must be used

# cauthtool -l /etc/ceph/keyring.admin 
	key = AQDLk5VOeHkHLxAAfGjcaUsOXOhJr7hZCNjXSQ==
	auid = 18446744073709551615

The base64 key there will be passed to the mount command, repeating on every host needing a filesystem present:

# mount -t ceph /mnt/ -o name=admin,secret=AQDLk5VOeHkHLxAAfGjcaUsOXOhJr7hZCNjXSQ==
error adding secret to kernel, key name client.admin: No such device

For some reason, that error message is always printed on my Fedora hosts, and despite that, the mount has actually succeeded

# grep /mnt /proc/mounts /mnt ceph rw,relatime,name=admin,secret= 0 0

Congratulations, /mnt is now a distributed filesystem. If you create a file on one host, it should appear on the other hosts & vica-verca.

RBD Volume setup

A shared filesystem is very nice, and can be used to hold regular virtual disk images in a variety of formats (raw, qcow2, etc). What I really wanted to try was the RBD virtual block device functionality in QEMU. Ceph includes a tool called rbd for manipulating those. The syntax of this tool is pretty self-explanatory

# rbd create --size 100 demo
# rbd ls
# rbd info demo
rbd image 'demo':
	size 102400 KB in 25 objects
	order 22 (4096 KB objects)
	block_name_prefix: rb.0.0
	parent:  (pool -1)

Alternatively RBD volume creation can be done using qemu-img …. at least once the Fedora QEMU package is fixed to enable RBD support.

# qemu-img create -f rbd rbd:rbd/demo  100M
Formatting 'rbd:rbd/foo', fmt=rbd size=104857600 cluster_size=0 
# qemu-img info rbd:rbd/demo
image: rbd:rbd/foo
file format: raw
virtual size: 100M (104857600 bytes)
disk size: unavailable

KVM guest setup

The syntax for configuring a RBD block device in libvirt, is very similar to that used for Sheepdog. In Sheepdog, every single virtualization node is also a storage node, so there is no hostname required. Not so for RBD. Here it is necessary to specify one or more host names, for the RBD servers.

<disk type='network' device='disk'>
  <driver name='qemu' type='raw'/>
  <source protocol='rbd' name='demo/wibble'>
    <host name='' port='6798'/>
    <host name='' port='6798'/>
    <host name='' port='6798'/>
  <target dev='vdb' bus='virtio'/>

More observant people might be wondering how QEMU gets permission to connect to the RBD server, given that the configuration earlier enabled authentication. This is thanks to the magic of the /etc/ceph/keyring.admin file which must exist on any virtualization server. Patches are currently being discussed which will allow authentication credentials to be set via libvirt, avoiding the need to store the credentials on the virtualization hosts permanently.