Ceph and Cinder multi-backend

Ceph's documentation is quite extensive. However, when trying and installing Ceph on a running Openstack platform, I met two main issues: How to deal with a multi-backend setup? And how to deal with several nova-compute nodes? This note will focus on the steps that I followed in order to have Ceph running as a Cinder backend (among other backends), using cephx authentication.

Ceph node

As described on Ceph's documentation, one has to create a pool on the Ceph nodes (Ceph's doc provides extensive documentation about the number of placement groups that should be used). The following command has to be launched on any Ceph node:

ceph osd pool create volumes 128

Because of cephx authentication, we have to create a new user with the appropriate rights for cinder and nova to be able to access Ceph's storage. The following command has to be launched on a Ceph node:

ceph auth get-or-create client.volumes mon 'allow r' osd 'allow class-read object_prefix rbd_children, allow rwx pool=volumes, allow rx pool=images'

The keyring (token used to identify Ceph users) has to be copied on cinder-volume nodes. The keyring file can be created with the following command on a Ceph node:

ceph auth get-or-create client.volumes > ceph.client.images.keyring

Cinder-volume nodes

The file created above has to be copied on the cinder-volume nodes, in the directory /etc/ceph with the propoer uid and gid:

chown cinder:cinder /etc/ceph/ceph.client.images.keyring

Ceph's configuration file /etc/ceph/ceph.conf has to be copied at the same location.

Then we have to install the following packages on these nodes:

sudo apt-get install python-ceph ceph-common

Cinder configuration file /etc/cinder/cinder.conf has to be updated, by setting a new backend. A new backend that we'll call ceph will be added to the enabled_backends parameter, and the corresponding backend section will be created:

enabled_backends = former-backend,ceph
[former-backend]
...
[ceph]
volume_driver = cinder.volume.drivers.rbd.RBDDriver
volume_backend_name = ceph
rbd_pool = volumes
glance_api_version = 2
rbd_user = volumes
rbd_secret_uuid = uuid_of_secret

The rbd_secret_uuid value cannot be set right now, this parameter will allow nova to mount Ceph block devices. We will update this value in a next step.

If the scheduler_driver parameter is not set to FilterScheduler, it has to be updated:

scheduler_driver = cinder.scheduler.filter_scheduler.FilterScheduler

Once the configuration file updated, cinder-volume service has to be restarted to load the new configuration:

sudo service cinder-volume restart

And a new volume-type has to be added to Cinder, with the following command, which has to be called with an adminitrator credentials (OS_USERNAME, OS_TENANT_NAME and OS_PASSWORD):

cinder type-create ceph
cinder type-key ceph set volume_backend_name=ceph

At that point, we should be able to create new Cinder volumes using Ceph as a backend, with the following command:

cinder create --volume-type ceph --display-name ceph-test 1
cinder list

Nova-compute nodes

Now we have to configure our nova-compute nodes to allow our VMs to mount Ceph block devices. To do that, we have to dump Ceph's authentication token to a file that we'll use on each nova-compute node. On a Ceph node:

ceph auth get-key client.volumes > client.volumes.key

We will also need a secret.xml file that will be used on each compute node, with the following initial content:

<secret ephemeral='no' private='no'>
  <usage type='ceph'>
    <name>client.volumes secret</name>
  </usage>
</secret>

Now we can copy these two files (client.volumes.key and secret.xml) on any nova-compute node. We'll call this node our first nova-compute node. On this first node we will define a secret with the following command:

virsh secret-define --file secret.xml

The UUID_OF_SECRET displayed has to be copied somewhere, since it will be used multiple times to configure nova-compute, as well as cinder-volume. We can then update the secret's value with the following command:

virsh secret-set-value --secret UUID_OF_SECRET --base64 $(cat client.volumes.key)

If using several nova-compute nodes, the secret.xml file has to be updated on the first nova-compute node (in order to ensure that the same UUID_OF_SECRET will be used on each nova-compute node), with the following command:

virsh secret-dumpxml UUID_OF_SECRET > secret.xml

Then with the new secret.xml file and the client.volumes.key file, the previous operation has to be repeated on each nova-compute node (except the first one that is already configured):

virsh secret-define --file secret.xml
virsh secret-set-value --secret UUID_OF_SECRET --base64 $(cat client.volumes.key)

Finally, cinder-volume configuration files /etc/cinder/cinder.conf have to be updated with the proper UUID_OF_SECRET value:

rbd_secret_uuid = UUID_OF_SECRET

And cinder-volume service restarted:

sudo service cinder-volume restart

After that point, any VM should be able to mount volumes using Ceph backend!

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