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Ceph

RBD

RADOS Block Device (RBD)

The Ceph RBD driver registers a driver named rbd with the libStorage driver manager and is used to connect and mount RADOS Block Devices from a Ceph cluster.

Requirements

  • The ceph and rbd binary executables must be installed on the host
  • The minimum required version of Ceph clients is Infernalis, v9.2.1
  • The rbd kernel module must be installed
  • A <clustername>.conf file must be present in its default location (/etc/ceph/). By default, this is /etc/ceph/ceph.conf.
  • The specified user (admin by default) must have a keyring present in /etc/ceph/.

Configuration

The following is an example with all possible fields configured. For a running example see the Examples section.

rbd:
  defaultPool: rbd
  testModule: true
  cephArgs: --cluster testcluster

Configuration Notes

  • The defaultPool parameter is optional, and defaults to "rbd". When set, all volume requests that do not reference a specific pool will use the defaultPool value as the destination storage pool.
  • The testModule parameter is optional, and defaults to "true". This setting indicates whether the libStorage client should test if the rbd kernel module is loaded, with the side-effect of loading it if is not already loaded. This setting should be disabled when the driver is executing inside of a container.
  • The cephArgs parameter is optional, and defaults to an empty string. When not empty, the value of cepArgs will be exported as the CEPH_ARGS environment variable when making calls to the rados and rbd binaries. The most common use of cephArgs is to set --cluster for an alternative cluster name, or to set --id to use a different CephX user than admin.

Runtime behavior

The Ceph RBD driver only works when the client and server are on the same node. There is no way for a centralized libStorage server to attach volumes to clients, therefore the libStorage server must be running on each node that wishes to mount RBD volumes.

The RBD driver uses the format of <pool>.<name> for the volume ID. This allows for the use of multiple pools by the driver. During a volume create, if the volume ID is given as <pool>.<name>, a volume named name will be created in the pool storage pool. If no pool is referenced, the defaultPool will be used.

!!! note When the rbd.cephArgs config option is set and contains any of the flags --id, --user, -n or --name, support for multiple pools is disabled and only the pool defined by rbd.defaultPool will be used.

Both pool and name may only contain alphanumeric characters, underscores, and dashes.

When querying volumes, the driver will return all RBDs present in all pools in the cluster, prefixing each volume with the appropriate <pool>. value.

All RBD creates are done using the default 4MB object size, and using the "layering" feature bit to ensure greatest compatibility with the kernel clients.

Activating the Driver

To activate the Ceph RBD driver please follow the instructions for activating storage drivers, using rbd as the driver name.

Troubleshooting

  • Make sure that ceph and rbd commands work without extra parameters for monitors. All monitor addresses must come from <clustername>.conf. Be sure to set/export CEPH_ARGS as appropriate based on whether rbd.cephArgs is set.
  • Check status of the ceph cluster with ceph -s command.

Examples

Below is a full config.yml that works with RBD

libstorage:
  # The libstorage.service property directs a libStorage client to direct its
  # requests to the given service by default. It is not used by the server.
  service: rbd
  server:
    services:
      rbd:
        driver: rbd
        rbd:
          defaultPool: rbd
          cephArgs: --id myuser

Caveats

  • Snapshot and copy functionality is not yet implemented
  • libStorage Server must be running on each host to mount/attach RBD volumes
  • There is not yet options for changing RBD create features
  • When using rbd.cephArgs, the specified user/ID must already be defined and its keyring in place at /etc/ceph/<clustername>.client.<id>.keyring, and any given cluster flag must have a config file at /etc/ceph/<clustername>.conf.
  • Volume pre-emption is not supported. Ceph does not provide a method to forcefully detach a volume from a remote host -- only a host can attach and detach volumes from itself.
  • RBD advisory locks are not yet in use. A volume is returned as "unavailable" if it has a watcher other than the requesting client. Until advisory locks are in place, it may be possible for a client to attach a volume that is already attached to another node. Mounting and writing to such a volume could lead to data corruption.