===============
 RBD Mirroring
===============

.. index:: Ceph Block Device; mirroring

RBD images can be asynchronously mirrored between two Ceph clusters. This
capability uses the RBD journaling image feature to ensure crash-consistent
replication between clusters. Mirroring is configured on a per-pool basis
within peer clusters and can be configured to automatically mirror all
images within a pool or only a specific subset of images. Mirroring is
configured using the ``rbd`` command. The ``rbd-mirror`` daemon is responsible
for pulling image updates from the remote, peer cluster and applying them to
the image within the local cluster.

.. note:: RBD mirroring requires the Ceph Jewel release or later.

.. important:: To use RBD mirroring, you must have two Ceph clusters, each
   running the ``rbd-mirror`` daemon.

Pool Configuration
==================

The following procedures demonstrate how to perform the basic administrative
tasks to configure mirroring using the ``rbd`` command. Mirroring is
configured on a per-pool basis within the Ceph clusters.

The pool configuration steps should be performed on both peer clusters. These
procedures assume two clusters, named "local" and "remote", are accessible from
a single host for clarity.

See the `rbd`_ manpage for additional details of how to connect to different
Ceph clusters.

.. note:: The cluster name in the following examples corresponds to a Ceph
   configuration file of the same name (e.g. /etc/ceph/remote.conf).  See the
   `ceph-conf`_ documentation for how to configure multiple clusters.

Enable Mirroring
----------------

To enable mirroring on a pool with ``rbd``, specify the ``mirror pool enable``
command, the pool name, and the mirroring mode::

        rbd mirror pool enable {pool-name} {mode}

The mirroring mode can either be ``pool`` or ``image``:

* **pool**:  When configured in ``pool`` mode, all images in the pool with the
  journaling feature enabled are mirrored.
* **image**: When configured in ``image`` mode, mirroring needs to be
  `explicitly enabled`_ on each image.

For example::

        rbd --cluster local mirror pool enable image-pool pool
        rbd --cluster remote mirror pool enable image-pool pool

Disable Mirroring
-----------------

To disable mirroring on a pool with ``rbd``, specify the ``mirror pool disable``
command and the pool name::

        rbd mirror pool disable {pool-name}

When mirroring is disabled on a pool in this way, mirroring will also be
disabled on any images (within the pool) for which mirroring was enabled
explicitly.

For example::

        rbd --cluster local mirror pool disable image-pool
        rbd --cluster remote mirror pool disable image-pool

Add Cluster Peer
----------------

In order for the ``rbd-mirror`` daemon to discover its peer cluster, the peer
needs to be registered to the pool. To add a mirroring peer Ceph cluster with
``rbd``, specify the ``mirror pool peer add`` command, the pool name, and a
cluster specification::

        rbd mirror pool peer add {pool-name} {client-name}@{cluster-name}

For example::

        rbd --cluster local mirror pool peer add image-pool client.remote@remote
        rbd --cluster remote mirror pool peer add image-pool client.local@local

Remove Cluster Peer
-------------------

To remove a mirroring peer Ceph cluster with ``rbd``, specify the
``mirror pool peer remove`` command, the pool name, and the peer UUID
(available from the ``rbd mirror pool info`` command)::

        rbd mirror pool peer remove {pool-name} {peer-uuid}

For example::

        rbd --cluster local mirror pool peer remove image-pool 55672766-c02b-4729-8567-f13a66893445
        rbd --cluster remote mirror pool peer remove image-pool 60c0e299-b38f-4234-91f6-eed0a367be08

Image Configuration
===================

Unlike pool configuration, image configuration only needs to be performed against
a single mirroring peer Ceph cluster.

Mirrored RBD images are designated as either primary or non-primary. This is a
property of the image and not the pool. Images that are designated as
non-primary cannot be modified.

Images are automatically promoted to primary when mirroring is first enabled on
an image (either implicitly if the pool mirror mode was **pool** and the image
has the journaling image feature enabled, or `explicitly enabled`_ by the
``rbd`` command).

Enable Image Journaling Support
-------------------------------

RBD mirroring uses the RBD journaling feature to ensure that the replicated
image always remains crash-consistent. Before an image can be mirrored to
a peer cluster, the journaling feature must be enabled. The feature can be
enabled at image creation time by providing the
``--image-feature exclusive-lock,journaling`` option to the ``rbd`` command.

Alternatively, the journaling feature can be dynamically enabled on
pre-existing RBD images. To enable journaling with ``rbd``, specify
the ``feature enable`` command, the pool and image name, and the feature name::

        rbd feature enable {pool-name}/{image-name} {feature-name}

For example::

        rbd --cluster local feature enable image-pool/image-1 journaling

.. note:: The journaling feature is dependent on the exclusive-lock feature. If
   the exclusive-lock feature is not already enabled, it should be enabled prior
   to enabling the journaling feature.

.. tip:: You can enable journaling on all new images by default by adding
   ``rbd default features = 125`` to your Ceph configuration file.

Enable Image Mirroring
----------------------

If the mirroring is configured in ``image`` mode for the image's pool, then it
is necessary to explicitly enable mirroring for each image within the pool.
To enable mirroring for a specific image with ``rbd``, specify the
``mirror image enable`` command along with the pool and image name::

        rbd mirror image enable {pool-name}/{image-name}

For example::

        rbd --cluster local mirror image enable image-pool/image-1

Disable Image Mirroring
-----------------------

To disable mirroring for a specific image with ``rbd``, specify the
``mirror image disable`` command along with the pool and image name::

        rbd mirror image disable {pool-name}/{image-name}

For example::

        rbd --cluster local mirror image disable image-pool/image-1

Image Promotion and Demotion
----------------------------

In a failover scenario where the primary designation needs to be moved to the
image in the peer Ceph cluster, access to the primary image should be stopped
(e.g. power down the VM or remove the associated drive from a VM), demote the
current primary image, promote the new primary image, and resume access to the
image on the alternate cluster.

.. note:: RBD only provides the necessary tools to facilitate an orderly
   failover of an image. An external mechanism is required to coordinate the
   full failover process (e.g. closing the image before demotion).

To demote an image to non-primary with ``rbd``, specify the
``mirror image demote`` command along with the pool and image name::

        rbd mirror image demote {pool-name}/{image-name}

For example::

        rbd --cluster local mirror image demote image-pool/image-1

To promote an image to primary with ``rbd``, specify the ``mirror image promote``
command along with the pool and image name::

        rbd mirror image promote {pool-name}/{image-name}

For example::

        rbd --cluster remote mirror image promote image-pool/image-1

.. tip:: Since the primary / non-primary status is per-image, it is possible to
   have two clusters split the IO load and stage failover / failback.

.. note:: Promotion can be forced using the ``--force`` option. Forced
   promotion is needed when the demotion cannot be propagated to the peer
   Ceph cluster (e.g. Ceph cluster failure, communication outage). This will
   result in a split-brain scenario between the two peers and the image will no
   longer be in-sync until a `force resync command`_ is issued.

Force Image Resync
------------------

If a split-brain event is detected by the ``rbd-daemon``, it will not attempt
to mirror the affected image until corrected. To resume mirroring for an image,
first `demote the image`_ determined to be out-of-date and then request a resync
to the primary image. To request an image resync with ``rbd``, specify the
``mirror image resync`` command along with the pool and image name::

        rbd mirror image resync {pool-name}/{image-name}

For example::

        rbd mirror image resync image-pool/image-1

.. note:: The ``rbd`` command only flags the image as requiring a resync. The
   local cluster's ``rbd-mirror`` daemon process is responsible for performing
   the resync asynchronously.

rbd-mirror Daemon
=================

The two ``rbd-mirror`` daemons are responsible for watching image journals on the
remote, peer cluster and replaying the journal events against the local
cluster. The RBD image journaling feature records all modifications to the
image in the order they occur. This ensures that a crash-consistent mirror of
the remote image is available locally.

The ``rbd-mirror`` daemon is available within the optional ``rbd-mirror``
distribution package.

.. important:: Each ``rbd-mirror`` daemon requires the ability to connect
   to both clusters simultaneously.
.. warning:: Only run a single ``rbd-mirror`` daemon per Ceph cluster. A
   future Ceph release will add support for horizontal scale-out of the
   ``rbd-mirror`` daemon.

.. _rbd: ../../man/8/rbd
.. _ceph-conf: ../../rados/configuration/ceph-conf/#running-multiple-clusters
.. _explicitly enabled: #enable-image-mirroring
.. _force resync command: #force-image-resync
.. _demote the image: #image-promotion-and-demotion