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301 lines
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301 lines
11 KiB
ReStructuredText
===============
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RBD Mirroring
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===============
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.. index:: Ceph Block Device; mirroring
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RBD images can be asynchronously mirrored between two Ceph clusters. This
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capability uses the RBD journaling image feature to ensure crash-consistent
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replication between clusters. Mirroring is configured on a per-pool basis
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within peer clusters and can be configured to automatically mirror all
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images within a pool or only a specific subset of images. Mirroring is
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configured using the ``rbd`` command. The ``rbd-mirror`` daemon is responsible
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for pulling image updates from the remote, peer cluster and applying them to
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the image within the local cluster.
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.. note:: RBD mirroring requires the Ceph Jewel release or later.
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.. important:: To use RBD mirroring, you must have two Ceph clusters, each
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running the ``rbd-mirror`` daemon.
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Pool Configuration
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==================
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The following procedures demonstrate how to perform the basic administrative
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tasks to configure mirroring using the ``rbd`` command. Mirroring is
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configured on a per-pool basis within the Ceph clusters.
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The pool configuration steps should be performed on both peer clusters. These
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procedures assume two clusters, named "local" and "remote", are accessible from
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a single host for clarity.
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See the `rbd`_ manpage for additional details of how to connect to different
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Ceph clusters.
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.. note:: The cluster name in the following examples corresponds to a Ceph
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configuration file of the same name (e.g. /etc/ceph/remote.conf). See the
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`ceph-conf`_ documentation for how to configure multiple clusters.
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Enable Mirroring
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----------------
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To enable mirroring on a pool with ``rbd``, specify the ``mirror pool enable``
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command, the pool name, and the mirroring mode::
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rbd mirror pool enable {pool-name} {mode}
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The mirroring mode can either be ``pool`` or ``image``:
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* **pool**: When configured in ``pool`` mode, all images in the pool with the
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journaling feature enabled are mirrored.
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* **image**: When configured in ``image`` mode, mirroring needs to be
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`explicitly enabled`_ on each image.
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For example::
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rbd --cluster local mirror pool enable image-pool pool
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rbd --cluster remote mirror pool enable image-pool pool
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Disable Mirroring
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-----------------
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To disable mirroring on a pool with ``rbd``, specify the ``mirror pool disable``
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command and the pool name::
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rbd mirror pool disable {pool-name}
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When mirroring is disabled on a pool in this way, mirroring will also be
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disabled on any images (within the pool) for which mirroring was enabled
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explicitly.
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For example::
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rbd --cluster local mirror pool disable image-pool
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rbd --cluster remote mirror pool disable image-pool
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Add Cluster Peer
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----------------
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In order for the ``rbd-mirror`` daemon to discover its peer cluster, the peer
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needs to be registered to the pool. To add a mirroring peer Ceph cluster with
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``rbd``, specify the ``mirror pool peer add`` command, the pool name, and a
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cluster specification::
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rbd mirror pool peer add {pool-name} {client-name}@{cluster-name}
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For example::
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rbd --cluster local mirror pool peer add image-pool client.remote@remote
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rbd --cluster remote mirror pool peer add image-pool client.local@local
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Remove Cluster Peer
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-------------------
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To remove a mirroring peer Ceph cluster with ``rbd``, specify the
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``mirror pool peer remove`` command, the pool name, and the peer UUID
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(available from the ``rbd mirror pool info`` command)::
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rbd mirror pool peer remove {pool-name} {peer-uuid}
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For example::
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rbd --cluster local mirror pool peer remove image-pool 55672766-c02b-4729-8567-f13a66893445
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rbd --cluster remote mirror pool peer remove image-pool 60c0e299-b38f-4234-91f6-eed0a367be08
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Image Configuration
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===================
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Unlike pool configuration, image configuration only needs to be performed against
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a single mirroring peer Ceph cluster.
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Mirrored RBD images are designated as either primary or non-primary. This is a
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property of the image and not the pool. Images that are designated as
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non-primary cannot be modified.
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Images are automatically promoted to primary when mirroring is first enabled on
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an image (either implicitly if the pool mirror mode was **pool** and the image
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has the journaling image feature enabled, or `explicitly enabled`_ by the
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``rbd`` command).
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Enable Image Journaling Support
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-------------------------------
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RBD mirroring uses the RBD journaling feature to ensure that the replicated
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image always remains crash-consistent. Before an image can be mirrored to
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a peer cluster, the journaling feature must be enabled. The feature can be
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enabled at image creation time by providing the
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``--image-feature exclusive-lock,journaling`` option to the ``rbd`` command.
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Alternatively, the journaling feature can be dynamically enabled on
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pre-existing RBD images. To enable journaling with ``rbd``, specify
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the ``feature enable`` command, the pool and image name, and the feature name::
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rbd feature enable {pool-name}/{image-name} {feature-name}
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For example::
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rbd --cluster local feature enable image-pool/image-1 journaling
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.. note:: The journaling feature is dependent on the exclusive-lock feature. If
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the exclusive-lock feature is not already enabled, it should be enabled prior
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to enabling the journaling feature.
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.. tip:: You can enable journaling on all new images by default by adding
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``rbd default features = 125`` to your Ceph configuration file.
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Enable Image Mirroring
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----------------------
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If the mirroring is configured in ``image`` mode for the image's pool, then it
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is necessary to explicitly enable mirroring for each image within the pool.
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To enable mirroring for a specific image with ``rbd``, specify the
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``mirror image enable`` command along with the pool and image name::
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rbd mirror image enable {pool-name}/{image-name}
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For example::
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rbd --cluster local mirror image enable image-pool/image-1
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Disable Image Mirroring
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-----------------------
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To disable mirroring for a specific image with ``rbd``, specify the
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``mirror image disable`` command along with the pool and image name::
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rbd mirror image disable {pool-name}/{image-name}
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For example::
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rbd --cluster local mirror image disable image-pool/image-1
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Image Promotion and Demotion
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----------------------------
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In a failover scenario where the primary designation needs to be moved to the
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image in the peer Ceph cluster, access to the primary image should be stopped
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(e.g. power down the VM or remove the associated drive from a VM), demote the
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current primary image, promote the new primary image, and resume access to the
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image on the alternate cluster.
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.. note:: RBD only provides the necessary tools to facilitate an orderly
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failover of an image. An external mechanism is required to coordinate the
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full failover process (e.g. closing the image before demotion).
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To demote a specific image to non-primary with ``rbd``, specify the
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``mirror image demote`` command along with the pool and image name::
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rbd mirror image demote {pool-name}/{image-name}
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For example::
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rbd --cluster local mirror image demote image-pool/image-1
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To demote all primary images within a pool to non-primary with ``rbd``, specify
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the ``mirror pool demote`` command along with the pool name::
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rbd mirror pool demote {pool-name}
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For example::
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rbd --cluster local mirror pool demote image-pool
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To promote a specific image to primary with ``rbd``, specify the
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``mirror image promote`` command along with the pool and image name::
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rbd mirror image promote [--force] {pool-name}/{image-name}
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For example::
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rbd --cluster remote mirror image promote image-pool/image-1
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To promote all non-primary images within a pool to primary with ``rbd``, specify
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the ``mirror pool promote`` command along with the pool name::
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rbd mirror pool promote [--force] {pool-name}
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For example::
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rbd --cluster local mirror pool promote image-pool
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.. tip:: Since the primary / non-primary status is per-image, it is possible to
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have two clusters split the IO load and stage failover / failback.
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.. note:: Promotion can be forced using the ``--force`` option. Forced
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promotion is needed when the demotion cannot be propagated to the peer
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Ceph cluster (e.g. Ceph cluster failure, communication outage). This will
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result in a split-brain scenario between the two peers and the image will no
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longer be in-sync until a `force resync command`_ is issued.
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Force Image Resync
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------------------
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If a split-brain event is detected by the ``rbd-mirror`` daemon, it will not
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attempt to mirror the affected image until corrected. To resume mirroring for an
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image, first `demote the image`_ determined to be out-of-date and then request a
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resync to the primary image. To request an image resync with ``rbd``, specify the
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``mirror image resync`` command along with the pool and image name::
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rbd mirror image resync {pool-name}/{image-name}
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For example::
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rbd mirror image resync image-pool/image-1
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.. note:: The ``rbd`` command only flags the image as requiring a resync. The
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local cluster's ``rbd-mirror`` daemon process is responsible for performing
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the resync asynchronously.
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Mirror Status
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=============
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The peer cluster replication status is stored for every primary mirrored image.
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This status can be retrieved using the ``mirror image status`` and
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``mirror pool status`` commands.
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To request the mirror image status with ``rbd``, specify the
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``mirror image status`` command along with the pool and image name::
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rbd mirror image status {pool-name}/{image-name}
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For example::
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rbd mirror image status image-pool/image-1
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To request the mirror pool summary status with ``rbd``, specify the
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``mirror pool status`` command along with the pool name::
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rbd mirror pool status {pool-name}
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For example::
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rbd mirror pool status image-pool
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.. note:: Adding ``--verbose`` option to the ``mirror pool status`` command will
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additionally output status details for every mirroring image in the pool.
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rbd-mirror Daemon
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=================
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The two ``rbd-mirror`` daemons are responsible for watching image journals on the
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remote, peer cluster and replaying the journal events against the local
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cluster. The RBD image journaling feature records all modifications to the
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image in the order they occur. This ensures that a crash-consistent mirror of
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the remote image is available locally.
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The ``rbd-mirror`` daemon is available within the optional ``rbd-mirror``
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distribution package.
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.. important:: Each ``rbd-mirror`` daemon requires the ability to connect
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to both clusters simultaneously.
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.. warning:: Only run a single ``rbd-mirror`` daemon per Ceph cluster. A
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future Ceph release will add support for horizontal scale-out of the
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``rbd-mirror`` daemon.
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.. _rbd: ../../man/8/rbd
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.. _ceph-conf: ../../rados/configuration/ceph-conf/#running-multiple-clusters
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.. _explicitly enabled: #enable-image-mirroring
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.. _force resync command: #force-image-resync
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.. _demote the image: #image-promotion-and-demotion
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