================== Ceph RBD mirroring ================== Overview -------- RADOS Block Device (RBD) mirroring is a process of asynchronous replication of Ceph block device images between two or more Ceph clusters. Mirroring ensures point-in-time consistent replicas of all changes to an image, including reads and writes, block device resizing, snapshots, clones, and flattening. RBD mirroring is mainly used for disaster recovery (i.e. having a secondary site as a failover). See `Upstream Ceph documentation on RBD mirroring`_ for complete information. This guide will show how to deploy two Ceph clusters with RBD mirroring between them with the use of the ceph-rbd-mirror charm. See the `charm's documentation`_ for basic information and charm limitations. RBD mirroring is only one aspect of datacentre redundancy. Refer to `Ceph RADOS Gateway Multisite Replication`_ and other work to arrive at a complete solution. .. note:: RBD mirroring makes use of the journaling feature of Ceph. This incurs an overhead for write activity on an RBD image that will adversely affect performance. See Florian Haas' performance analysis of `RBD mirror`_ from Cephalocon Barcelona 2019. Requirements ------------ The two Ceph clusters will correspond to sites 'a' and 'b' and each cluster will reside within a separate model (models 'site-a' and 'site-b'). The deployment will require the use of `Cross model relations`_. Deployment characteristics: * each cluster will have 7 units: * 3 x ceph-osd * 3 x ceph-mon * 1 x ceph-rbd-mirror * application names will be used to distinguish between applications in site 'a' from those in site 'b' (e.g. ceph-mon-a and ceph-mon-b) * the ceph-osd units will use block device ``/dev/vdd`` for their OSD volumes .. note:: The two Ceph clusters can optionally be placed within the same model, and thus obviate the need for cross model relations. This topology is not generally considered to be a real world scenario. Deployment ---------- For site 'a' the following configuration is placed into file ``site-a.yaml``: .. code-block:: yaml ceph-mon-a: monitor-count: 3 expected-osd-count: 3 source: distro ceph-osd-a: osd-devices: /dev/vdd source: distro ceph-rbd-mirror-a: source: distro Create the model and deploy the software for each site: * Site 'a' .. code-block:: none juju add-model site-a juju deploy -n 3 --config site-a.yaml ceph-osd ceph-osd-a juju deploy -n 3 --config site-a.yaml ceph-mon ceph-mon-a juju deploy --config site-a.yaml ceph-rbd-mirror ceph-rbd-mirror-a * Site 'b' An analogous configuration file is used (i.e. replace 'a' with 'b'): .. code-block:: none juju add-model site-b juju deploy -n 3 --config site-b.yaml ceph-osd ceph-osd-b juju deploy -n 3 --config site-b.yaml ceph-mon ceph-mon-b juju deploy --config site-b.yaml ceph-rbd-mirror ceph-rbd-mirror-b Add two local relations for each site: * Site 'a' .. code-block:: none juju add-relation -m site-a ceph-mon-a:osd ceph-osd-a:mon juju add-relation -m site-a ceph-mon-a:rbd-mirror ceph-rbd-mirror-a:ceph-local * Site 'b' .. code-block:: none juju add-relation -m site-b ceph-mon-b:osd ceph-osd-b:mon juju add-relation -m site-b ceph-mon-b:rbd-mirror ceph-rbd-mirror-b:ceph-local Export a ceph-rbd-mirror endpoint (by means of an "offer") for each site. This will enable us to create the inter-site (cross model) relations: * Site 'a' .. code-block:: none juju switch site-a juju offer ceph-rbd-mirror-a:ceph-remote Output: .. code-block:: console Application "ceph-rbd-mirror-a" endpoints [ceph-remote] available at "admin/site-a.ceph-rbd-mirror-a" * Site 'b' .. code-block:: none juju switch site-b juju offer ceph-rbd-mirror-b:ceph-remote Output: .. code-block:: console Application "ceph-rbd-mirror-b" endpoints [ceph-remote] available at "admin/site-b.ceph-rbd-mirror-b" Add the two inter-site relations by referring to the offer URLs (included in the output above) as if they were applications in the local model: .. code-block:: none juju add-relation -m site-a ceph-mon-a admin/site-b.ceph-rbd-mirror-b juju add-relation -m site-b ceph-mon-b admin/site-a.ceph-rbd-mirror-a Verify the output of :command:`juju status` for each model: .. code-block:: none juju status -m site-a Output: .. code-block:: console Model Controller Cloud/Region Version SLA Timestamp site-a maas-prod-1 acme-1/default 2.8.1 unsupported 20:00:41Z SAAS Status Store URL ceph-rbd-mirror-b waiting icarus-maas admin/site-b.ceph-rbd-mirror-b App Version Status Scale Charm Store Rev OS Notes ceph-mon-a 15.2.3 active 3 ceph-mon jujucharms 49 ubuntu ceph-osd-a 15.2.3 active 3 ceph-osd jujucharms 304 ubuntu ceph-rbd-mirror-a 15.2.3 waiting 1 ceph-rbd-mirror jujucharms 12 ubuntu Unit Workload Agent Machine Public address Ports Message ceph-mon-a/0* active idle 0/lxd/0 10.0.0.57 Unit is ready and clustered ceph-mon-a/1 active idle 1/lxd/0 10.0.0.58 Unit is ready and clustered ceph-mon-a/2 active idle 2/lxd/0 10.0.0.59 Unit is ready and clustered ceph-osd-a/0* active idle 0 10.0.0.69 Unit is ready (1 OSD) ceph-osd-a/1 active idle 1 10.0.0.19 Unit is ready (1 OSD) ceph-osd-a/2 active idle 2 10.0.0.20 Unit is ready (1 OSD) ceph-rbd-mirror-a/0* waiting idle 3 10.0.0.22 Waiting for pools to be created Machine State DNS Inst id Series AZ Message 0 started 10.0.0.69 virt-node-08 focal default Deployed 0/lxd/0 started 10.0.0.57 juju-bb0dc1-0-lxd-0 focal default Container started 1 started 10.0.0.19 virt-node-10 focal default Deployed 1/lxd/0 started 10.0.0.58 juju-bb0dc1-1-lxd-0 focal default Container started 2 started 10.0.0.20 virt-node-11 focal default Deployed 2/lxd/0 started 10.0.0.59 juju-bb0dc1-2-lxd-0 focal default Container started 3 started 10.0.0.22 virt-node-03 focal default Deployed Offer Application Charm Rev Connected Endpoint Interface Role ceph-rbd-mirror-a ceph-rbd-mirror-a ceph-rbd-mirror 12 1/1 ceph-remote ceph-rbd-mirror requirer .. code-block:: none juju status -m site-b Output: .. code-block:: console Model Controller Cloud/Region Version SLA Timestamp site-b maas-prod-1 acme-1/default 2.8.1 unsupported 20:02:58Z SAAS Status Store URL ceph-rbd-mirror-a waiting icarus-maas admin/site-a.ceph-rbd-mirror-a App Version Status Scale Charm Store Rev OS Notes ceph-mon-b 15.2.3 active 3 ceph-mon jujucharms 49 ubuntu ceph-osd-b 15.2.3 active 3 ceph-osd jujucharms 304 ubuntu ceph-rbd-mirror-b 15.2.3 waiting 1 ceph-rbd-mirror jujucharms 12 ubuntu Unit Workload Agent Machine Public address Ports Message ceph-mon-b/0* active idle 0/lxd/0 10.0.0.60 Unit is ready and clustered ceph-mon-b/1 active idle 1/lxd/0 10.0.0.61 Unit is ready and clustered ceph-mon-b/2 active idle 2/lxd/0 10.0.0.62 Unit is ready and clustered ceph-osd-b/0* active idle 0 10.0.0.21 Unit is ready (1 OSD) ceph-osd-b/1 active idle 1 10.0.0.54 Unit is ready (1 OSD) ceph-osd-b/2 active idle 2 10.0.0.55 Unit is ready (1 OSD) ceph-rbd-mirror-b/0* waiting idle 3 10.0.0.56 Waiting for pools to be created Machine State DNS Inst id Series AZ Message 0 started 10.0.0.21 virt-node-02 focal default Deployed 0/lxd/0 started 10.0.0.60 juju-3ef7c5-0-lxd-0 focal default Container started 1 started 10.0.0.54 virt-node-04 focal default Deployed 1/lxd/0 started 10.0.0.61 juju-3ef7c5-1-lxd-0 focal default Container started 2 started 10.0.0.55 virt-node-05 focal default Deployed 2/lxd/0 started 10.0.0.62 juju-3ef7c5-2-lxd-0 focal default Container started 3 started 10.0.0.56 virt-node-06 focal default Deployed Offer Application Charm Rev Connected Endpoint Interface Role ceph-rbd-mirror-b ceph-rbd-mirror-b ceph-rbd-mirror 12 1/1 ceph-remote ceph-rbd-mirror requirer There are no Ceph pools created by default. The next section ('Pool creation') provides guidance. Pool creation ------------- RBD pools can be created by either a supporting charm (through the Ceph broker protocol) or manually by the operator: #. A charm-created pool (e.g. the glance or nova-compute charms) will automatically be detected and acted upon (i.e. a remote pool will be set up in the peer cluster). #. A manually-created pool, whether done via the ceph-mon application or through Ceph directly, will require an action to be run on the ceph-rbd-mirror application leader in order for the remote pool to come online. For example, to create a pool manually in site 'a' and have ceph-rbd-mirror (of site 'a') initialise a pool in site 'b': .. code-block:: none juju run-action --wait -m site-a ceph-mon-a/leader create-pool name=mypool app-name=rbd juju run-action --wait -m site-a ceph-rbd-mirror-a/leader refresh-pools This can be verified by listing the pools in site 'b': .. code-block:: none juju run-action --wait -m site-b ceph-mon-b/leader list-pools .. note:: Automatic peer-pool creation (for a charm-created pool) is based on the local pool being labelled with a Ceph 'rbd' tag. This Ceph-internal labelling occurs when the newly-created local pool is associated with the RBD application. This last feature is supported starting with Ceph Luminous (OpenStack Queens). Failover and fallback --------------------- To manage failover and fallback, the ``demote`` and ``promote`` actions are applied to the ceph-rbd-mirror application leader. For instance, to fail over from site 'a' to site 'b' the former is demoted and the latter is promoted. The rest of the commands are status checks: .. code-block:: none juju run-action --wait -m site-a ceph-rbd-mirror-a/leader status verbose=true juju run-action --wait -m site-b ceph-rbd-mirror-b/leader status verbose=true juju run-action --wait -m site-a ceph-rbd-mirror-a/leader demote juju run-action --wait -m site-a ceph-rbd-mirror-a/leader status verbose=true juju run-action --wait -m site-b ceph-rbd-mirror-b/leader status verbose=true juju run-action --wait -m site-b ceph-rbd-mirror-b/leader promote To fall back to site 'a' the actions are reversed: .. code-block:: none juju run-action --wait -m site-b ceph-rbd-mirror-b/leader demote juju run-action --wait -m site-a ceph-rbd-mirror-a/leader promote .. note:: With Ceph Luminous (and greater), the mirror status information may not be accurate. Specifically, the ``entries_behind_master`` counter may never get to '0' even though the image has been fully synchronised. Recovering from abrupt shutdown ------------------------------- It is possible that an abrupt shutdown and/or an interruption to communication channels may lead to a "split-brain" condition. This may cause the mirroring daemon in each cluster to claim to be the primary. In such cases, the operator must make a call as to which daemon is correct. Generally speaking, this means deciding which cluster has the most recent data. Elect a primary by applying the ``demote`` and ``promote`` actions to the appropriate ceph-rbd-mirror leader. After doing so, the ``resync-pools`` action must be run on the secondary cluster leader. The ``promote`` action may require a force option. Here, we make site 'a' be the primary by demoting site 'b' and promoting site 'a': .. code-block:: none juju run-action --wait -m site-b ceph-rbd-mirror/leader demote juju run-action --wait -m site-a ceph-rbd-mirror/leader promote force=true juju run-action --wait -m site-a ceph-rbd-mirror/leader status verbose=true juju run-action --wait -m site-b ceph-rbd-mirror/leader status verbose=true juju run-action --wait -m site-b ceph-rbd-mirror/leader resync-pools i-really-mean-it=true .. note:: When using Ceph Luminous, the mirror state information will not be accurate after recovering from unclean shutdown. Regardless of the output of the status information, you will be able to write to images after a forced promote. .. LINKS .. _charm's documentation: https://opendev.org/openstack/charm-ceph-rbd-mirror/src/branch/master/src/README.md .. _Ceph RADOS Gateway Multisite replication: https://docs.openstack.org/project-deploy-guide/charm-deployment-guide/latest/app-rgw-multisite.html .. _Upstream Ceph documentation on RBD mirroring: https://docs.ceph.com/docs/mimic/rbd/rbd-mirroring/ .. _RBD mirror: https://fghaas.github.io/cephalocon2019-rbdmirror/#/7/6 .. _Cross model relations: https://juju.is/docs/cross-model-relations