Ceph RBD mirroring¶
This page has been identified as being affected by the breaking changes introduced between versions 2.9.x and 3.x of the Juju client. Read support note Breaking changes between Juju 2.9.x and 3.x before continuing.
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 the 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 multi-site replication and other work to arrive at a complete solution.
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.
For more information on performance aspects see Florian Haas’ talk Geographical Redundancy with rbd-mirror (video) given at Cephalocon Barcelona 2019.
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.
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 the applications in each site (e.g. site-a-ceph-mon and site-b-ceph-mon)
the ceph-osd units will use block device
/dev/vddfor their OSD volumes
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.
For site ‘a’ the following configuration is placed into file
site-a-ceph-mon: monitor-count: 3 expected-osd-count: 3 source: distro site-a-ceph-osd: osd-devices: /dev/vdd source: distro site-a-ceph-rbd-mirror: source: distro
Create the model and deploy the software for each site:
juju add-model site-a juju deploy -n 3 --config site-a.yaml ceph-osd site-a-ceph-osd juju deploy -n 3 --config site-a.yaml ceph-mon site-a-ceph-mon juju deploy --config site-a.yaml ceph-rbd-mirror site-a-ceph-rbd-mirror
An analogous configuration file is used (i.e. replace ‘a’ with ‘b’):
juju add-model site-b juju deploy -n 3 --config site-b.yaml ceph-osd site-b-ceph-osd juju deploy -n 3 --config site-b.yaml ceph-mon site-b-ceph-mon juju deploy --config site-b.yaml ceph-rbd-mirror site-b-ceph-rbd-mirror
Add two local relations for each site:
juju integrate -m site-a site-a-ceph-mon:osd site-a-ceph-osd:mon juju integrate -m site-a site-a-ceph-mon:rbd-mirror site-a-ceph-rbd-mirror:ceph-local
juju integrate -m site-b site-b-ceph-mon:osd site-b-ceph-osd:mon juju integrate -m site-b site-b-ceph-mon:rbd-mirror site-b-ceph-rbd-mirror: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:
juju switch site-a juju offer site-a-ceph-rbd-mirror:ceph-remote
Application "site-a-ceph-rbd-mirror" endpoints [ceph-remote] available at "admin/site-a.site-a-ceph-rbd-mirror"
juju switch site-b juju offer site-b-ceph-rbd-mirror:ceph-remote
Application "site-b-ceph-rbd-mirror" endpoints [ceph-remote] available at "admin/site-b.site-b-ceph-rbd-mirror"
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:
juju integrate -m site-a site-a-ceph-mon admin/site-b.site-b-ceph-rbd-mirror juju integrate -m site-b site-b-ceph-mon admin/site-a.site-a-ceph-rbd-mirror
Verify the output of juju status for each model:
juju status -m site-a --relations
Model Controller Cloud/Region Version SLA Timestamp site-a prod-1 openstack/default 2.8.9 unsupported 16:00:39Z SAAS Status Store URL site-b-ceph-rbd-mirror waiting serverstack admin/site-b.site-b-ceph-rbd-mirror App Version Status Scale Charm Store Rev OS Notes site-a-ceph-mon 15.2.8 active 3 ceph-mon jujucharms 53 ubuntu site-a-ceph-osd 15.2.8 active 3 ceph-osd jujucharms 308 ubuntu site-a-ceph-rbd-mirror 15.2.8 waiting 1 ceph-rbd-mirror jujucharms 15 ubuntu Unit Workload Agent Machine Public address Ports Message site-a-ceph-mon/0 active idle 0 10.5.0.4 Unit is ready and clustered site-a-ceph-mon/1 active idle 1 10.5.0.14 Unit is ready and clustered site-a-ceph-mon/2* active idle 2 10.5.0.7 Unit is ready and clustered site-a-ceph-osd/0 active idle 0 10.5.0.4 Unit is ready (1 OSD) site-a-ceph-osd/1 active idle 1 10.5.0.14 Unit is ready (1 OSD) site-a-ceph-osd/2* active idle 2 10.5.0.7 Unit is ready (1 OSD) site-a-ceph-rbd-mirror/0* waiting idle 3 10.5.0.11 Waiting for pools to be created Machine State DNS Inst id Series AZ Message 0 started 10.5.0.4 4f3e4d94-5003-4998-ab30-11fc3c845a7a focal nova ACTIVE 1 started 10.5.0.14 7682822e-4469-41e1-b938-225c067f9f82 focal nova ACTIVE 2 started 10.5.0.7 786e7d84-3f94-4cd6-9493-72026d629fcf focal nova ACTIVE 3 started 10.5.0.11 715c8738-e41e-4be2-8638-560206b2c434 focal nova ACTIVE Offer Application Charm Rev Connected Endpoint Interface Role site-a-ceph-rbd-mirror site-a-ceph-rbd-mirror ceph-rbd-mirror 15 1/1 ceph-remote ceph-rbd-mirror requirer Relation provider Requirer Interface Type Message site-a-ceph-mon:mon site-a-ceph-mon:mon ceph peer site-a-ceph-mon:osd site-a-ceph-osd:mon ceph-osd regular site-a-ceph-mon:rbd-mirror site-a-ceph-rbd-mirror:ceph-local ceph-rbd-mirror regular site-a-ceph-mon:rbd-mirror site-b-ceph-rbd-mirror:ceph-remote ceph-rbd-mirror regular Model Controller Cloud/Region Version SLA Timestamp site-a maas-prod-1 acme-1/default 2.8.1 unsupported 20:00:41Z
juju status -m site-b --relations
Model Controller Cloud/Region Version SLA Timestamp site-b prod-1 openstack/default 2.8.9 unsupported 16:05:39Z SAAS Status Store URL site-a-ceph-rbd-mirror waiting serverstack admin/site-a.site-a-ceph-rbd-mirror App Version Status Scale Charm Store Rev OS Notes site-b-ceph-mon 15.2.8 active 3 ceph-mon jujucharms 53 ubuntu site-b-ceph-osd 15.2.8 active 3 ceph-osd jujucharms 308 ubuntu site-b-ceph-rbd-mirror 15.2.8 waiting 1 ceph-rbd-mirror jujucharms 15 ubuntu Unit Workload Agent Machine Public address Ports Message site-b-ceph-mon/0 active idle 0 10.5.0.3 Unit is ready and clustered site-b-ceph-mon/1 active idle 1 10.5.0.20 Unit is ready and clustered site-b-ceph-mon/2* active idle 2 10.5.0.8 Unit is ready and clustered site-b-ceph-osd/0 active idle 0 10.5.0.3 Unit is ready (1 OSD) site-b-ceph-osd/1 active idle 1 10.5.0.20 Unit is ready (1 OSD) site-b-ceph-osd/2* active idle 2 10.5.0.8 Unit is ready (1 OSD) site-b-ceph-rbd-mirror/0* waiting idle 3 10.5.0.12 Waiting for pools to be created Machine State DNS Inst id Series AZ Message 0 started 10.5.0.3 2caf61f7-8675-4cd9-a3c4-cc68a0cb3f2d focal nova ACTIVE 1 started 10.5.0.20 d1b3bd0b-1631-4bd3-abba-14a366b3d752 focal nova ACTIVE 2 started 10.5.0.8 84eb5db2-d673-4d36-82b4-902463362704 focal nova ACTIVE 3 started 10.5.0.12 c40e1247-7b7d-4b84-ab3a-8b72c22f096e focal nova ACTIVE Offer Application Charm Rev Connected Endpoint Interface Role site-b-ceph-rbd-mirror site-b-ceph-rbd-mirror ceph-rbd-mirror 15 1/1 ceph-remote ceph-rbd-mirror requirer Relation provider Requirer Interface Type Message site-b-ceph-mon:mon site-b-ceph-mon:mon ceph peer site-b-ceph-mon:osd site-b-ceph-osd:mon ceph-osd regular site-b-ceph-mon:rbd-mirror site-a-ceph-rbd-mirror:ceph-remote ceph-rbd-mirror regular site-b-ceph-mon:rbd-mirror site-b-ceph-rbd-mirror:ceph-local ceph-rbd-mirror regular
There are no Ceph pools created by default. The next section (‘Pool creation’) provides guidance.
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’:
juju run -m site-a site-a-ceph-mon/leader create-pool name=mypool app-name=rbd juju run -m site-a site-a-ceph-rbd-mirror/leader refresh-pools
This can be verified by listing the pools in site ‘b’:
juju run -m site-b site-b-ceph-mon/leader list-pools
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
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:
juju run -m site-a site-a-ceph-rbd-mirror/leader status verbose=true juju run -m site-b site-b-ceph-rbd-mirror/leader status verbose=true juju run -m site-a site-a-ceph-rbd-mirror/leader demote juju run -m site-a site-a-ceph-rbd-mirror/leader status verbose=true juju run -m site-b site-b-ceph-rbd-mirror/leader status verbose=true juju run -m site-b site-b-ceph-rbd-mirror/leader promote
To fall back to site ‘a’ the actions are reversed:
juju run -m site-b site-b-ceph-rbd-mirror/leader demote juju run -m site-a site-a-ceph-rbd-mirror/leader promote
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
promote actions to the
appropriate ceph-rbd-mirror leader. After doing so, the
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’:
juju run -m site-b site-b-ceph-rbd-mirror/leader demote juju run -m site-a site-a-ceph-rbd-mirror/leader promote force=true juju run -m site-a site-a-ceph-rbd-mirror/leader status verbose=true juju run -m site-b site-b-ceph-rbd-mirror/leader status verbose=true juju run -m site-b site-b-ceph-rbd-mirror/leader resync-pools i-really-mean-it=true
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.