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 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 Multisite Replication and other work to arrive at a complete solution.
Performance considerations¶
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.
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 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
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:
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:
Site ‘a’
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
Site ‘b’
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:
Site ‘a’
juju add-relation -m site-a site-a-ceph-mon:osd site-a-ceph-osd:mon juju add-relation -m site-a site-a-ceph-mon:rbd-mirror site-a-ceph-rbd-mirror:ceph-local
Site ‘b’
juju add-relation -m site-b site-b-ceph-mon:osd site-b-ceph-osd:mon juju add-relation -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:
Site ‘a’
juju switch site-a juju offer site-a-ceph-rbd-mirror:ceph-remote
Output:
Application "site-a-ceph-rbd-mirror" endpoints [ceph-remote] available at "admin/site-a.site-a-ceph-rbd-mirror"Site ‘b’
juju switch site-b juju offer site-b-ceph-rbd-mirror:ceph-remote
Output:
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 add-relation -m site-a site-a-ceph-mon admin/site-b.site-b-ceph-rbd-mirror
juju add-relation -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
Output:
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
Output:
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.
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’:
juju run-action --wait -m site-a site-a-ceph-mon/leader create-pool name=mypool app-name=rbd juju run-action --wait -m site-a site-a-ceph-rbd-mirror/leader refresh-pools
This can be verified by listing the pools in site ‘b’:
juju run-action --wait -m site-b site-b-ceph-mon/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:
juju run-action --wait -m site-a site-a-ceph-rbd-mirror/leader status verbose=true
juju run-action --wait -m site-b site-b-ceph-rbd-mirror/leader status verbose=true
juju run-action --wait -m site-a site-a-ceph-rbd-mirror/leader demote
juju run-action --wait -m site-a site-a-ceph-rbd-mirror/leader status verbose=true
juju run-action --wait -m site-b site-b-ceph-rbd-mirror/leader status verbose=true
juju run-action --wait -m site-b site-b-ceph-rbd-mirror/leader promote
To fall back to site ‘a’ the actions are reversed:
juju run-action --wait -m site-b site-b-ceph-rbd-mirror/leader demote
juju run-action --wait -m site-a site-a-ceph-rbd-mirror/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’:
juju run-action --wait -m site-b site-b-ceph-rbd-mirror/leader demote
juju run-action --wait -m site-a site-a-ceph-rbd-mirror/leader promote force=true
juju run-action --wait -m site-a site-a-ceph-rbd-mirror/leader status verbose=true
juju run-action --wait -m site-b site-b-ceph-rbd-mirror/leader status verbose=true
juju run-action --wait -m site-b 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.
