charm-specs/specs/stein/approved/ceph-rbd-mirror.rst

Ceph RBD Mirror Charm

Problem Description

RBD image mirroring can be used to provide a solution for Ceph cluster disaster recovery. Ceph has a daemon called rbd-mirror which can be placed on a primary and a backup cluster and provide asynchronous replication of RBD images in a given pool.

rbd-mirror can work in two modes:

• pool (all images in a given pool are synchronized);
• image (per-image synchronization).

The scenario targeted by this spec involves an operator when it comes to performing promote/demote actions and the DR procedure is operator-driven as opposed to a full automatic failover in the event of a outage at the primary site.

Note

Promotion/demotion of pools will be operator driven

Note

RADOS objects are not mirrored so for mirroring radosgw objects which use RADOS objects or gnocchi metrics stored in RADOS objects different backup mechanisms are required - this spec covers RBD images only.

Note

RBD mirroring relies on the use of the exclusive-lock and journaling features of RBD; these are only supported in the userspace integration libraries as used by libvirt and qemu for native KVM virtualization. This requirement excludes the use of this feature with LXD based clouds which disable the majority of RBD features for compatibility with the Linux kernel RBD driver.

Note

The initial RBD mirror charm will only support mirroring of whole pools.

Proposed Change

High Level Design

As rbd-mirror is a separate package and the service itself acts as an RBD client it makes sense to implement the target functionality in a separate principle charm (ceph-rbd-mirror). The charm will accept parameters on which pools to replicate and be able to be related to multiple ceph-mon applications in separate clusters.

The charm will relate to a local ceph cluster and a remote ceph cluster typically using a cross model relation.

A new interface type ('rbd-mirror') will be created to support this integration; this will be provided by the ceph-mon charm, and consumed by the new ceph-rbd-mirror charm for both local and remote cluster connections.

Each rbd-mirror daemon requires a key for connectivity to the local cluster (named uniquely for the daemon) and a key for connectivity to the remote cluster (named globally for all rbd-mirror daemons). Multiple ceph configurations will also be maintained on the ceph-rbd-mirror units -'ceph' to reference the local cluster and 'remote' to reference the remote cluster. Configuration files and keys will be prefixed inline with this naming - for example:

$ ls /etc/ceph
    ceph.conf
    ceph.client.rbd-mirror.<hostname>.keyring
    remote.conf
    remote.client.rbd-mirror.keyring

In order to support resilience and scale-out of rbd mirroring, multiple units of the charm may be deployed; as a result this feature will only be supported with Ceph Luminous or later (which support multiple instances of the rbd-mirror service).

Deployment and Scalability Considerations

From the deployment perspective the charm units should have high-bandwidth and low-latency L3 connectivity to access and replication networks of both clusters to be able to keep up with the changes to Ceph pools it tries to replicate. At minimum, static routes will need to be configured on the node running rbd-mirror daemon but that is outside of the scope of this spec.

Multiple units of the ceph-rbd-mirror charm may be used to scale out replication traffic.

Alternatives

No alternative solutions have been considered.

Dependencies

This feature relies on use of a Juju version which supports cross model relations.

Implementation

Assignee(s)

Primary assignee:

<tbd>

Gerrit Topic

Use Gerrit topic "rbd-mirror" for all patches related to this spec.

git-review -t rbd-mirror

Work Items

• Implement a new reactive charm called ceph-rbd-mirror.
• Implement the following relation:
  • rbd-mirror - ceph-mon (local and cross model).
• Add "cluster" endpoint to extra-bindings in metadata.yaml to allow binding the "cluster" endpoint to a ceph replication space.
• ceph-mon relations should retrieve cluster details and cephx keys via the broker protocol implemented in Ceph charms (code reuse).
• Add config options to specify pool names for replication.
• Automate creation of pools on a backup cluster if they are not present.
• Add actions to promote and demote pools.
• Enable RBD journaling feature as documented in rbd-mirror docs.
• Write unit tests.
• Write functional tests via zaza framework.

Repositories

A new git repository will be required for the ceph-rbd-mirror charm:

git://git.openstack.org/openstack/charm-ceph-rbd-mirror

Documentation

The ceph-rbd-mirror charm should contain a README with instructions on deploying the charm and on limitations related to scalability and networking.

Security

• Users created for replication must not have admin privileges - they only need to be able to write to the pools they require on the target cluster. This is supported through the existing group based permissions system in the ceph-mon broker using the 'rbd' profile for mon and osd permissions.

Testing

Code written or changed will be covered by unit tests; functional testing will be done using Zaza.