Replication GroupsΒΆ

The storage hosts on Ceph systems are organized into replication groups to provide redundancy.

Each replication group contains a number of hosts, referred to as peers. Each peer independently replicates the same data. StarlingX supports a minimum of two peers and a maximum of three peers per replication group. This replication factor is defined when the Ceph storage backend is added.

For a system with two peers per replication group, up to four replication groups are supported. For a system with three peers per replication group, up to three replication groups are supported.

For best performance, StarlingX recommends a balanced storage capacity, in which each peer has sufficient resources to meet the operational requirements of the system.

A replication group is considered healthy when all its peers are available. When only a minimum number of peers are available (as indicated by the min_replication value reported for the group), the group continues to provide storage services but without full replication, and a HEALTH_WARN state is declared. When the number of available peers falls below the min_replication value, the group no longer provides storage services, and a HEALTH_ERR state is declared. The min_replication value is always one less than the replication factor for the group.

It is not possible to lock more than one peer at a time in a replication group.

Replication groups are created automatically. When a new storage host is added and an incomplete replication group exists, the host is added to the existing group. If all existing replication groups are complete, then a new incomplete replication group is defined and the host becomes its first member.


Ceph relies on monitoring to detect when to switch from a primary OSD to a replicated OSD. The Ceph parameter osd heartbeat grace sets the amount of time required to wait before switching OSDs when the primary OSD is not responding. StarlingX currently uses the default value of 20 seconds. This means that a Ceph filesystem may not respond to I/O for up to 20 seconds when a storage node or OSD goes out of service. For more information, see the Ceph documentation:

Replication groups are shown on the Hosts Inventory page in association with the storage hosts. You can also use the following CLI commands to obtain information about replication groups:

~(keystone_admin)]$ system cluster-list
| uuid                                 | cluster_uuid | type | name     | deployment_model |
| 335766eb-8564-4f4d-8665-681f73d13dfb | None         | ceph | ceph_clu | controller-nodes |
|                                      |              |      | ster     |                  |
|                                      |              |      |          |                  |
~(keystone_admin)]$ system cluster-show 335766eb-968e-44fc-9ca7-907f93c772a1

| Property           | Value                                  |
| uuid               | 335766eb-968e-44fc-9ca7-907f93c772a1   |
| cluster_uuid       | None                                   |
| type               | ceph                                   |
| name               | ceph_cluster                           |
| replication_groups | ["group-0:['storage-0', 'storage-1']"] |
| storage_tiers      | ['storage (in-use)']                   |
| deployment_model   | controller-nodes                       |