Chapter 6. Placement group splitting and merging

Splitting

Red Hat Ceph Storage can split existing placement groups (PGs) into smaller PGs, which increases the total number of PGs for a given pool. Splitting existing placement groups (PGs) allows a small Red Hat Ceph Storage cluster to scale over time as storage requirements increase. The PG auto-scaling feature can increase the pg_num value, which causes the existing PGs to split as the storage cluster expands. If the PG auto-scaling feature is disabled, then you can manually increase the pg_num value, which triggers the PG split process to begin. For example, increasing the pg_num value from 4 to 16, will split into four pieces. Increasing the pg_num value will also increase the pgp_num value, but the pgp_num value increases at a gradual rate. This gradual increase is done to minimize the impact to a storage cluster’s performance and to a client’s workload, because migrating object data adds a significant load to the system. By default, Ceph queues and moves no more than 5% of the object data that is in a "misplaced" state. This default percentage can be adjusted with the target_max_misplaced_ratio option.