Part II. Placement Groups (PGs)

Placement Groups (PGs) are invisible to Ceph clients, but they play an important role in Ceph Storage Clusters.

A Ceph Storage Cluster may require many thousands of OSDs to reach an exabyte level of storage capacity. Ceph clients store objects in pools, which are a logical subset of the overall cluster. The number of objects stored in a pool may easily run into the millions and beyond. A system with millions of objects or more cannot realistically track placement on a per-object basis and still perform well. Ceph assigns objects to placement groups, and placement groups to OSDs to make re-balancing dynamic and efficient.