When a Ceph cluster receives requests for data storage, it splits into sections known as placement groups (PG). However, CRUSH data is first broken down into a set of objects, and based on the hash operation on object names, replication levels and total number of placement groups in the system, placement group IDs are generated. A placement group is a logical collection of objects that are replicated on OSDs to provide reliability in a storage system. Depending on the replication level of your Ceph pool, each placement group is replicated and distributed on more than one OSD of a Ceph cluster. You can consider a placement group as a logical container holding multiple objects such that this logical container is mapped to multiple OSDs. The placement groups are essential for the scalability and performance of a Ceph storage system.

Without placement groups, it will be difficult to manage and track tens of millions of objects that are replicated and spread over hundreds of OSDs...