The Ceph components

Before we dive in, let's take a look at some key components that make up a Ceph cluster.

Physical node

A physical node is the actual server hardware that holds Object Storage Daemon (OSDs), monitors, and MDSs.

Maps

In Ceph, maps hold information such as a list of participating nodes in a cluster and their locations, data paths, and a list of OSDs with certain data chunks. There are several maps in a Ceph cluster: a cluster map, OSD map for a list of OSDs, monitor map for known monitor nodes, Placement Group (PG) map for the location of objects or data chunks, and a CRUSH map to determine the storage and retrieval of data by computing the data storage location.

Cluster map

A cluster map is a map of devices and buckets that comprise a Ceph cluster. Ceph uses a bucket hierarchy to define nodes or node locations, such as a room, rack, shelf, and host. For example, let's say there are four disk drives used as four OSDs in the following bucket hierarchy:

In the previous example, osd...