#Core Concepts

#Rook Operator

The Rook operator is a simple container that has all that is needed to bootstrap and monitor the storage cluster. The operator will start and monitor Ceph monitor pods, the Ceph OSD daemons to provide RADOS storage, as well as start and manage other Ceph daemons. The operator manages CRDs for pools, object stores (S3/Swift), and filesystems by initializing the pods and other resources necessary to run the services.

The operator will monitor the storage daemons to ensure the cluster is healthy. Ceph mons will be started or failed over when necessary, and other adjustments are made as the cluster grows or shrinks. The operator will also watch for desired state changes specified in the Ceph custom resources (CRs) and apply the changes.

Rook automatically configures the Ceph-CSI driver to mount the storage to your pods. The rook/ceph image includes all necessary tools to manage the cluster.

#Ceph CSI

Ceph CSI plugins implement an interface between a CSI-enabled Container Orchestrator (CO) and Ceph clusters. They enable dynamically provisioning Ceph volumes and attaching them to workloads.

#Ceph module functions

Module	Function
MON	The monitor (MON) is the most important component in a Ceph cluster. It manages the Ceph cluster and maintains the status of the entire cluster. The MON ensures that related components of a cluster can be synchronized at the same time. It functions as the leader of the cluster and is responsible for collecting, updating, and publishing cluster information.
MGR	The manager (MGR) is a monitoring system that provides collection, storage, analysis (including alarming), and visualization functions. It makes certain cluster parameters available for external systems.
OSD	Object storage daemons (OSDs) store the actual user data. Every OSD is usually bound to one physical drive. The OSDs handle the read/write requests from clients.
MDS	The Ceph Metadata Server (MDS) tracks the file hierarchy and stores metadata used only for CephFS. The RBD and RGW do not require metadata. The MDS does not directly provide data services for clients.
RGW	The RADOS gateway (RGW) is a Ceph object gateway that provides RESTful APIs compatible with S3 and Swift. The RGW also supports multi-tenant and OpenStack Identity service (Keystone).
RADOS	Reliable Autonomic Distributed Object Store (RADOS) is the heart of a Ceph storage cluster. Everything in Ceph is stored by RADOS in the form of objects irrespective of their data types. The RADOS layer ensures data consistency and reliability through data replication, fault detection and recovery, and data recovery across cluster nodes.
LIBRADOS	Librados is a method that simplifies access to RADOS. Currently, it supports programming languages PHP, Ruby, Java, Python, C, and C++. It provides RADOS, a local interface of the Ceph storage cluster, and is the base component of other services such as the RADOS block device (RBD) and RADOS gateway (RGW). In addition, it provides the Portable Operating System Interface (POSIX) for the Ceph file system (CephFS). The Librados API can be used to directly access RADOS, enabling developers to create their own interfaces for accessing the Ceph cluster storage.
RBD	The RADOS block device (RBD) is the Ceph block device that provides block storage for external systems. It can be mapped, formatted, and mounted like a drive to a server.
CephFS	The CephFS provides a POSlX-compatible distributed file system of any size. It depends on the Ceph MDS to track the file hierarchy, namely the metadata.