• OSDs are The building blocks of the Ceph storage cluster.
• OSDs connects a storage device (such as a hard disk ) to the Ceph storage cluster. 
• An individual storage server may run multiple OSD daemons and provide multiple OSDs to the cluster.
• The CRUSH algorithm is used to store objects in OSDs using placement groups
• The replication of objects to multiple OSDs is handled automatically
• One OSD is the primary OSD for the object's placement group
• Ceph clients always contact the primary OSD in the acting set when it reads or writes data. So primary OSD provides.
•  Serves all I/O requests
• Replicates and protects the data
• Checks the coherence of the data
• Rebalances the data
• Recovers the data

• Other OSDs are secondary OSDs.

• Always acts under the control of the primary OSD
• Capable of becoming the primary OSD

• Each OSD has its own OSD journal. The OSD journal is not related to the file system journal but is used to improve the performance of write operations to the OSD.

• Each writes operation to the Ceph cluster is acknowledged to the client after all involved OSD journals have recorded the write request. The OSD then commits the operation to its backing file system.
• Every few seconds, the OSD stops writing new requests to its journal to apply the contents of the OSD journal to the backing file system. It then trims the committed requests from the journal to reclaim space on the journal's storage device.
• OSD journals use raw volumes on the OSD nodes and should be configured on a separate device, if possible a fast device such as an SSD, for performance-oriented and/or write-intensive environments.
  

Ceph Managers (MGRs)

• Ceph Managers (MGRs) provide for the collection of cluster statistics.
• It's recommended that you deploy at least two Ceph managers for each cluster, each running in a separate failure domain.
• If a manager is not available in your cluster, it will have no impact on client I/O operations. However, it will cause attempts to query cluster statistics to fail.
• The manager daemon centralizes access to all data collected from the cluster and can provide a simple web dashboard to storage administrators on TCP port 7000 (by default).

Metadata Server (MDS)

• The Ceph Metadata Server (MDS) is a service which provides POSIX-compliant, shared file system metadata management, which supports both directory hierarchy and file metadata, including ownership, time stamps, and mode.
• The MDS uses RADOS instead of local storage to store metadata, and has no access to file content, because it is only required for file access. 
• The MDS is therefore a Ceph component but not a RADOS component.
• The MDS enables CephFS to interact with the Ceph Object Store, mapping an inode to an object, and remembering where data is stored within a tree. Clients accessing a CephFS file system first make a request to an MDS, which provides the information needed to get files from the correct OSDs.