Ceph Fundamentals

Ceph turns a group of servers and storage devices into one distributed storage system. It can present block devices, a shared file system, and object APIs from the same underlying cluster.

That range is useful, but it can hide the central idea. Ceph stores data as objects in RADOS, its distributed object store. The block, file, and object services translate client requests into RADOS operations.

Use this mental model:

applications
    -> RBD block devices | CephFS files | RGW object APIs
        -> RADOS pools and placement groups
            -> OSD daemons and storage devices

Ceph is not a storage appliance with one controller. Clients obtain a cluster map from monitors. They use that map and the CRUSH algorithm to calculate where data belongs. They then communicate with the responsible OSD daemons.

Why Ceph exists

A single storage server has finite capacity and creates a failure boundary. Traditional shared-storage designs often place controllers in the data path. Those controllers must handle every request and remain available.

Ceph distributes storage and placement work across the cluster. You add capacity by adding OSDs and hosts. CRUSH changes the placement calculation when the topology changes, so the cluster can move affected placement groups instead of consulting a central object-location table.

This design targets large, fault-tolerant storage systems. It also creates operational work. You must plan failure domains, networks, media, capacity, protection, upgrades, monitoring, and recovery.

The RADOS foundation

RADOS is Ceph's reliable, autonomic distributed object store. All client interfaces ultimately store data as RADOS objects in pools.

An object has an identifier, binary data, and metadata. OSD daemons store those objects and handle reads, writes, replication, recovery, and scrubbing.

A pool is a logical partition of the RADOS object space. A pool defines placement and protection settings. It also forms an authorization boundary for many client capabilities.

Ceph maps every object to a placement group. A placement group collects many objects into a unit that Ceph can place, peer, recover, and rebalance. CRUSH then maps the placement group to an ordered set of OSDs.

Ceph Fundamentals

Why Ceph exists

The RADOS foundation

Relevant careers