CEPH GLOSSARY

Ceph is growing rapidly. As firms deploy Ceph, the technical terms such as “RADOS”, “RBD,” “RGW” and so forth require corresponding marketing terms that explain what each component does. The terms in this glossary are intended to complement the existing technical terminology.

Sometimes more than one term applies to a definition. Generally, the first term reflects a term consistent with Ceph’s marketing, and secondary terms reflect either technical terms or legacy ways of referring to Ceph systems.

Ceph Project

The aggregate term for the people, software, mission and infrastructure of Ceph.

cephx

The Ceph authentication protocol. Cephx operates like Kerberos, but it has no single point of failure.

Ceph
Ceph Platform

All Ceph software, which includes any piece of code hosted at  http://github.com/ceph.

Ceph System
Ceph Stack

A collection of two or more components of Ceph.

Ceph Node
Node
Host

Any single machine or server in a Ceph System.

Ceph Storage Cluster
Ceph Object Store
RADOS
RADOS Cluster
Reliable Autonomic Distributed Object Store

The core set of storage software which stores the user’s data (MON+OSD).

Ceph Cluster Map
cluster map

The set of maps comprising the monitor map, OSD map, PG map, MDS map and CRUSH map. See  Cluster Map for details.

Ceph Object Storage

The object storage “product”, service or capabilities, which consists essentially of a Ceph Storage Cluster and a Ceph Object Gateway.

Ceph Object Gateway
RADOS Gateway
RGW

The S3/Swift gateway component of Ceph.

Ceph Block Device
RBD

The block storage component of Ceph.

Ceph Block Storage

The block storage “product,” service or capabilities when used in conjunction with  librbd, a hypervisor such as QEMU or Xen, and a hypervisor abstraction layer such as  libvirt.

Ceph Filesystem
CephFS
Ceph FS

The POSIX filesystem components of Ceph.

Cloud Platforms
Cloud Stacks

Third party cloud provisioning platforms such as OpenStack, CloudStack, OpenNebula, ProxMox, etc.

Object Storage Device
OSD

A physical or logical storage unit ( e.g., LUN). Sometimes, Ceph users use the term “OSD” to refer to  Ceph OSD Daemon, though the proper term is “Ceph OSD”.

Object Storage Devices.

Ceph OSD Daemon
Ceph OSD

The Ceph OSD software, which interacts with a logical disk ( OSD). Sometimes, Ceph users use the term “OSD” to refer to “Ceph OSD Daemon”, though the proper term is “Ceph OSD”.

Ceph Monitor
MON

The Ceph monitor software.

Ceph Metadata Server
MDS

The Ceph metadata software.

Ceph Clients
Ceph Client

The collection of Ceph components which can access a Ceph Storage Cluster. These include the Ceph Object Gateway, the Ceph Block Device, the Ceph Filesystem, and their corresponding libraries, kernel modules, and FUSEs.

Ceph Kernel Modules

The collection of kernel modules which can be used to interact with the Ceph System (e.g,.  ceph.ko,  rbd.ko).

Ceph Client Libraries

The collection of libraries that can be used to interact with components of the Ceph System.

Ceph Release

Any distinct numbered version of Ceph.

Ceph Point Release

Any ad-hoc release that includes only bug or security fixes.

Ceph Interim Release

Versions of Ceph that have not yet been put through quality assurance testing, but may contain new features.

Ceph Release Candidate

A major version of Ceph that has undergone initial quality assurance testing and is ready for beta testers.

Ceph Stable Release

A major version of Ceph where all features from the preceding interim releases have been put through quality assurance testing successfully.

Ceph Test Framework
Teuthology

The collection of software that performs scripted tests on Ceph.

CRUSH

Controlled Replication Under Scalable Hashing. It is the algorithm Ceph uses to compute object storage locations.

ruleset

A set of CRUSH data placement rules that applies to a particular pool(s).

Pool
Pools

Pools are logical partitions for storing objects.

CLUSTER MAP

Ceph depends upon Ceph Clients and Ceph OSD Daemons having knowledge of the cluster topology, which is inclusive of 5 maps collectively referred to as the “Cluster Map”:

1. The Monitor Map: Contains the cluster fsid, the position, name address and port of each monitor. It also indicates the current epoch, when the map was created, and the last time it changed. To view a monitor map, execute ceph mon dump.
2. The OSD Map: Contains the cluster fsid, when the map was created and last modified, a list of pools, replica sizes, PG numbers, a list of OSDs and their status (e.g., up, in). To view an OSD map, execute ceph osd dump.
3. The PG Map: Contains the PG version, its time stamp, the last OSD map epoch, the full ratios, and details on each placement group such as the PG ID, the Up Set, the Acting Set, the state of the PG (e.g., active + clean), and data usage statistics for each pool.
4. The CRUSH Map: Contains a list of storage devices, the failure domain hierarchy (e.g., device, host, rack, row, room, etc.), and rules for traversing the hierarchy when storing data. To view a CRUSH map, execute ceph osd getcrushmap -o {filename}; then, decompile it by executing crushtool -d {comp-crushmap-filename} -o {decomp-crushmap-filename}. You can view the decompiled map in a text editor or with cat.
5. The MDS Map: Contains the current MDS map epoch, when the map was created, and the last time it changed. It also contains the pool for storing metadata, a list of metadata servers, and which metadata servers are up and in. To view an MDS map, execute ceph mds dump.

Each map maintains an iterative history of its operating state changes. Ceph Monitors maintain a master copy of the cluster map including the cluster members, state, changes, and the overall health of the Ceph Storage Cluster.