ceph/doc/glossary.rst

===============
 Ceph Glossary
===============

.. glossary::

        Application
                More properly called a :term:`client`, an application is any program
                external to Ceph that uses a Ceph Cluster to store and
                replicate data.

	:ref:`BlueStore<rados_config_storage_devices_bluestore>`
                OSD BlueStore is a storage back end used by OSD daemons, and
                was designed specifically for use with Ceph. BlueStore was
                introduced in the Ceph Kraken release. The Luminous release of
                Ceph promoted BlueStore to the default OSD back end,
                supplanting FileStore. As of the Reef release, FileStore is no
                longer available as a storage back end.
                
                BlueStore stores objects directly on raw block devices or
                partitions, and does not interact with mounted file systems.
                BlueStore uses RocksDB's key/value database to map object names
                to block locations on disk.

        Bucket
                In the context of :term:`RGW`, a bucket is a group of objects.
                In a filesystem-based analogy in which objects are the
                counterpart of files, buckets are the counterpart of
                directories. :ref:`Multisite sync
                policies<radosgw-multisite-sync-policy>` can be set on buckets,
                to provide fine-grained control of data movement from one zone
                to another zone. 
                
                The concept of the bucket has been taken from AWS S3. See also
                `the AWS S3 page on creating buckets <https://docs.aws.amazon.com/AmazonS3/latest/userguide/creating-buckets-s3.html>`_
                and `the AWS S3 'Buckets Overview' page <https://docs.aws.amazon.com/AmazonS3/latest/userguide/UsingBucket.html>`_.

                OpenStack Swift uses the term "containers" for what RGW and AWS call "buckets". 
                See `the OpenStack Storage API overview page <https://docs.openstack.org/swift/latest/api/object_api_v1_overview.html>`_.
                
	Ceph
                Ceph is a distributed network storage and file system with
                distributed metadata management and POSIX semantics.

        `ceph-ansible <https://docs.ceph.com/projects/ceph-ansible/en/latest/index.html>`_
                A GitHub repository, supported from the Jewel release to the
                Quincy release, that facilitates the installation of a Ceph
                cluster.
                
	Ceph Block Device
                Also called "RADOS Block Device" and :term:`RBD`. A software
                instrument that orchestrates the storage of block-based data in
                Ceph. Ceph Block Device splits block-based application data
                into "chunks". RADOS stores these chunks as objects. Ceph Block
                Device orchestrates the storage of those objects across the
                storage cluster. 

	Ceph Block Storage
                One of the three kinds of storage supported by Ceph (the other
                two are object storage and file storage). Ceph Block Storage is
                the block storage "product", which refers to block-storage
                related services and capabilities when used in conjunction with
                the collection of (1) ``librbd`` (a python module that provides
                file-like access to :term:`RBD` images), (2) a hypervisor such
                as QEMU or Xen, and (3) a hypervisor abstraction layer such as
                ``libvirt``.

	:ref:`Ceph Client <architecture_ceph_clients>`
                Any of the Ceph components that can access a Ceph Storage
                Cluster. This includes the Ceph Object Gateway, the Ceph Block
                Device, the Ceph File System, and their corresponding
                libraries. It also includes kernel modules, and FUSEs
                (Filesystems in USERspace).

	Ceph Client Libraries
                The collection of libraries that can be used to interact with
                components of the Ceph Cluster.

	Ceph Cluster Map
                See :term:`Cluster Map`

	Ceph Dashboard
                :ref:`The Ceph Dashboard<mgr-dashboard>` is a built-in
                web-based Ceph management and monitoring application through
                which you can inspect and administer various resources within
                the cluster. It is implemented as a :ref:`ceph-manager-daemon`
                module.

	Ceph File System
                See :term:`CephFS`

	:ref:`CephFS<ceph-file-system>`
                The **Ceph F**\ile **S**\ystem, or CephFS, is a
                POSIX-compliant file system built on top of Ceph’s distributed
                object store, RADOS.  See :ref:`CephFS Architecture
                <arch-cephfs>` for more details.

        :ref:`ceph-fuse <man-ceph-fuse>`
                :ref:`ceph-fuse <man-ceph-fuse>` is a FUSE ("**F**\ilesystem in
                **USE**\rspace") client for CephFS. ceph-fuse mounts a Ceph FS
                ata  specified mount point. 

	Ceph Interim Release
                See :term:`Releases`.

	Ceph Kernel Modules
                The collection of kernel modules that can be used to interact
                with the Ceph Cluster (for example: ``ceph.ko``, ``rbd.ko``).

	:ref:`Ceph Manager<ceph-manager-daemon>`
                The Ceph manager daemon (ceph-mgr) is a daemon that runs
                alongside monitor daemons to provide monitoring and interfacing
                to external monitoring and management systems. Since the
                Luminous release (12.x), no Ceph cluster functions properly
                unless it contains a running ceph-mgr daemon. 

	Ceph Manager Dashboard
                See :term:`Ceph Dashboard`.

	Ceph Metadata Server
                See :term:`MDS`.

	Ceph Monitor
                A daemon that maintains a map of the state of the cluster. This
                "cluster state" includes the monitor map, the manager map, the
                OSD map, and the CRUSH map. A Ceph cluster must contain a
                minimum of three running monitors in order to be both redundant
                and highly-available. Ceph monitors and the nodes on which they
                run are often referred to as "mon"s. See :ref:`Monitor Config
                Reference <monitor-config-reference>`.

	Ceph Node
               A Ceph node is a unit of the Ceph Cluster that communicates with
               other nodes in the Ceph Cluster in order to replicate and
               redistribute data. All of the nodes together are called the
               :term:`Ceph Storage Cluster`. Ceph nodes include :term:`OSD`\s,
               :term:`Ceph Monitor`\s, :term:`Ceph Manager`\s, and
               :term:`MDS`\es. The term "node" is usually equivalent to "host"
               in the Ceph documentation. If you have a running Ceph Cluster,
               you can list all of the nodes in it by running the command
               ``ceph node ls all``.
                
	:ref:`Ceph Object Gateway<object-gateway>`
                An object storage interface built on top of librados. Ceph
                Object Gateway provides a RESTful gateway between applications
                and Ceph storage clusters.

	Ceph Object Storage
                See :term:`Ceph Object Store`.

	Ceph Object Store
                A Ceph Object Store consists of a :term:`Ceph Storage Cluster`
                and a :term:`Ceph Object Gateway` (RGW).

	:ref:`Ceph OSD<rados_configuration_storage-devices_ceph_osd>`
                Ceph **O**\bject **S**\torage **D**\aemon. The Ceph OSD
                software, which interacts with logical disks (:term:`OSD`).
                Around 2013, there was an attempt by "research and industry"
                (Sage's own words) to insist on using the term "OSD" to mean
                only "Object Storage Device", but the Ceph community has always
                persisted in using the term to mean "Object Storage Daemon" and
                no less an authority than Sage Weil himself confirms in
                November of 2022 that "Daemon is more accurate for how Ceph is
                built" (private correspondence between Zac Dover and Sage Weil,
                07 Nov 2022). 

	Ceph OSD Daemon
                See :term:`Ceph OSD`.

	Ceph OSD Daemons
                See :term:`Ceph OSD`.

	Ceph Platform
                All Ceph software, which includes any piece of code hosted at
                `https://github.com/ceph`_.

	Ceph Point Release
                See :term:`Releases`.

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

	Ceph Release
                See :term:`Releases`.

	Ceph Release Candidate
                See :term:`Releases`.

	Ceph Stable Release
                See :term:`Releases`.

	Ceph Stack
		A collection of two or more components of Ceph.

	:ref:`Ceph Storage Cluster<arch-ceph-storage-cluster>`
                The collection of :term:`Ceph Monitor`\s, :term:`Ceph
                Manager`\s, :term:`Ceph Metadata Server`\s, and :term:`OSD`\s
                that work together to store and replicate data for use by
                applications, Ceph Users, and :term:`Ceph Client`\s. Ceph
                Storage Clusters receive data from :term:`Ceph Client`\s.

	CephX
                The Ceph authentication protocol. CephX authenticates users and
                daemons. CephX operates like Kerberos, but it has no single
                point of failure. See the :ref:`High-availability
                Authentication section<arch_high_availability_authentication>`
                of the Architecture document and the :ref:`CephX Configuration
                Reference<rados-cephx-config-ref>`. 

	Client
                A client is any program external to Ceph that uses a Ceph
                Cluster to store and replicate data. 

	Cloud Platforms
	Cloud Stacks
                Third party cloud provisioning platforms such as OpenStack,
                CloudStack, OpenNebula, and Proxmox VE.

	Cluster Map
                The set of maps consisting of the monitor map, OSD map, PG map,
                MDS map, and CRUSH map, which together report the state of the
                Ceph cluster. See :ref:`the "Cluster Map" section of the
                Architecture document<architecture_cluster_map>` for details.

        Crimson
                A next-generation OSD architecture whose main aim is the
                reduction of latency costs incurred due to cross-core
                communications. A re-design of the OSD reduces lock
                contention by reducing communication between shards in the data
                path. Crimson improves upon the performance of classic Ceph
                OSDs by eliminating reliance on thread pools. See `Crimson:
                Next-generation Ceph OSD for Multi-core Scalability
                <https://ceph.io/en/news/blog/2023/crimson-multi-core-scalability/>`_.
                See the :ref:`Crimson developer
                documentation<crimson_dev_doc>`.

	CRUSH
                **C**\ontrolled **R**\eplication **U**\nder **S**\calable
                **H**\ashing. The algorithm that Ceph uses to compute object
                storage locations. See `CRUSH: Controlled, Scalable,
                Decentralized Placement of Replicated Data
                <https://ceph.com/assets/pdfs/weil-crush-sc06.pdf>`_.

	CRUSH rule
                The CRUSH data placement rule that applies to a particular
                pool or pools.

        DAS
                **D**\irect-\ **A**\ttached **S**\torage. Storage that is
                attached directly to the computer accessing it, without passing
                through a network.  Contrast with NAS and SAN.

	:ref:`Dashboard<mgr-dashboard>`
                A built-in web-based Ceph management and monitoring application
                to administer various aspects and objects of the cluster. The
                dashboard is implemented as a Ceph Manager module. See
                :ref:`mgr-dashboard` for more details.

	Dashboard Module
                Another name for :term:`Dashboard`.

	Dashboard Plugin
        Flapping OSD
                An OSD that is repeatedly marked ``up`` and then ``down`` in
                rapid succession. See :ref:`rados_tshooting_flapping_osd`.

        FQDN
                **F**\ully **Q**\ualified **D**\omain **N**\ame. A domain name
                that is applied to a node in a network and that specifies the
                node's exact location in the tree hierarchy of the DNS.

                In the context of Ceph cluster administration, FQDNs are often
                applied to hosts. In this documentation, the term "FQDN" is
                used mostly to distinguish between FQDNs and relatively simpler
                hostnames, which do not specify the exact location of the host
                in the tree hierarchy of the DNS but merely name the host.

	Host
                Any single machine or server in a Ceph Cluster. See :term:`Ceph
                Node`.

        Hybrid OSD  
                Refers to an OSD that has both HDD and SSD drives.

        librados
                An API that can be used to create a custom interface to a Ceph
                storage cluster. ``librados`` makes it possible to interact
                with Ceph Monitors and with OSDs. See :ref:`Introduction to
                librados <librados-intro>`. See :ref:`librados (Python)
                <librados-python>`.

	LVM tags
                **L**\ogical **V**\olume **M**\anager tags. Extensible metadata
                for LVM volumes and groups. They are used to store
                Ceph-specific information about devices and its relationship
                with OSDs.

	MDS
                The Ceph **M**\eta\ **D**\ata **S**\erver daemon. Also referred
                to as "ceph-mds". The Ceph metadata server daemon must be
                running in any Ceph cluster that runs the CephFS file system.
                The MDS stores all filesystem metadata. :term:`Client`\s work
                together with either a single MDS or a group of MDSes to
                maintain a distributed metadata cache that is required by
                CephFS.

                See :ref:`Deploying Metadata Servers<cephfs_add_remote_mds>`.

                See the :ref:`ceph-mds man page<ceph_mds_man>`.

	MGR
                The Ceph manager software, which collects all the state from
                the whole cluster in one place.

	:ref:`MON<arch_monitor>`
		The Ceph monitor software.

        Monitor Store
                The persistent storage that is used by the Monitor. This
                includes the Monitor's RocksDB and all related files in
                ``/var/lib/ceph``.

	Node
                See :term:`Ceph Node`.

	Object Storage
                Object storage is one of three kinds of storage relevant to
                Ceph. The other two kinds of storage relevant to Ceph are file
                storage and block storage. Object storage is the category of
                storage most fundamental to Ceph.

	Object Storage Device
                See :term:`OSD`.

        OMAP
                "object map". A key-value store (a database) that is used to
                reduce the time it takes to read data from and to write to the
                Ceph cluster. RGW bucket indexes are stored as OMAPs.
                Erasure-coded pools cannot store RADOS OMAP data structures.
               
                Run the command ``ceph osd df`` to see your OMAPs.

                See Eleanor Cawthon's 2012 paper `A Distributed Key-Value Store
                using Ceph
                <https://ceph.io/assets/pdfs/CawthonKeyValueStore.pdf>`_ (17
                pages).

        OpenStack Swift
                In the context of Ceph, OpenStack Swift is one of the two APIs
                supported by the Ceph Object Store. The other API supported by
                the Ceph Object Store is S3.

                See `the OpenStack Storage API overview page
                <https://docs.openstack.org/swift/latest/api/object_api_v1_overview.html>`_.

	OSD
                Probably :term:`Ceph OSD`, but not necessarily. Sometimes
                (especially in older correspondence, and especially in
                documentation that is not written specifically for Ceph), "OSD"
                means "**O**\bject **S**\torage **D**\evice", which refers to a
                physical or logical storage unit (for example: LUN). The Ceph
                community has always used the term "OSD" to refer to
                :term:`Ceph OSD Daemon` despite an industry push in the
                mid-2010s to insist that "OSD" should refer to "Object Storage
                Device", so it is important to know which meaning is intended. 

        OSD, flapping
                See :term:`Flapping OSD`.

	OSD FSID 
                The OSD fsid is a unique identifier that is used to identify an
                OSD. It is found in the OSD path in a file called ``osd_fsid``.
                The term ``FSID`` is used interchangeably with ``UUID``.

	OSD ID 
                The OSD id an integer unique to each OSD (each OSD has a unique
                OSD ID). Each OSD id is generated by the monitors during the
                creation of its associated OSD.

	OSD UUID 
                The OSD UUID is the unique identifier of an OSD. This term is
                used interchangeably with ``FSID``.

        Period
                In the context of :term:`RGW`, a period is the configuration
                state of the :term:`Realm`. The period stores the configuration
                state of a multi-site configuration. When the period is updated,
                the "epoch" is said thereby to have been changed.

        Placement Groups (PGs)
                Placement groups (PGs) are subsets of each logical Ceph pool.
                Placement groups perform the function of placing objects (as a
                group) into OSDs. Ceph manages data internally at
                placement-group granularity: this scales better than would
                managing individual (and therefore more numerous) RADOS
                objects. A cluster that has a larger number of placement groups
                (for example, 100 per OSD) is better balanced than an otherwise
                identical cluster with a smaller number of placement groups. 
                
                Ceph's internal RADOS objects are each mapped to a specific
                placement group, and each placement group belongs to exactly
                one Ceph pool. 

        PLP 
                **P**\ower **L**\oss **P**\rotection. A technology that
                protects the data of solid-state drives by using capacitors to
                extend the amount of time available for transferring data from
                the DRAM cache to the SSD's permanent memory. Consumer-grade
                SSDs are rarely equipped with PLP.

	:ref:`Pool<rados_pools>`

		A pool is a logical partition used to store objects.

	Pools
                See :term:`pool`.

	:ref:`Primary Affinity <rados_ops_primary_affinity>`
                The characteristic of an OSD that governs the likelihood that
                a given OSD will be selected as the primary OSD (or "lead
                OSD") in an acting set. Primary affinity was introduced in
                Firefly (v. 0.80). See :ref:`Primary Affinity
                <rados_ops_primary_affinity>`.

        :ref:`Prometheus <mgr-prometheus>`
                An open-source monitoring and alerting toolkit. Ceph offers a
                :ref:`"Prometheus module" <mgr-prometheus>`, which provides a
                Prometheus exporter that passes performance counters from a
                collection point in ``ceph-mgr`` to Prometheus.

        Quorum	
                Quorum is the state that exists when a majority of the
                :ref:`Monitors<arch_monitor>` in the cluster are ``up``. A
                minimum of three :ref:`Monitors<arch_monitor>` must exist in
                the cluster in order for Quorum to be possible.

	RADOS
                **R**\eliable **A**\utonomic **D**\istributed **O**\bject
                **S**\tore. RADOS is the object store that provides a scalable
                service for variably-sized objects. The RADOS object store is
                the core component of a Ceph cluster.  `This blog post from
                2009
                <https://ceph.io/en/news/blog/2009/the-rados-distributed-object-store/>`_
                provides a beginner's introduction to RADOS. Readers interested
                in a deeper understanding of RADOS are directed to `RADOS: A
                Scalable, Reliable Storage Service for Petabyte-scale Storage
                Clusters <https://ceph.io/assets/pdfs/weil-rados-pdsw07.pdf>`_.

	RADOS Cluster
                A proper subset of the Ceph Cluster consisting of
                :term:`OSD`\s, :term:`Ceph Monitor`\s, and :term:`Ceph
                Manager`\s.
                
	RADOS Gateway
                See :term:`RGW`.

	RBD
                **R**\ADOS **B**\lock **D**\evice. See :term:`Ceph Block
                Device`.

        :ref:`Realm<rgw-realms>`
                In the context of RADOS Gateway (RGW), a realm is a globally
                unique namespace that consists of one or more zonegroups.

        Releases

	        Ceph Interim Release
                        A version of Ceph that has not yet been put through
                        quality assurance testing. May contain new features.

                Ceph Point Release
                        Any ad hoc release that includes only bug fixes and
                        security fixes.

                Ceph Release
                        Any distinct numbered version of Ceph.

                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.

	Reliable Autonomic Distributed Object Store
                The core set of storage software which stores the user's data
                (MON+OSD). See also :term:`RADOS`.

	:ref:`RGW<object-gateway>`
                **R**\ADOS **G**\ate\ **w**\ay.

                Also called "Ceph Object Gateway". The component of Ceph that
                provides a gateway to both the Amazon S3 RESTful API and the
                OpenStack Swift API. 

        S3
                In the context of Ceph, S3 is one of the two APIs supported by
                the Ceph Object Store. The other API supported by the Ceph
                Object Store is OpenStack Swift.

                See `the Amazon S3 overview page
                <https://aws.amazon.com/s3/>`_.

        scrubs

                The processes by which Ceph ensures data integrity. During the
                process of scrubbing, Ceph generates a catalog of all objects
                in a placement group, then ensures that none of the objects are
                missing or mismatched by comparing each primary object against
                its replicas, which are stored across other OSDs. Any PG
                is determined to have a copy of an object that is different
                than the other copies or is missing entirely is marked
                "inconsistent" (that is, the PG is marked "inconsistent"). 

                There are two kinds of scrubbing: light scrubbing and deep
                scrubbing (also called "shallow scrubbing" and "deep scrubbing",
                respectively). Light scrubbing is performed daily and does
                nothing more than confirm that a given object exists and that
                its metadata is correct. Deep scrubbing is performed weekly and
                reads the data and uses checksums to ensure data integrity.

                See :ref:`Scrubbing <rados_config_scrubbing>` in the RADOS OSD
                Configuration Reference Guide and page 141 of *Mastering Ceph,
                second edition* (Fisk, Nick. 2019).

        secrets
                Secrets are credentials used to perform digital authentication
                whenever privileged users must access systems that require
                authentication. Secrets can be passwords, API keys, tokens, SSH
                keys, private certificates, or encryption keys.

        SDS
                **S**\oftware-**d**\efined **S**\torage.

	systemd oneshot
                A systemd ``type`` where a command is defined in ``ExecStart``
                which will exit upon completion (it is not intended to
                daemonize)

        Swift
                See :term:`OpenStack Swift`.

	Teuthology
		The collection of software that performs scripted tests on Ceph.

        User
                An individual or a system actor (for example, an application)
                that uses Ceph clients to interact with the :term:`Ceph Storage
                Cluster`. See :ref:`User<rados-ops-user>` and :ref:`User
                Management<user-management>`.

        Zone
                In the context of :term:`RGW`, a zone is a logical group that
                consists of one or more :term:`RGW` instances.  A zone's
                configuration state is stored in the :term:`period`. See
                :ref:`Zones<radosgw-zones>`.

.. _https://github.com/ceph: https://github.com/ceph
.. _Cluster Map: ../architecture#cluster-map