doc/glossary.rst
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===============
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.
cephadm
The tool that deploys and manages a Ceph cluster as containers
on a set of hosts. ``cephadm bootstrap`` creates a new cluster
on one host, and the ``ceph orch`` commands then add hosts and
daemons through the orchestrator. See :ref:`cephadm`.
CephX
The Ceph authentication protocol. CephX authenticates users and
daemons. CephX operates like Kerberos, but it has no single
point of failure and allows authentication to a class of
services. 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.
CRC
*C*\yclic *R*\edundancy *C*\heck. An algorithm used to detect
errors in data transmission. CRC cannot correct errors.
Crimson
A next-generation OSD architecture whose 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
The dashboard plugin was a Mimic-era web application that
visualized information and statistics about the Ceph cluster
using a web server hosted by the :ref:`Ceph
Manager<ceph-manager-daemon>`.
See `the Mimic-era Dashboard Plugin documentation
<https://docs.ceph.com/en/mimic/mgr/dashboard/>`_.
DC
**D**\ata **C**\enter.
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.
Keyring
A file that holds one or more Ceph user names and their secret
keys. Clients and daemons present a key from a keyring to
authenticate with :term:`CephX`. The bootstrap process writes
the administrator's keyring to
``/etc/ceph/ceph.client.admin.keyring``. See :ref:`User
Management <user-management>`.
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, by default, cannot store RADOS omap data
structures. This functionality can be enabled in non-Crimson
erasure-coded pools by enabling EC optimizations.
Run the command ``ceph osd df`` to see the storage space used by
omaps on each OSD.
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/scalability-high-availability#cluster-map