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==========================
 Beginner's Guide
==========================

The purpose of A Beginner's Guide to Ceph is to make Ceph comprehensible.

Ceph is a clustered and distributed storage manager. If that's too cryptic,
then just think of Ceph as a computer program that stores data and uses a
network to make sure that there is a backup copy of the data.

Components of Ceph
==================

Storage Interfaces
------------------

Ceph offers several "storage interfaces", which is another
way of saying "ways of storing data". These storage interfaces include:

- CephFS (a file system)
- RBD (block devices)
- RGW (an object store)

Deep down, though, all three of these are really RADOS object stores. CephFS
and RBD are just presenting themselves as file systems and block devices.

Storage Manager: What is It?
----------------------------

Ceph is a clustered and distributed storage manager that offers data
redundancy. This sentence might be too cryptic for first-time readers of the
Ceph Beginner's Guide, so let's explain all of the terms in it:

- **Storage manager.** Ceph is a storage manager. This means that Ceph is
  software that helps storage resources store data. Storage resources come in
  several forms: hard disk drives (HDD), solid-state drives (SSD), magnetic
  tape, floppy disks, punched tape, Hollerith-style punch cards, and magnetic
  drum memory are all forms of storage resources. In this beginner's guide,
  we'll focus on hard disk drives (HDD) and solid-state drives (SSD).
- **Clustered and distributed storage manager.** Ceph is a clustered and
  distributed storage manager. This means that the storage manager is deployed
  not just on a single server but on several servers that work together as a
  system: the data that is stored and the infrastructure that supports it is
  spread across multiple servers and is not centralized in a single server. To
  better understand what distributed means in this context, it might be helpful
  to describe what it is not: it is not a system like a traditional enterprise
  storage array, which is a system that exposes a single logical disk over the
  network in a 1:1 (one-to-one) mapping.
- **Data Redundancy.** Having a second copy of your data somewhere.

Ceph Monitor
------------

The Ceph Monitor is one of the daemons essential to the functioning of a Ceph
cluster. Monitors know the location of all the data in the Ceph cluster.
Monitors maintain maps of the cluster state, and those maps make it possible
for Ceph daemons to work together. These maps include the Monitor map, the OSD
map, the MDS map, and the CRUSH map. At least three Monitors are required for
the daemons to be resistant to failures. A majority of the Monitors must be in
the "up" state in order for them to reach quorum. Quorum is a state that is
necessary for a Ceph cluster to work properly.

Manager
-------

The Ceph Manager is one of the daemons essential to the functioning of the Ceph
cluster. Managers are in charge of various Ceph cluster management and
monitoring tasks that are provided by Manager modules. These tasks include
orchestration, the Ceph dashboard web GUI, balancing the data and load evenly
in the Ceph cluster, keeping track of runtime metrics, and providing
connectivity to non-native clients. Offloading less than critical and
resource-intensive tasks from Monitors to Managers simplifies scaling the
Ceph cluster.

OSD
---

Object Storage Daemons (OSDs) store objects.

An OSD is a process that runs on a storage server. The OSD is responsible for
managing a single unit of storage, which is usually a single disk.

Pools
-----

A pool is an abstraction that can be designated as either "replicated" or
"erasure coded". In Ceph, the method of data protection is set at the pool
level. Ceph offers and supports two types of data protection: replication and
erasure coding. Objects are stored in pools. "A storage pool is a collection of
storage volumes. A storage volume is the basic unit of storage, such as
allocated space on a disk or a single tape cartridge. The server uses the
storage volumes to store backed-up, archived, or space-managed files." (IBM
Tivoli Storage Manager, Version 7.1, "Storage Pools")

Placement Groups
----------------

Placement groups are a part of pools.

MDS
---

A metadata server (MDS) is necessary for the proper functioning of CephFS.
See :ref:`orchestrator-cli-cephfs` and :ref:`arch-cephfs`.

Vstart Cluster Installation and Configuration Procedure
=======================================================

#. Clone the ``ceph/ceph`` repository:

   .. prompt:: bash #

      git clone git@github.com:ceph/ceph

#. Update the submodules in the ``ceph/ceph`` repository:

   .. prompt:: bash #

      git submodule update --init --recursive --progress

#. Run ``install-deps.sh`` from within the directory into which you cloned the
   ``ceph/ceph`` repository:

   .. prompt:: bash #

      ./install-deps.sh

#. Install the ``python3-routes`` package:

   .. prompt:: bash #

      apt install python3-routes

#. Move into the ``ceph`` directory. You will know that you are in the correct
   directory if it contains the file ``do_cmake.sh``:

   .. prompt:: bash #

      cd ceph

#. Run the ``do_cmake.sh`` script:

   .. prompt:: bash #

      ./do_cmake.sh

#. The ``do_cmake.sh`` script creates a ``build/`` directory. Move into the
   ``build/`` directory:

   .. prompt:: bash #

      cd build

#. Use ``ninja`` to build the development environment:

   .. prompt:: bash #

      ninja -j3

   .. note:: This step takes a long time to run. The ``ninja -j3`` command
      kicks off a process consisting of 2289 steps. This step took over three
      hours when I ran it on an Intel NUC with an i7 in September of 2024.

#. Install the Ceph development environment:

   .. prompt:: bash #

      ninja install

   This step does not take as long as the previous step.

#. Build the vstart cluster:

   .. prompt:: bash #

      ninja vstart

#. Start the vstart cluster:

   .. prompt:: bash #

      ../src/vstart.sh --debug --new -x --localhost --bluestore

   .. note:: Run this command from within the ``ceph/build`` directory.




Links
-----

#. `Ceph Wiki (requires Ceph Redmine Tracker account) <https://tracker.ceph.com/projects/ceph/wiki>`_
#. `Sage Weil's 27 June 2019 "Intro To Ceph" tech talk (1h27m) <https://www.youtube.com/watch?v=PmLPbrf-x9g>`_
#. `Sage Weil's 2018 talk "Ceph, the Future of Storage" (27m) <https://www.youtube.com/watch?v=szE4Hg1eXoA>`_