ceph/doc/start/intro.rst
Alexander Chuzhoy efc8969ec3 Doc: Correcting the default number of copies.
According to the "quick ceph deploy" page, the default number of replicas in the Ceph configuration file is 3

Signed-off-by: Sasha Chuzhoy <sashac88@hotmail.com>
2015-08-11 18:40:00 -04:00

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===============
Intro to Ceph
===============
Whether you want to provide :term:`Ceph Object Storage` and/or :term:`Ceph Block
Device` services to :term:`Cloud Platforms`, deploy a :term:`Ceph Filesystem` or
use Ceph for another purpose, all :term:`Ceph Storage Cluster` deployments begin
with setting up each :term:`Ceph Node`, your network and the Ceph Storage
Cluster. A Ceph Storage Cluster requires at least one Ceph Monitor and at least
two Ceph OSD Daemons. The Ceph Metadata Server is essential when running Ceph
Filesystem clients.
.. ditaa:: +---------------+ +---------------+ +---------------+
| OSDs | | Monitor | | MDS |
+---------------+ +---------------+ +---------------+
- **Ceph OSDs**: A :term:`Ceph OSD Daemon` (Ceph OSD) stores data, handles data
replication, recovery, backfilling, rebalancing, and provides some monitoring
information to Ceph Monitors by checking other Ceph OSD Daemons for a
heartbeat. A Ceph Storage Cluster requires at least two Ceph OSD Daemons to
achieve an ``active + clean`` state when the cluster makes two copies of your
data (Ceph makes 3 copies by default, but you can adjust it).
- **Monitors**: A :term:`Ceph Monitor` maintains maps of the cluster state,
including the monitor map, the OSD map, the Placement Group (PG) map, and the
CRUSH map. Ceph maintains a history (called an "epoch") of each state change
in the Ceph Monitors, Ceph OSD Daemons, and PGs.
- **MDSs**: A :term:`Ceph Metadata Server` (MDS) stores metadata on behalf of
the :term:`Ceph Filesystem` (i.e., Ceph Block Devices and Ceph Object Storage
do not use MDS). Ceph Metadata Servers make it feasible for POSIX file system
users to execute basic commands like ``ls``, ``find``, etc. without placing
an enormous burden on the Ceph Storage Cluster.
Ceph stores a client's data as objects within storage pools. Using the CRUSH
algorithm, Ceph calculates which placement group should contain the object,
and further calculates which Ceph OSD Daemon should store the placement group.
The CRUSH algorithm enables the Ceph Storage Cluster to scale, rebalance, and
recover dynamically.
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<style type="text/css">div.body h3{margin:5px 0px 0px 0px;}</style>
<table cellpadding="10"><colgroup><col width="50%"><col width="50%"></colgroup><tbody valign="top"><tr><td><h3>Recommendations</h3>
To begin using Ceph in production, you should review our hardware
recommendations and operating system recommendations.
.. toctree::
:maxdepth: 2
Hardware Recommendations <hardware-recommendations>
OS Recommendations <os-recommendations>
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</td><td><h3>Get Involved</h3>
You can avail yourself of help or contribute documentation, source
code or bugs by getting involved in the Ceph community.
.. toctree::
:maxdepth: 2
get-involved
documenting-ceph
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</td></tr></tbody></table>