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reef: doc/architecture: edit "High Avail. Monitors" #53451

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reef: doc/architecture: edit "High Avail. Monitors" #53451

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35 changes: 20 additions & 15 deletions doc/architecture.rst
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Expand Up @@ -179,21 +179,26 @@ information recording the overall health of the Ceph Storage Cluster.
High Availability Monitors
~~~~~~~~~~~~~~~~~~~~~~~~~~

Before Ceph Clients can read or write data, they must contact a Ceph Monitor
to obtain the most recent copy of the cluster map. A Ceph Storage Cluster
can operate with a single monitor; however, this introduces a single
point of failure (i.e., if the monitor goes down, Ceph Clients cannot
read or write data).

For added reliability and fault tolerance, Ceph supports a cluster of monitors.
In a cluster of monitors, latency and other faults can cause one or more
monitors to fall behind the current state of the cluster. For this reason, Ceph
must have agreement among various monitor instances regarding the state of the
cluster. Ceph always uses a majority of monitors (e.g., 1, 2:3, 3:5, 4:6, etc.)
and the `Paxos`_ algorithm to establish a consensus among the monitors about the
current state of the cluster.

For details on configuring monitors, see the `Monitor Config Reference`_.
A Ceph Client must contact a Ceph Monitor and obtain a current copy of the
cluster map in order to read data from or to write data to the Ceph cluster.

It is possible for a Ceph cluster to function properly with only a single
monitor, but a Ceph cluster that has only a single monitor has a single point
of failure: if the monitor goes down, Ceph clients will be unable to read data
from or write data to the cluster.

Ceph leverages a cluster of monitors in order to increase reliability and fault
tolerance. When a cluster of monitors is used, however, one or more of the
monitors in the cluster can fall behind due to latency or other faults. Ceph
mitigates these negative effects by requiring multiple monitor instances to
agree about the state of the cluster. To establish consensus among the monitors
regarding the state of the cluster, Ceph uses the `Paxos`_ algorithm and a
majority of monitors (for example, one in a cluster that contains only one
monitor, two in a cluster that contains three monitors, three in a cluster that
contains five monitors, four in a cluster that contains six monitors, and so
on).

See the `Monitor Config Reference`_ for more detail on configuring monitors.

.. index:: architecture; high availability authentication

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