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doc: add-or-rm-mons.rst: better explanation on number of monitors

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Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
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Joao Eduardo Luis 2014-01-22 12:21:36 +00:00
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1 changed files with 21 additions and 5 deletions
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doc/rados/operations/add-or-rm-mons.rst
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@ -11,10 +11,26 @@ Adding Monitors
Ceph monitors are light-weight processes that maintain a master copy of the
cluster map. You can run a cluster with 1 monitor. We recommend at least 3
monitors for a production cluster. Ceph monitors use PAXOS to establish
consensus about the master cluster map, which requires a majority of
monitors running to establish a quorum for consensus about the cluster map
(e.g., 1; 2 out of 3; 3 out of 4 or 5; 4 out of 6; etc.).
monitors for a production cluster. Ceph monitors use a variation of the
`Paxos_` protocol to establish consensus about maps and other critical
information across the cluster. Due to the nature of Paxos, Ceph requires
a majority of monitors running to establish a quorum (thus establishing
consensus).
It is advisable to run an odd-number of monitors but not mandatory. An
odd-number of monitors has a higher resiliency to failures than an
even-number of monitors. For instance, on a 2 monitor deployment, no
failures can be tolerated in order to maintain a quorum; with 3 monitors,
one failure can be tolerated; in a 4 monitor deployment, one failure can
be tolerated; with 5 monitors, two failures can be tolerated. This is
why an odd-number is advisable. Summarizing, Ceph needs a majority of
monitors to be running (and able to communicate with each other), but that
majority can be achieved using a single monitor, or 2 out of 2 monitors,
2 out of 3, 3 out of 4, etc.
For an initial deployment of a multi-node Ceph cluster, it is advisable to
deploy three monitors, increasing the number two at a time if a valid need
for more than three exists.
Since monitors are light-weight, it is possible to run them on the same
host as an OSD; however, we recommend running them on separate hosts,
@ -343,4 +359,4 @@ the monitors should operate successfully.
.. _Manual Deployment: ../../../install/manual-deployment
.. _Monitor Bootstrap: ../../../dev/mon-bootstrap
.. _Monitor Bootstrap: ../../../dev/mon-bootstrap