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doc: Clarified text to reflect requirement for majority, not odd number of monitors.

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fixes: 3464

Signed-off-by: John Wilkins <john.wilkins@inktank.com>
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John Wilkins 2012-11-08 11:59:15 -08:00
parent 937345219d
commit 82ebbaba31
1 changed files with 20 additions and 16 deletions
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36
doc/cluster-ops/add-or-rm-mons.rst
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@ -11,22 +11,23 @@ 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, so you must have an odd number of
monitors to establish a quorum for consensus about the cluster map.
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; 3 out of 5; 4 out of 6; etc.).
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.
.. important:: You must have an odd number of monitors in your cluster
to establish a quorum.
.. important:: A *majority* of monitors in your cluster must be able to
reach each other in order to establish a quorum.
Deploy your Hardware
--------------------
If you are adding a new host when adding a new monitor,
see `Hardware Recommendations`_ for details on minimum recommendations
for monitor hardware. To add a monitor host to your cluster, first make sure you have
an up-to-date version of Linux installed (typically Ubuntu 12.04 precise).
If you are adding a new host when adding a new monitor, see `Hardware
Recommendations`_ for details on minimum recommendations for monitor hardware.
To add a monitor host to your cluster, first make sure you have an up-to-date
version of Linux installed (typically Ubuntu 12.04 precise).
Add your monitor host to a rack in your cluster, connect it to the network
and ensure that it has network connectivity.
@ -59,9 +60,10 @@ Adding a Monitor (Manual)
-------------------------
This procedure creates a ``ceph-mon`` data directory, retrieves the monitor map
and monitor keyring, and adds a ``ceph-mon`` daemon to your cluster. If you
this results in an even number of daemons, you may add another monitor by
repeating this procedure until you have an odd number of ``ceph-mon`` daemons.
and monitor keyring, and adds a ``ceph-mon`` daemon to your cluster. If
this results in only two monitor daemons, you may add more monitors by
repeating this procedure until you have a sufficient number of ``ceph-mon``
daemons to achieve a quorum.
#. Create the default directory on your new monitor. ::
@ -110,16 +112,18 @@ repeating this procedure until you have an odd number of ``ceph-mon`` daemons.
Removing Monitors
=================
When you remove monitors from a cluster, Ceph monitors use PAXOS to establish
consensus about the master cluster map, so you must have an odd number of
monitors to establish a quorum for consensus about the cluster map.
When you remove monitors from a cluster, consider that Ceph monitors use
PAXOS to establish consensus about the master cluster map. You must have
a sufficient number of monitors to establish a quorum for consensus about
the cluster map.
Removing a Monitor (Manual)
---------------------------
This procedure removes a ``ceph-mon`` daemon from your cluster. If this
procedure results in an even number of daemons, you may add or remove another
monitor until you have an odd number of ``ceph-mon`` daemons.
procedure results in only two monitor daemons, you may add or remove another
monitor until you have a number of ``ceph-mon`` daemons that can achieve a
quorum.
#. Stop the monitor. ::