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-==========================
- Adding/Removing Monitors
-==========================
-
-When you have a cluster up and running, you may add or remove monitors
-from the cluster at runtime. To bootstrap a monitor, see `Manual Deployment`_
-or `Monitor Bootstrap`_.
-
-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 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,
-because fsync issues with the kernel may impair performance.
-
-.. note:: 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 14.04 or RHEL 7).
-
-Add your monitor host to a rack in your cluster, connect it to the network
-and ensure that it has network connectivity.
-
-.. _Hardware Recommendations: ../../../start/hardware-recommendations
-
-Install the Required Software
------------------------------
-
-For manually deployed clusters, you must install Ceph packages
-manually. See `Installing Packages`_ for details.
-You should configure SSH to a user with password-less authentication
-and root permissions.
-
-.. _Installing Packages: ../../../install/install-storage-cluster
-
-
-.. _Adding a Monitor (Manual):
-
-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
-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.
-
-At this point you should define your monitor's id. Traditionally, monitors
-have been named with single letters (``a``, ``b``, ``c``, ...), but you are
-free to define the id as you see fit. For the purpose of this document,
-please take into account that ``{mon-id}`` should be the id you chose,
-without the ``mon.`` prefix (i.e., ``{mon-id}`` should be the ``a``
-on ``mon.a``).
-
-#. Create the default directory on the machine that will host your
- new monitor. ::
-
- ssh {new-mon-host}
- sudo mkdir /var/lib/ceph/mon/ceph-{mon-id}
-
-#. Create a temporary directory ``{tmp}`` to keep the files needed during
- this process. This directory should be different from the monitor's default
- directory created in the previous step, and can be removed after all the
- steps are executed. ::
-
- mkdir {tmp}
-
-#. Retrieve the keyring for your monitors, where ``{tmp}`` is the path to
- the retrieved keyring, and ``{key-filename}`` is the name of the file
- containing the retrieved monitor key. ::
-
- ceph auth get mon. -o {tmp}/{key-filename}
-
-#. Retrieve the monitor map, where ``{tmp}`` is the path to
- the retrieved monitor map, and ``{map-filename}`` is the name of the file
- containing the retrieved monitor monitor map. ::
-
- ceph mon getmap -o {tmp}/{map-filename}
-
-#. Prepare the monitor's data directory created in the first step. You must
- specify the path to the monitor map so that you can retrieve the
- information about a quorum of monitors and their ``fsid``. You must also
- specify a path to the monitor keyring::
-
- sudo ceph-mon -i {mon-id} --mkfs --monmap {tmp}/{map-filename} --keyring {tmp}/{key-filename}
-
-
-#. Start the new monitor and it will automatically join the cluster.
- The daemon needs to know which address to bind to, either via
- ``--public-addr {ip:port}`` or by setting ``mon addr`` in the
- appropriate section of ``ceph.conf``. For example::
-
- ceph-mon -i {mon-id} --public-addr {ip:port}
-
-
-Removing Monitors
-=================
-
-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):
-
-Removing a Monitor (Manual)
----------------------------
-
-This procedure removes a ``ceph-mon`` daemon from your cluster. If this
-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. ::
-
- service ceph -a stop mon.{mon-id}
-
-#. Remove the monitor from the cluster. ::
-
- ceph mon remove {mon-id}
-
-#. Remove the monitor entry from ``ceph.conf``.
-
-
-Removing Monitors from an Unhealthy Cluster
--------------------------------------------
-
-This procedure removes a ``ceph-mon`` daemon from an unhealthy
-cluster, for example a cluster where the monitors cannot form a
-quorum.
-
-
-#. Stop all ``ceph-mon`` daemons on all monitor hosts. ::
-
- ssh {mon-host}
- service ceph stop mon || stop ceph-mon-all
- # and repeat for all mons
-
-#. Identify a surviving monitor and log in to that host. ::
-
- ssh {mon-host}
-
-#. Extract a copy of the monmap file. ::
-
- ceph-mon -i {mon-id} --extract-monmap {map-path}
- # in most cases, that's
- ceph-mon -i `hostname` --extract-monmap /tmp/monmap
-
-#. Remove the non-surviving or problematic monitors. For example, if
- you have three monitors, ``mon.a``, ``mon.b``, and ``mon.c``, where
- only ``mon.a`` will survive, follow the example below::
-
- monmaptool {map-path} --rm {mon-id}
- # for example,
- monmaptool /tmp/monmap --rm b
- monmaptool /tmp/monmap --rm c
-
-#. Inject the surviving map with the removed monitors into the
- surviving monitor(s). For example, to inject a map into monitor
- ``mon.a``, follow the example below::
-
- ceph-mon -i {mon-id} --inject-monmap {map-path}
- # for example,
- ceph-mon -i a --inject-monmap /tmp/monmap
-
-#. Start only the surviving monitors.
-
-#. Verify the monitors form a quorum (``ceph -s``).
-
-#. You may wish to archive the removed monitors' data directory in
- ``/var/lib/ceph/mon`` in a safe location, or delete it if you are
- confident the remaining monitors are healthy and are sufficiently
- redundant.
-
-.. _Changing a Monitor's IP address:
-
-Changing a Monitor's IP Address
-===============================
-
-.. important:: Existing monitors are not supposed to change their IP addresses.
-
-Monitors are critical components of a Ceph cluster, and they need to maintain a
-quorum for the whole system to work properly. To establish a quorum, the
-monitors need to discover each other. Ceph has strict requirements for
-discovering monitors.
-
-Ceph clients and other Ceph daemons use ``ceph.conf`` to discover monitors.
-However, monitors discover each other using the monitor map, not ``ceph.conf``.
-For example, if you refer to `Adding a Monitor (Manual)`_ you will see that you
-need to obtain the current monmap for the cluster when creating a new monitor,
-as it is one of the required arguments of ``ceph-mon -i {mon-id} --mkfs``. The
-following sections explain the consistency requirements for Ceph monitors, and a
-few safe ways to change a monitor's IP address.
-
-
-Consistency Requirements
-------------------------
-
-A monitor always refers to the local copy of the monmap when discovering other
-monitors in the cluster. Using the monmap instead of ``ceph.conf`` avoids
-errors that could break the cluster (e.g., typos in ``ceph.conf`` when
-specifying a monitor address or port). Since monitors use monmaps for discovery
-and they share monmaps with clients and other Ceph daemons, the monmap provides
-monitors with a strict guarantee that their consensus is valid.
-
-Strict consistency also applies to updates to the monmap. As with any other
-updates on the monitor, changes to the monmap always run through a distributed
-consensus algorithm called `Paxos`_. The monitors must agree on each update to
-the monmap, such as adding or removing a monitor, to ensure that each monitor in
-the quorum has the same version of the monmap. Updates to the monmap are
-incremental so that monitors have the latest agreed upon version, and a set of
-previous versions, allowing a monitor that has an older version of the monmap to
-catch up with the current state of the cluster.
-
-If monitors discovered each other through the Ceph configuration file instead of
-through the monmap, it would introduce additional risks because the Ceph
-configuration files are not updated and distributed automatically. Monitors
-might inadvertently use an older ``ceph.conf`` file, fail to recognize a
-monitor, fall out of a quorum, or develop a situation where `Paxos`_ is not able
-to determine the current state of the system accurately. Consequently, making
-changes to an existing monitor's IP address must be done with great care.
-
-
-Changing a Monitor's IP address (The Right Way)
------------------------------------------------
-
-Changing a monitor's IP address in ``ceph.conf`` only is not sufficient to
-ensure that other monitors in the cluster will receive the update. To change a
-monitor's IP address, you must add a new monitor with the IP address you want
-to use (as described in `Adding a Monitor (Manual)`_), ensure that the new
-monitor successfully joins the quorum; then, remove the monitor that uses the
-old IP address. Then, update the ``ceph.conf`` file to ensure that clients and
-other daemons know the IP address of the new monitor.
-
-For example, lets assume there are three monitors in place, such as ::
-
- [mon.a]
- host = host01
- addr = 10.0.0.1:6789
- [mon.b]
- host = host02
- addr = 10.0.0.2:6789
- [mon.c]
- host = host03
- addr = 10.0.0.3:6789
-
-To change ``mon.c`` to ``host04`` with the IP address ``10.0.0.4``, follow the
-steps in `Adding a Monitor (Manual)`_ by adding a new monitor ``mon.d``. Ensure
-that ``mon.d`` is running before removing ``mon.c``, or it will break the
-quorum. Remove ``mon.c`` as described on `Removing a Monitor (Manual)`_. Moving
-all three monitors would thus require repeating this process as many times as
-needed.
-
-
-Changing a Monitor's IP address (The Messy Way)
------------------------------------------------
-
-There may come a time when the monitors must be moved to a different network, a
-different part of the datacenter or a different datacenter altogether. While it
-is possible to do it, the process becomes a bit more hazardous.
-
-In such a case, the solution is to generate a new monmap with updated IP
-addresses for all the monitors in the cluster, and inject the new map on each
-individual monitor. This is not the most user-friendly approach, but we do not
-expect this to be something that needs to be done every other week. As it is
-clearly stated on the top of this section, monitors are not supposed to change
-IP addresses.
-
-Using the previous monitor configuration as an example, assume you want to move
-all the monitors from the ``10.0.0.x`` range to ``10.1.0.x``, and these
-networks are unable to communicate. Use the following procedure:
-
-#. Retrieve the monitor map, where ``{tmp}`` is the path to
- the retrieved monitor map, and ``{filename}`` is the name of the file
- containing the retrieved monitor monitor map. ::
-
- ceph mon getmap -o {tmp}/{filename}
-
-#. The following example demonstrates the contents of the monmap. ::
-
- $ monmaptool --print {tmp}/{filename}
-
- monmaptool: monmap file {tmp}/{filename}
- epoch 1
- fsid 224e376d-c5fe-4504-96bb-ea6332a19e61
- last_changed 2012-12-17 02:46:41.591248
- created 2012-12-17 02:46:41.591248
- 0: 10.0.0.1:6789/0 mon.a
- 1: 10.0.0.2:6789/0 mon.b
- 2: 10.0.0.3:6789/0 mon.c
-
-#. Remove the existing monitors. ::
-
- $ monmaptool --rm a --rm b --rm c {tmp}/{filename}
-
- monmaptool: monmap file {tmp}/{filename}
- monmaptool: removing a
- monmaptool: removing b
- monmaptool: removing c
- monmaptool: writing epoch 1 to {tmp}/{filename} (0 monitors)
-
-#. Add the new monitor locations. ::
-
- $ monmaptool --add a 10.1.0.1:6789 --add b 10.1.0.2:6789 --add c 10.1.0.3:6789 {tmp}/{filename}
-
- monmaptool: monmap file {tmp}/{filename}
- monmaptool: writing epoch 1 to {tmp}/{filename} (3 monitors)
-
-#. Check new contents. ::
-
- $ monmaptool --print {tmp}/{filename}
-
- monmaptool: monmap file {tmp}/{filename}
- epoch 1
- fsid 224e376d-c5fe-4504-96bb-ea6332a19e61
- last_changed 2012-12-17 02:46:41.591248
- created 2012-12-17 02:46:41.591248
- 0: 10.1.0.1:6789/0 mon.a
- 1: 10.1.0.2:6789/0 mon.b
- 2: 10.1.0.3:6789/0 mon.c
-
-At this point, we assume the monitors (and stores) are installed at the new
-location. The next step is to propagate the modified monmap to the new
-monitors, and inject the modified monmap into each new monitor.
-
-#. First, make sure to stop all your monitors. Injection must be done while
- the daemon is not running.
-
-#. Inject the monmap. ::
-
- ceph-mon -i {mon-id} --inject-monmap {tmp}/{filename}
-
-#. Restart the monitors.
-
-After this step, migration to the new location is complete and
-the monitors should operate successfully.
-
-
-.. _Manual Deployment: ../../../install/manual-deployment
-.. _Monitor Bootstrap: ../../../dev/mon-bootstrap
-.. _Paxos: http://en.wikipedia.org/wiki/Paxos_(computer_science)