.. _rados-troubleshooting-mon:

==========================
 Troubleshooting Monitors
==========================

.. index:: monitor, high availability

Even if a cluster experiences monitor-related problems, the cluster is not
necessarily in danger of going down. If a cluster has lost multiple monitors,
it can still remain up and running as long as there are enough surviving
monitors to form a quorum.
   
If your cluster is having monitor-related problems, we recommend that you
consult the following troubleshooting information.

Initial Troubleshooting
=======================

The first steps in the process of troubleshooting Ceph Monitors involve making
sure that the Monitors are running and that they are able to communicate with
the network and on the network. Follow the steps in this section to rule out
the simplest causes of Monitor malfunction.

#. **Make sure that the Monitors are running.**

    Make sure that the Monitor (*mon*) daemon processes (``ceph-mon``) are
    running. It might be the case that the mons have not be restarted after an
    upgrade. Checking for this simple oversight can save hours of painstaking
    troubleshooting. 
    
    It is also important to make sure that the manager daemons (``ceph-mgr``)
    are running. Remember that typical cluster configurations provide one
    Manager (``ceph-mgr``) for each Monitor (``ceph-mon``).

    .. note:: In releases prior to v1.12.5, Rook will not run more than two
       managers.

#. **Make sure that you can reach the Monitor nodes.**

    In certain rare cases, ``iptables`` rules might be blocking access to
    Monitor nodes or TCP ports. These rules might be left over from earlier
    stress testing or rule development. To check for the presence of such
    rules, SSH into each Monitor node and use ``telnet`` or ``nc`` or a similar
    tool to attempt to connect to each of the other Monitor nodes on ports
    ``tcp/3300`` and ``tcp/6789``. 

#. **Make sure that the "ceph status" command runs and receives a reply from the cluster.**

    If the ``ceph status`` command receives a reply from the cluster, then the
    cluster is up and running. Monitors answer to a ``status`` request only if
    there is a formed quorum. Confirm that one or more ``mgr`` daemons are
    reported as running. In a cluster with no deficiencies, ``ceph status``
    will report that all ``mgr`` daemons are running.

    If the ``ceph status`` command does not receive a reply from the cluster,
    then there are probably not enough Monitors ``up`` to form a quorum. If the
    ``ceph -s`` command is run with no further options specified, it connects
    to an arbitrarily selected Monitor. In certain cases, however, it might be
    helpful to connect to a specific Monitor (or to several specific Monitors
    in sequence) by adding the ``-m`` flag to the command: for example, ``ceph
    status -m mymon1``.

#. **None of this worked. What now?**

    If the above solutions have not resolved your problems, you might find it
    helpful to examine each individual Monitor in turn. Even if no quorum has
    been formed, it is possible to contact each Monitor individually and
    request its status by using the ``ceph tell mon.ID mon_status`` command
    (here ``ID`` is the Monitor's identifier).

    Run the ``ceph tell mon.ID mon_status`` command for each Monitor in the
    cluster. For more on this command's output, see :ref:`Understanding
    mon_status
    <rados_troubleshoting_troubleshooting_mon_understanding_mon_status>`.

    There is also an alternative method for contacting each individual Monitor:
    SSH into each Monitor node and query the daemon's admin socket. See
    :ref:`Using the Monitor's Admin
    Socket<rados_troubleshoting_troubleshooting_mon_using_admin_socket>`.

.. _rados_troubleshoting_troubleshooting_mon_using_admin_socket:

Using the monitor's admin socket
================================

A monitor's admin socket allows you to interact directly with a specific daemon
by using a Unix socket file. This socket file is found in the monitor's ``run``
directory. 

The admin socket's default directory is ``/var/run/ceph/ceph-mon.ID.asok``. It
is possible to override the admin socket's default location. If the default
location has been overridden, then the admin socket will be elsewhere. This is
often the case when a cluster's daemons are deployed in containers. 

To find the directory of the admin socket, check either your ``ceph.conf`` for
an alternative path or run the following command:
    
.. prompt:: bash $

   ceph-conf --name mon.ID --show-config-value admin_socket

The admin socket is available for use only when the Monitor daemon is running.
Every time the Monitor is properly shut down, the admin socket is removed.  If
the Monitor is not running and yet the admin socket persists, it is likely that
the Monitor has been improperly shut down. If the Monitor is not running, it
will be impossible to use the admin socket, and the ``ceph`` command is likely
to return ``Error 111: Connection Refused``.

To access the admin socket, run a ``ceph tell`` command of the following form
(specifying the daemon that you are interested in):

.. prompt:: bash $

   ceph tell mon.<id> mon_status

This command passes a ``help`` command to the specified running Monitor daemon
``<id>`` via its admin socket. If you know the full path to the admin socket
file, this can be done more directly by running the following command:

.. prompt:: bash $

   ceph --admin-daemon <full_path_to_asok_file> <command>

Running ``ceph help`` shows all supported commands that are available through
the admin socket. See especially ``config get``, ``config show``, ``mon stat``,
and ``quorum_status``.

.. _rados_troubleshoting_troubleshooting_mon_understanding_mon_status:

Understanding mon_status
========================

The status of a Monitor (as reported by the ``ceph tell mon.X mon_status``
command) can be obtained via the admin socket. The ``ceph tell mon.X
mon_status`` command outputs a great deal of information about the monitor
(including the information found in the output of the ``quorum_status``
command).

.. note:: The command ``ceph tell mon.X mon_status`` is not meant to be input
   literally. The ``X`` portion of ``mon.X`` is meant to be replaced with a
   value specific to your Ceph cluster when you run the command.

To understand this command's output, let us consider the following example, in
which we see the output of ``ceph tell mon.c mon_status``::

  { "name": "c",
    "rank": 2,
    "state": "peon",
    "election_epoch": 38,
    "quorum": [
          1,
          2],
    "outside_quorum": [],
    "extra_probe_peers": [],
    "sync_provider": [],
    "monmap": { "epoch": 3,
        "fsid": "5c4e9d53-e2e1-478a-8061-f543f8be4cf8",
        "modified": "2013-10-30 04:12:01.945629",
        "created": "2013-10-29 14:14:41.914786",
        "mons": [
              { "rank": 0,
                "name": "a",
                "addr": "127.0.0.1:6789\/0"},
              { "rank": 1,
                "name": "b",
                "addr": "127.0.0.1:6790\/0"},
              { "rank": 2,
                "name": "c",
                "addr": "127.0.0.1:6795\/0"}]}}

This output reports that there are three monitors in the monmap (``a``, ``b``,
and ``c``), that quorum is formed by only two monitors, and that ``c`` is a
``peon``.

**Which monitor is out of quorum?**

  The answer is ``a`` (that is, ``mon.a``). ``mon.a`` is out of quorum.

**How do we know, in this example, that mon.a is out of quorum?**

  We know that ``mon.a`` is out of quorum because it has rank ``0``, and
  Monitors with rank ``0`` are by definition out of quorum.

  If we examine the ``quorum`` set, we can see that there are clearly two
  monitors in the set: ``1`` and ``2``. But these are not monitor names. They
  are monitor ranks, as established in the current ``monmap``. The ``quorum``
  set does not include the monitor that has rank ``0``, and according to the
  ``monmap`` that monitor is ``mon.a``.

**How are monitor ranks determined?**

  Monitor ranks are calculated (or recalculated) whenever monitors are added to
  or removed from the cluster. The calculation of ranks follows a simple rule:
  the **greater** the ``IP:PORT`` combination, the **lower** the rank. In this
  case, because ``127.0.0.1:6789`` (``mon.a``) is numerically less than the
  other two ``IP:PORT`` combinations (which are ``127.0.0.1:6790`` for "Monitor
  b" and ``127.0.0.1:6795`` for "Monitor c"), ``mon.a`` has the highest rank:
  namely, rank ``0``.
  

Most Common Monitor Issues
===========================

The Cluster Has Quorum but at Least One Monitor is Down
-------------------------------------------------------

When the cluster has quorum but at least one monitor is down, ``ceph health
detail`` returns a message similar to the following::

      $ ceph health detail
      [snip]
      mon.a (rank 0) addr 127.0.0.1:6789/0 is down (out of quorum)

**How do I troubleshoot a Ceph cluster that has quorum but also has at least one monitor down?**

  #. Make sure that ``mon.a`` is running.

  #. Make sure that you can connect to ``mon.a``'s node from the
     other Monitor nodes. Check the TCP ports as well. Check ``iptables`` and
     ``nf_conntrack`` on all nodes and make sure that you are not
     dropping/rejecting connections.

  If this initial troubleshooting doesn't solve your problem, then further
  investigation is necessary.

  First, check the problematic monitor's ``mon_status`` via the admin
  socket as explained in `Using the monitor's admin socket`_ and
  `Understanding mon_status`_.

  If the Monitor is out of the quorum, then its state will be one of the
  following: ``probing``, ``electing`` or ``synchronizing``. If the state of
  the Monitor is ``leader`` or ``peon``, then the Monitor believes itself to be
  in quorum but the rest of the cluster believes that it is not in quorum. It
  is possible that a Monitor that is in one of the ``probing``, ``electing``,
  or ``synchronizing`` states has entered the quorum during the process of
  troubleshooting. Check ``ceph status`` again to determine whether the Monitor
  has entered quorum during your troubleshooting. If the Monitor remains out of
  the quorum, then proceed with the investigations described in this section of
  the documentation.
  

**What does it mean when a Monitor's state is ``probing``?**

  If ``ceph health detail`` shows that a Monitor's state is
  ``probing``, then the Monitor is still looking for the other Monitors. Every
  Monitor remains in this state for some time when it is started. When a
  Monitor has connected to the other Monitors specified in the ``monmap``, it
  ceases to be in the ``probing`` state. The amount of time that a Monitor is
  in the ``probing`` state depends upon the parameters of the cluster of which
  it is a part. For example, when a Monitor is a part of a single-monitor
  cluster (never do this in production), the monitor passes through the probing
  state almost instantaneously. In a multi-monitor cluster, the Monitors stay
  in the ``probing`` state until they find enough monitors to form a quorum
  |---| this means that if two out of three Monitors in the cluster are
  ``down``, the one remaining Monitor stays in the ``probing``  state
  indefinitely until you bring one of the other monitors up.

  If quorum has been established, then the Monitor daemon should be able to
  find the other Monitors quickly, as long as they can be reached. If a Monitor
  is stuck in the ``probing`` state and you have exhausted the procedures above
  that describe the troubleshooting of communications between the Monitors,
  then it is possible that the problem Monitor is trying to reach the other
  Monitors at a wrong address. ``mon_status`` outputs the ``monmap`` that is
  known to the monitor: determine whether the other Monitors' locations as
  specified in the ``monmap`` match the locations of the Monitors in the
  network. If they do not, see :ref:`Recovering a Monitor's Broken monmap
  <rados_troubleshooting_troubleshooting_mon_recovering_broken_monmap>`. If
  the locations of the Monitors as specified in the ``monmap`` match the
  locations of the Monitors in the network, then the persistent ``probing``
  state could  be related to severe clock skews among the monitor nodes.  See
  `Clock Skews`_.  If the information in `Clock Skews`_ does not bring the
  Monitor out of the ``probing`` state, then prepare your system logs and ask
  the Ceph community for help. See `Preparing your logs`_ for information about
  the proper preparation of logs.


**What does it mean when a Monitor's state is ``electing``?**

  If ``ceph health detail`` shows that a Monitor's state is ``electing``, the
  monitor is in the middle of an election. Elections typically complete
  quickly, but sometimes the monitors can get stuck in what is known as an
  *election storm*. See :ref:`Monitor Elections <dev_mon_elections>` for more
  on monitor elections.
  
  The presence of election storm might indicate clock skew among the monitor
  nodes. See `Clock Skews`_ for more information. 
  
  If your clocks are properly synchronized, search the mailing lists and bug
  tracker for issues similar to your issue. The ``electing`` state is not
  likely to persist. In versions of Ceph after the release of Cuttlefish, there
  is no obvious reason other than clock skew that explains why an ``electing``
  state would persist.  
  
  It is possible to investigate the cause of a persistent ``electing`` state if
  you put the problematic Monitor into a ``down`` state while you investigate.
  This is possible only if there are enough surviving Monitors to form quorum. 

**What does it mean when a Monitor's state is ``synchronizing``?**

  If ``ceph health detail`` shows that the Monitor is ``synchronizing``, the
  monitor is catching up with the rest of the cluster so that it can join the
  quorum. The amount of time that it takes for the Monitor to synchronize with
  the rest of the quorum is a function of the size of the cluster's monitor
  store, the cluster's size, and the state of the cluster. Larger and degraded
  clusters generally keep Monitors in the ``synchronizing`` state longer than
  do smaller, new clusters.

  A Monitor that changes its state from ``synchronizing`` to ``electing`` and
  then back to ``synchronizing`` indicates a problem: the cluster state may be
  advancing (that is, generating new maps) too fast for the synchronization
  process to keep up with the pace of the creation of the new maps. This issue
  presented more frequently prior to the Cuttlefish release than it does in
  more recent releases, because the synchronization process has since been
  refactored and enhanced to avoid this dynamic. If you experience this in
  later versions, report the issue in the `Ceph bug tracker
  <https://tracker.ceph.com>`_. Prepare and provide logs to substantiate any
  bug you raise. See `Preparing your logs`_ for information about the proper
  preparation of logs.

**What does it mean when a Monitor's state is ``leader`` or ``peon``?**

  During normal Ceph operations when the cluster is in the ``HEALTH_OK`` state,
  one monitor in the Ceph cluster is in the ``leader`` state and the rest of
  the monitors are in the ``peon`` state. The state of a given monitor can be
  determined by examining the value of the state key returned by the command
  ``ceph tell <mon_name> mon_status``.

  If ``ceph health detail`` shows that the Monitor is in the ``leader`` state
  or in the ``peon`` state, it is likely that clock skew is present. Follow the
  instructions in `Clock Skews`_. If you have followed those instructions and
  ``ceph health detail`` still shows that the Monitor is in the ``leader``
  state or the ``peon`` state, report the issue in the `Ceph bug tracker
  <https://tracker.ceph.com>`_. If you raise an issue, provide logs to
  substantiate it. See `Preparing your logs`_ for information about the
  proper preparation of logs.

.. _rados_troubleshooting_troubleshooting_mon_recovering_broken_monmap:

Recovering a Monitor's Broken "monmap"
--------------------------------------

A monmap can be retrieved by using a command of the form ``ceph tell mon.c
mon_status``, as described in :ref:`Understanding mon_status
<rados_troubleshoting_troubleshooting_mon_understanding_mon_status>`.

Here is an example of a ``monmap``::

      epoch 3
      fsid 5c4e9d53-e2e1-478a-8061-f543f8be4cf8
      last_changed 2013-10-30 04:12:01.945629
      created 2013-10-29 14:14:41.914786
      0: 127.0.0.1:6789/0 mon.a
      1: 127.0.0.1:6790/0 mon.b
      2: 127.0.0.1:6795/0 mon.c

This ``monmap`` is in working order, but your ``monmap`` might not be in
working order. The ``monmap`` in a given node might be outdated because the
node was down for a long time, during which the cluster's Monitors changed.

There are two ways to update a Monitor's outdated ``monmap``: 

A. **Scrap the monitor and redeploy.**

    Do this only if you are certain that you will not lose the information kept
    by the Monitor that you scrap. Make sure that you have other Monitors in
    good condition, so that the new Monitor will be able to synchronize with
    the surviving Monitors. Remember that destroying a Monitor can lead to data
    loss if there are no other copies of the Monitor's contents. 

B. **Inject a monmap into the monitor.**

    It is possible to fix a Monitor that has an outdated ``monmap`` by
    retrieving an up-to-date ``monmap`` from surviving Monitors in the cluster
    and injecting it into the Monitor that has a corrupted or missing
    ``monmap``.

    Implement this solution by carrying out the following procedure:

    #. Retrieve the ``monmap`` in one of the two following ways:

       a. **IF THERE IS A QUORUM OF MONITORS:** 
       
          Retrieve the ``monmap`` from the quorum:

             .. prompt:: bash

                ceph mon getmap -o /tmp/monmap

       b. **IF THERE IS NO QUORUM OF MONITORS:** 
       
          Retrieve the ``monmap`` directly from a Monitor that has been stopped
          :

             .. prompt:: bash

                ceph-mon -i ID-FOO --extract-monmap /tmp/monmap

          In this example, the ID of the stopped Monitor is ``ID-FOO``.

    #. Stop the Monitor into which the ``monmap`` will be injected:

       .. prompt:: bash 

          service ceph -a stop mon.{mon-id}

    #. Inject the monmap into the stopped Monitor:

       .. prompt:: bash

          ceph-mon -i ID --inject-monmap /tmp/monmap

    #. Start the Monitor.

       .. warning:: Injecting a ``monmap`` into a Monitor  can cause serious
          problems. Injecting a ``monmap`` overwrites the latest existing
          ``monmap`` stored on the monitor.  Be careful!

Clock Skews
-----------

The Paxos consensus algorithm requires close time synchroniziation, which means
that clock skew among the monitors in the quorum can have a serious effect on
monitor operation. The resulting behavior can be puzzling. To avoid this issue,
run a clock synchronization tool on your monitor nodes: for example, use
``Chrony`` or the legacy ``ntpd`` utility. Configure each monitor nodes so that
the `iburst` option is in effect and so that each monitor has multiple peers,
including the following: 

* Each other
* Internal ``NTP`` servers
* Multiple external, public pool servers

.. note:: The ``iburst`` option sends a burst of eight packets instead of the
   usual single packet, and is used during the process of getting two peers
   into initial synchronization.

Furthermore, it is advisable to synchronize *all* nodes in your cluster against
internal and external servers, and perhaps even against your monitors. Run
``NTP`` servers on bare metal: VM-virtualized clocks are not suitable for
steady timekeeping. See `https://www.ntp.org <https://www.ntp.org>`_ for more
information about the Network Time Protocol (NTP). Your organization might
already have quality internal ``NTP`` servers available.  Sources for ``NTP``
server appliances include the following:

* Microsemi (formerly Symmetricom) `https://microsemi.com <https://www.microsemi.com/product-directory/3425-timing-synchronization>`_
* EndRun `https://endruntechnologies.com <https://endruntechnologies.com/products/ntp-time-servers>`_
* Netburner `https://www.netburner.com <https://www.netburner.com/products/network-time-server/pk70-ex-ntp-network-time-server>`_

Clock Skew Questions and Answers
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

**What's the maximum tolerated clock skew?**

  By default, monitors allow clocks to drift up to a maximum of 0.05 seconds
  (50 milliseconds).

**Can I increase the maximum tolerated clock skew?**

  Yes, but we strongly recommend against doing so. The maximum tolerated clock
  skew is configurable via the ``mon-clock-drift-allowed`` option, but it is
  almost certainly a bad idea to make changes to this option. The clock skew
  maximum is in place because clock-skewed monitors cannot be relied upon. The
  current default value has proven its worth at alerting the user before the
  monitors encounter serious problems. Changing this value might cause
  unforeseen effects on the stability of the monitors and overall cluster
  health.

**How do I know whether there is a clock skew?**

  The monitors will warn you via the cluster status ``HEALTH_WARN``. When clock
  skew is present, the ``ceph health detail`` and ``ceph status`` commands
  return an output resembling the following::

      mon.c addr 10.10.0.1:6789/0 clock skew 0.08235s > max 0.05s (latency 0.0045s)

  In this example, the monitor ``mon.c`` has been flagged as suffering from 
  clock skew.

  In Luminous and later releases, it is possible to check for a clock skew by
  running the ``ceph time-sync-status`` command. Note that the lead monitor
  typically has the numerically lowest IP address. It will always show ``0``:
  the reported offsets of other monitors are relative to the lead monitor, not
  to any external reference source.

**What should I do if there is a clock skew?**

  Synchronize your clocks. Using an NTP client might help. However, if you
  are already using an NTP client and you still encounter clock skew problems,
  determine whether the NTP server that you are using is remote to your network
  or instead hosted on your network. Hosting your own NTP servers tends to
  mitigate clock skew problems.


Client Can't Connect or Mount
-----------------------------

If a client can't connect to the cluster or mount, check your iptables. Some
operating-system install utilities add a ``REJECT`` rule to ``iptables``.
``iptables`` rules will reject all clients other than ``ssh`` that try to
connect to the host. If your monitor host's iptables have a ``REJECT`` rule in
place, clients that connect from a separate node will fail, and this will raise
a timeout error. Look for ``iptables`` rules that reject clients that are
trying to connect to Ceph daemons. For example::

    REJECT all -- anywhere anywhere reject-with icmp-host-prohibited

It might also be necessary to add rules to iptables on your Ceph hosts to
ensure that clients are able to access the TCP ports associated with your Ceph
monitors (default: port 6789) and Ceph OSDs (default: 6800 through 7568). For
example::

    iptables -A INPUT -m multiport -p tcp -s {ip-address}/{netmask} --dports 6789,6800:7568 -j ACCEPT


Monitor Store Failures
======================

Symptoms of store corruption
----------------------------

Ceph Monitors maintain the :term:`Cluster Map` in a key-value store. If
key-value store corruption causes a Monitor to fail, then the Monitor log might
contain one of the following error messages::

  Corruption: error in middle of record

or::

  Corruption: 1 missing files; e.g.: /var/lib/ceph/mon/mon.foo/store.db/1234567.ldb

Recovery using healthy monitor(s)
---------------------------------

If the cluster contains surviving Monitors, the corrupted Monitor can be
:ref:`replaced <adding-and-removing-monitors>` with a new Monitor. After the
new Monitor boots, it will synchronize with a healthy peer. After the new
Monitor is fully synchronized, it will be able to serve clients.

.. _mon-store-recovery-using-osds:

Recovery using OSDs
-------------------

Even if all monitors fail at the same time, it is possible to recover the
Monitor store by using information that is stored in OSDs. You are encouraged
to deploy at least three (and preferably five) Monitors in a Ceph cluster. In
such a deployment, complete Monitor failure is unlikely. However, unplanned
power loss in a data center whose disk settings or filesystem settings are
improperly configured could cause the underlying filesystem to fail and this
could kill all of the monitors. In such a case, data in the OSDs can be used to
recover the Monitors. The following is a script that can be used in such a case
to recover the Monitors:

.. code-block:: bash

  ms=/root/mon-store
  mkdir $ms
  
  # collect the cluster map from stopped OSDs
  for host in $hosts; do
    rsync -avz $ms/. user@$host:$ms.remote
    rm -rf $ms
    ssh user@$host <<EOF
      for osd in /var/lib/ceph/osd/ceph-*; do
        ceph-objectstore-tool --data-path \$osd --no-mon-config --op update-mon-db --mon-store-path $ms.remote
      done
  EOF
    rsync -avz user@$host:$ms.remote/. $ms
  done
  
  # rebuild the monitor store from the collected map, if the cluster does not
  # use cephx authentication, we can skip the following steps to update the
  # keyring with the caps, and there is no need to pass the "--keyring" option.
  # i.e. just use "ceph-monstore-tool $ms rebuild" instead
  ceph-authtool /path/to/admin.keyring -n mon. \
    --cap mon 'allow *'
  ceph-authtool /path/to/admin.keyring -n client.admin \
    --cap mon 'allow *' --cap osd 'allow *' --cap mds 'allow *'
  # add one or more ceph-mgr's key to the keyring. in this case, an encoded key
  # for mgr.x is added, you can find the encoded key in
  # /etc/ceph/${cluster}.${mgr_name}.keyring on the machine where ceph-mgr is
  # deployed
  ceph-authtool /path/to/admin.keyring --add-key 'AQDN8kBe9PLWARAAZwxXMr+n85SBYbSlLcZnMA==' -n mgr.x \
    --cap mon 'allow profile mgr' --cap osd 'allow *' --cap mds 'allow *'
  # If your monitors' ids are not sorted by ip address, please specify them in order.
  # For example. if mon 'a' is 10.0.0.3, mon 'b' is 10.0.0.2, and mon 'c' is  10.0.0.4,
  # please passing "--mon-ids b a c".
  # In addition, if your monitors' ids are not single characters like 'a', 'b', 'c', please
  # specify them in the command line by passing them as arguments of the "--mon-ids"
  # option. if you are not sure, please check your ceph.conf to see if there is any
  # sections named like '[mon.foo]'. don't pass the "--mon-ids" option, if you are
  # using DNS SRV for looking up monitors.
  ceph-monstore-tool $ms rebuild -- --keyring /path/to/admin.keyring --mon-ids alpha beta gamma
  
  # make a backup of the corrupted store.db just in case!  repeat for
  # all monitors.
  mv /var/lib/ceph/mon/mon.foo/store.db /var/lib/ceph/mon/mon.foo/store.db.corrupted

  # move rebuild store.db into place.  repeat for all monitors.
  mv $ms/store.db /var/lib/ceph/mon/mon.foo/store.db
  chown -R ceph:ceph /var/lib/ceph/mon/mon.foo/store.db

This script performs the following steps:

#. Collect the map from each OSD host.
#. Rebuild the store.
#. Fill the entities in the keyring file with appropriate capabilities.
#. Replace the corrupted store on ``mon.foo`` with the recovered copy.


Known limitations
~~~~~~~~~~~~~~~~~

The above recovery tool is unable to recover the following information:

- **Certain added keyrings**: All of the OSD keyrings added using the ``ceph
  auth add`` command are recovered from the OSD's copy, and the
  ``client.admin`` keyring is imported using ``ceph-monstore-tool``. However,
  the MDS keyrings and all other keyrings will be missing in the recovered
  Monitor store. It might be necessary to manually re-add them.

- **Creating pools**: If any RADOS pools were in the process of being created,
  that state is lost. The recovery tool operates on the assumption that all
  pools have already been created. If there are PGs that are stuck in the
  ``unknown`` state after the recovery for a partially created pool, you can
  force creation of the *empty* PG by running the ``ceph osd force-create-pg``
  command. This creates an *empty* PG, so take this action only if you are
  certain that the pool is empty.

- **MDS Maps**: The MDS maps are lost.

Everything Failed! Now What?
============================

Reaching out for help
---------------------

You can find help on IRC in #ceph and #ceph-devel on OFTC (server
irc.oftc.net), or at ``dev@ceph.io`` and ``ceph-users@lists.ceph.com``. Make
sure that you have prepared your logs and that you have them ready upon
request.

The upstream Ceph Slack workspace can be joined at this address:
https://ceph-storage.slack.com/

See https://ceph.io/en/community/connect/ for current (as of December 2023)
information on getting in contact with the upstream Ceph community.

Preparing your logs
-------------------

The default location for Monitor logs is ``/var/log/ceph/ceph-mon.FOO.log*``.
It is possible that the location of the Monitor logs has been changed from the
default. If the location of the Monitor logs has been changed from the default
location, find the location of the Monitor logs by running the following
command:

.. prompt:: bash

   ceph-conf --name mon.FOO --show-config-value log_file

The amount of information in the logs is determined by the debug levels in the
cluster's configuration files. If Ceph is using the default debug levels, then
your logs might be missing important information that would help the upstream
Ceph community address your issue.

Raise debug levels to make sure that your Monitor logs contain relevant
information. Here we are interested in information from the Monitors.  As with
other components, the Monitors have different parts that output their debug
information on different subsystems.

If you are an experienced Ceph troubleshooter, we recommend raising the debug
levels of the most relevant subsystems. This approach might not be easy for
beginners. In most cases, however, enough information to address the issue will
be logged if the following debug levels are entered::

      debug mon = 10
      debug ms = 1

Sometimes these debug levels do not yield enough information. In such cases,
members of the upstream Ceph community will ask you to make additional changes
to these or to other debug levels. In any case, it is better for us to receive
at least some useful information than to receive an empty log.


Do I need to restart a monitor to adjust debug levels?
------------------------------------------------------

No. It is not necessary to restart a Monitor when adjusting its debug levels. 

There are two different methods for adjusting debug levels. One method is used
when there is quorum. The other is used when there is no quorum. 

**Adjusting debug levels when there is a quorum**

  Either inject the debug option into the specific monitor that needs to 
  be debugged::

        ceph tell mon.FOO config set debug_mon 10/10

  Or inject it into all monitors at once::

        ceph tell mon.* config set debug_mon 10/10


**Adjusting debug levels when there is no quorum**

  Use the admin socket of the specific monitor that needs to be debugged
  and directly adjust the monitor's configuration options::

      ceph daemon mon.FOO config set debug_mon 10/10

**Returning debug levels to their default values**

To return the debug levels to their default values, run the above commands
using the debug level ``1/10`` rather than the debug level ``10/10``. To check
a Monitor's current values, use the admin socket and run either of the
following commands:

  .. prompt:: bash

     ceph daemon mon.FOO config show

or:

  .. prompt:: bash

     ceph daemon mon.FOO config get 'OPTION_NAME'



I Reproduced the problem with appropriate debug levels. Now what?
-----------------------------------------------------------------

Send the upstream Ceph community only the portions of your logs that are
relevant to your Monitor problems. Because it might not be easy for you to
determine which portions are relevant, the upstream Ceph community accepts
complete and unabridged logs. But don't send logs containing hundreds of
thousands of lines with no additional clarifying information. One common-sense
way to help the Ceph community help you is to write down the current time and
date when you are reproducing the problem and then extract portions of your
logs based on that information.

Contact the upstream Ceph community on the mailing lists or IRC or Slack, or by
filing a new issue on the `tracker`_.

.. _tracker: http://tracker.ceph.com/projects/ceph/issues/new

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