ceph/doc/rados/troubleshooting/troubleshooting-mon.rst

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

.. index:: monitor, high availability

When a cluster encounters monitor-related troubles there's a tendency to
panic, and some times with good reason. You should keep in mind that losing
a monitor, or a bunch of them, don't necessarily mean that your cluster is
down, as long as a majority is up, running and with a formed quorum.
Regardless of how bad the situation is, the first thing you should do is to
calm down, take a breath and try answering our initial troubleshooting script.


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


**Are the monitors running?**

  First of all, we need to make sure the monitors are running. You would be
  amazed by how often people forget to run the monitors, or restart them after
  an upgrade. There's no shame in that, but let's try not losing a couple of
  hours chasing an issue that is not there.

**Are you able to connect to the monitor's servers?**

  Doesn't happen often, but sometimes people do have ``iptables`` rules that
  block accesses to monitor servers or monitor ports. Usually leftovers from
  monitor stress-testing that were forgotten at some point. Try ssh'ing into
  the server and, if that succeeds, try connecting to the monitor's port
  using you tool of choice (telnet, nc,...).

**Does ceph -s run and obtain a reply from the cluster?**

  If the answer is yes then your cluster is up and running.  One thing you
  can take for granted is that the monitors will only answer to a ``status``
  request if there is a formed quorum.

  If ``ceph -s`` blocked however, without obtaining a reply from the cluster
  or showing a lot of ``fault`` messages, then it is likely that your monitors
  are either down completely or just a portion is up -- a portion that is not
  enough to form a quorum (keep in mind that a quorum if formed by a majority
  of monitors).

**What if ceph -s doesn't finish?**

  If you haven't gone through all the steps so far, please go back and do.

  For those running on Emperor 0.72-rc1 and forward, you will be able to
  contact each monitor individually asking them for their status, regardless
  of a quorum being formed. This an be achieved using ``ceph ping mon.ID``,
  ID being the monitor's identifier. You should perform this for each monitor
  in the cluster. In section `Understanding mon_status`_ we will explain how
  to interpret the output of this command.

  For the rest of you who don't tread on the bleeding edge, you will need to
  ssh into the server and use the monitor's admin socket. Please jump to
  `Using the monitor's admin socket`_.

For other specific issues, keep on reading.


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

The admin socket allows you to interact with a given daemon directly using a
Unix socket file. This file can be found in your monitor's ``run`` directory.
By default, the admin socket will be kept in ``/var/run/ceph/ceph-mon.ID.asok``
but this can vary if you defined it otherwise. If you don't find it there,
please check your ``ceph.conf`` for an alternative path or run::

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

Please bear in mind that the admin socket will only be available while the
monitor is running. When the monitor is properly shutdown, the admin socket
will be removed. If however the monitor is not running and the admin socket
still persists, it is likely that the monitor was improperly shutdown.
Regardless, if the monitor is not running, you will not be able to use the
admin socket, with ``ceph`` likely returning ``Error 111: Connection Refused``.

Accessing the admin socket is as simple as telling the ``ceph`` tool to use
the ``asok`` file.  In pre-Dumpling Ceph, this can be achieved by::

  ceph --admin-daemon /var/run/ceph/ceph-mon.<id>.asok <command>

while in Dumpling and beyond you can use the alternate (and recommended)
format::

  ceph daemon mon.<id> <command>

Using ``help`` as the command to the ``ceph`` tool will show you the
supported commands available through the admin socket. Please take a look
at ``config get``, ``config show``, ``mon_status`` and ``quorum_status``,
as those can be enlightening when troubleshooting a monitor.


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

``mon_status`` can be obtained through the ``ceph`` tool when you have
a formed quorum, or via the admin socket if you don't. This command will
output a multitude of information about the monitor, including the same
output you would get with ``quorum_status``.

Take the following example of ``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"}]}}

A couple of things are obvious: we have three monitors in the monmap (*a*, *b*
and *c*), the quorum is formed by only two monitors, and *c* is in the quorum
as a *peon*.

Which monitor is out of the quorum?

  The answer would be **a**.

Why?

  Take a look at the ``quorum`` set. We have two monitors in this set: *1*
  and *2*. These are not monitor names. These are monitor ranks, as established
  in the current monmap. We are missing the monitor with rank 0, and according
  to the monmap that would be ``mon.a``.

By the way, how are ranks established?

  Ranks are (re)calculated whenever you add or remove monitors and follow a
  simple rule: the **greater** the ``IP:PORT`` combination, the **lower** the
  rank is. In this case, considering that ``127.0.0.1:6789`` is lower than all
  the remaining ``IP:PORT`` combinations, ``mon.a`` has rank 0.

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

Have Quorum but at least one Monitor is down
---------------------------------------------

When this happens, depending on the version of Ceph you are running,
you should be seeing something similar to::

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

How to troubleshoot this?

  First, make sure ``mon.a`` is running.

  Second, make sure you are able to connect to ``mon.a``'s server from the
  other monitors' servers. Check the ports as well. Check ``iptables`` on
  all your monitor nodes and make sure you're not dropping/rejecting
  connections.

  If this initial troubleshooting doesn't solve your problems, then it's
  time to go deeper.

  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`_.

  Considering the monitor is out of the quorum, its state should be one of
  ``probing``, ``electing`` or ``synchronizing``. If it happens to be either
  ``leader`` or ``peon``, then the monitor believes to be in quorum, while
  the remaining cluster is sure it is not; or maybe it got into the quorum
  while we were troubleshooting the monitor, so check you ``ceph -s`` again
  just to make sure. Proceed if the monitor is not yet in the quorum.

What if the state is ``probing``?

  This means the monitor is still looking for the other monitors. Every time
  you start a monitor, the monitor will stay in this state for some time
  while trying to find the rest of the monitors specified in the ``monmap``.
  The time a monitor will spend in this state can vary. For instance, when on
  a single-monitor cluster, the monitor will pass through the probing state
  almost instantaneously, since there are no other monitors around. On a
  multi-monitor cluster, the monitors will stay in this state until they
  find enough monitors to form a quorum -- this means that if you have 2 out
  of 3 monitors down, the one remaining monitor will stay in this state
  indefinitively until you bring one of the other monitors up.

  If you have a quorum, however, the monitor should be able to find the
  remaining monitors pretty fast, as long as they can be reached. If your
  monitor is stuck probing and you've gone through with all the communication
  troubleshooting, then there is a fair chance that the monitor is trying
  to reach the other monitors on a wrong address. ``mon_status`` outputs the
  ``monmap`` known to the monitor: check if the other monitor's locations
  match reality. If they don't, jump to
  `Recovering a Monitor's Broken monmap`_; if they do, then it may be related
  to severe clock skews amongst the monitor nodes and you should refer to
  `Clock Skews`_ first, but if that doesn't solve your problem then it is
  the time to prepare some logs and reach out to the community (please refer
  to `Preparing your logs`_ on how to best prepare your logs).


What if state is ``electing``?

  This means the monitor is in the middle of an election. These should be
  fast to complete, but at times the monitors can get stuck electing. This
  is usually a sign of a clock skew among the monitor nodes; jump to
  `Clock Skews`_ for more infos on that. If all your clocks are properly
  synchronized, it is best if you prepare some logs and reach out to the
  community. This is not a state that is likely to persist and aside from
  (*really*) old bugs there isn't an obvious reason besides clock skews on
  why this would happen.

What if state is ``synchronizing``?

  This means the monitor is synchronizing with the rest of the cluster in
  order to join the quorum. The synchronization process is as faster as
  smaller your monitor store is, so if you have a big store it may
  take a while. Don't worry, it should be finished soon enough.

  However, if you notice that the monitor jumps from ``synchronizing`` to
  ``electing`` and then back to ``synchronizing``, then you do have a
  problem: the cluster state is advancing (i.e., generating new maps) way
  too fast for the synchronization process to keep up. This used to be a
  thing in early Cuttlefish, but since then the synchronization process was
  quite refactored and enhanced to avoid just this sort of behavior. If this
  happens in later versions let us know. And bring some logs
  (see `Preparing your logs`_).

What if state is ``leader`` or ``peon``?

  This should not happen. There is a chance this might happen however, and
  it has a lot to do with clock skews -- see `Clock Skews`_. If you're not
  suffering from clock skews, then please prepare your logs (see
  `Preparing your logs`_) and reach out to us.


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

This is how a ``monmap`` usually looks like, depending on the number of
monitors::


      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 may not be what you have however. For instance, in some versions of
early Cuttlefish there was this one bug that could cause your ``monmap``
to be nullified.  Completely filled with zeros. This means that not even
``monmaptool`` would be able to read it because it would find it hard to
make sense of only-zeros. Some other times, you may end up with a monitor
with a severely outdated monmap, thus being unable to find the remaining
monitors (e.g., say ``mon.c`` is down; you add a new monitor ``mon.d``,
then remove ``mon.a``, then add a new monitor ``mon.e`` and remove
``mon.b``; you will end up with a totally different monmap from the one
``mon.c`` knows).

In this sort of situations, you have two possible solutions:

Scrap the monitor and create a new one

  You should only take this route if you are positive that you won't
  lose the information kept by that monitor; that you have other monitors
  and that they are running just fine so that your new monitor is able
  to synchronize from the remaining monitors. Keep in mind that destroying
  a monitor, if there are no other copies of its contents, may lead to
  loss of data.

Inject a monmap into the monitor

  Usually the safest path. You should grab the monmap from the remaining
  monitors and inject it into the monitor with the corrupted/lost monmap.

  These are the basic steps:

  1. Is there a formed quorum? If so, grab the monmap from the quorum::

      $ ceph mon getmap -o /tmp/monmap

  2. No quorum? Grab the monmap directly from another monitor (this
     assumes the monitor you're grabbing the monmap from has id ID-FOO
     and has been stopped)::

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

  3. Stop the monitor you're going to inject the monmap into.

  4. Inject the monmap::

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

  5. Start the monitor

  Please keep in mind that the ability to inject monmaps is a powerful
  feature that can cause havoc with your monitors if misused as it will
  overwrite the latest, existing monmap kept by the monitor.


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

Monitors can be severely affected by significant clock skews across the
monitor nodes. This usually translates into weird behavior with no obvious
cause. To avoid such issues, you should run a clock synchronization tool
on your monitor nodes.


What's the maximum tolerated clock skew?

  By default the monitors will allow clocks to drift up to ``0.05 seconds``.


Can I increase the maximum tolerated clock skew?

  This value is configurable via the ``mon-clock-drift-allowed`` option, and
  although you *CAN* it doesn't mean you *SHOULD*. The clock skew mechanism
  is in place because clock skewed monitor may not properly behave. We, as
  developers and QA afficcionados, are comfortable with the current default
  value, as it will alert the user before the monitors get out hand. Changing
  this value without testing it first may cause unforeseen effects on the
  stability of the monitors and overall cluster healthiness, although there is
  no risk of dataloss.


How do I know there's a clock skew?

  The monitors will warn you in the form of a ``HEALTH_WARN``. ``ceph health
  detail`` should show something in the form of::

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

  That means that ``mon.c`` has been flagged as suffering from a clock skew.


What should I do if there's a clock skew?

  Synchronize your clocks. Running an NTP client may help. If you are already
  using one and you hit this sort of issues, check if you are using some NTP
  server remote to your network and consider hosting your own NTP server on
  your network.  This last option tends to reduce the amount of issues with
  monitor clock skews.


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

Check your IP tables. Some OS install utilities add a ``REJECT`` rule to
``iptables``. The rule rejects all clients trying to connect to the host except
for ``ssh``. If your monitor host's IP tables have such a ``REJECT`` rule in
place, clients connecting from a separate node will fail to mount with a timeout
error. You need to address ``iptables`` rules that reject clients trying to
connect to Ceph daemons.  For example, you would need to address rules that look
like this appropriately::

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

You may also need to add rules to IP tables on your Ceph hosts to ensure
that clients can access the ports associated with your Ceph monitors (i.e., port
6789 by default) and Ceph OSDs (i.e., 6800 through 7300 by default). For
example::

	iptables -A INPUT -m multiport -p tcp -s {ip-address}/{netmask} --dports 6789,6800:7300 -j ACCEPT


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

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

You can find us on IRC at #ceph and #ceph-devel at OFTC (server irc.oftc.net)
and on ``ceph-devel@vger.kernel.org`` and ``ceph-users@lists.ceph.com``. Make
sure you have grabbed your logs and have them ready if someone asks: the faster
the interaction and lower the latency in response, the better chances everyone's
time is optimized.


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

Monitor logs are, by default, kept in ``/var/log/ceph/ceph-mon.FOO.log*``. We
may want them. However, your logs may not have the necessary information. If
you don't find your monitor logs at their default location, you can check
where they should be by running::

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

The amount of information in the logs are subject to the debug levels being
enforced by your configuration files. If you have not enforced a specific
debug level then Ceph is using the default levels and your logs may not
contain important information to track down you issue.
A first step in getting relevant information into your logs will be to raise
debug levels. In this case we will be interested in the information from the
monitor.
Similarly to what happens on other components, different parts of the monitor
will output their debug information on different subsystems.

You will have to raise the debug levels of those subsystems more closely
related to your issue. This may not be an easy task for someone unfamiliar
with troubleshooting Ceph. For most situations, setting the following options
on your monitors will be enough to pinpoint a potential source of the issue::

      debug mon = 10
      debug ms = 1

If we find that these debug levels are not enough, there's a chance we may
ask you to raise them or even define other debug subsystems to obtain infos
from -- but at least we started off with some useful information, instead
of a massively empty log without much to go on with.

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

No. You may do it in one of two ways:

You have quorum

  Either inject the debug option into the monitor you want to debug::

        ceph tell mon.FOO injectargs --debug_mon 10/10

  or into all monitors at once::

        ceph tell mon.* injectargs --debug_mon 10/10

No quourm

  Use the monitor's admin socket and directly adjust the configuration
  options::

      ceph daemon mon.FOO config set debug_mon 10/10


Going back to default values is as easy as rerunning the above commands
using the debug level ``1/10`` instead.  You can check your current
values using the admin socket and the following commands::

      ceph daemon mon.FOO config show

or::

      ceph daemon mon.FOO config get 'OPTION_NAME'


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

Ideally you would send us only the relevant portions of your logs.
We realise that figuring out the corresponding portion may not be the
easiest of tasks. Therefore, we won't hold it to you if you provide the
full log, but common sense should be employed. If your log has hundreds
of thousands of lines, it may get tricky to go through the whole thing,
specially if we are not aware at which point, whatever your issue is,
happened. For instance, when reproducing, keep in mind to write down
current time and date and to extract the relevant portions of your logs
based on that.

Finally, you should reach out to us on the mailing lists, on IRC or file
a new issue on the `tracker`_.

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