ceph/doc/cephfs/troubleshooting.rst

=================
 Troubleshooting
=================

Slow/stuck operations
=====================

If you are experiencing apparent hung operations, the first task is to identify
where the problem is occurring: in the client, the MDS, or the network connecting
them. Start by looking to see if either side has stuck operations
(:ref:`slow_requests`, below), and narrow it down from there.

We can get hints about what's going on by dumping the MDS cache ::

  ceph daemon mds.<name> dump cache /tmp/dump.txt

.. note:: The file `dump.txt` is on the machine executing the MDS and for systemd
	  controlled MDS services, this is in a tmpfs in the MDS container.
	  Use `nsenter(1)` to locate `dump.txt` or specify another system-wide path.

If high logging levels are set on the MDS, that will almost certainly hold the
information we need to diagnose and solve the issue.

RADOS Health
============

If part of the CephFS metadata or data pools is unavailable and CephFS is not
responding, it is probably because RADOS itself is unhealthy. Resolve those
problems first (:doc:`../../rados/troubleshooting/index`).

The MDS
=======

If an operation is hung inside the MDS, it will eventually show up in ``ceph health``,
identifying "slow requests are blocked". It may also identify clients as
"failing to respond" or misbehaving in other ways. If the MDS identifies
specific clients as misbehaving, you should investigate why they are doing so.

Generally it will be the result of

#. Overloading the system (if you have extra RAM, increase the "mds cache size"
   config from its default 100000; having a larger active file set than your MDS
   cache is the #1 cause of this!).

#. Running an older (misbehaving) client.

#. Underlying RADOS issues.

Otherwise, you have probably discovered a new bug and should report it to
the developers!

.. _slow_requests:

Slow requests (MDS)
-------------------
You can list current operations via the admin socket by running::

  ceph daemon mds.<name> dump_ops_in_flight

from the MDS host. Identify the stuck commands and examine why they are stuck.
Usually the last "event" will have been an attempt to gather locks, or sending
the operation off to the MDS log. If it is waiting on the OSDs, fix them. If
operations are stuck on a specific inode, you probably have a client holding
caps which prevent others from using it, either because the client is trying
to flush out dirty data or because you have encountered a bug in CephFS'
distributed file lock code (the file "capabilities" ["caps"] system).

If it's a result of a bug in the capabilities code, restarting the MDS
is likely to resolve the problem.

If there are no slow requests reported on the MDS, and it is not reporting
that clients are misbehaving, either the client has a problem or its
requests are not reaching the MDS.

.. _ceph_fuse_debugging:

ceph-fuse debugging
===================

ceph-fuse also supports ``dump_ops_in_flight``. See if it has any and where they are
stuck.

Debug output
------------

To get more debugging information from ceph-fuse, try running in the foreground
with logging to the console (``-d``) and enabling client debug
(``--debug-client=20``), enabling prints for each message sent
(``--debug-ms=1``).

If you suspect a potential monitor issue, enable monitor debugging as well
(``--debug-monc=20``).

.. _kernel_mount_debugging:

Kernel mount debugging
======================

If there is an issue with the kernel client, the most important thing is
figuring out whether the problem is with the kernel client or the MDS. Generally,
this is easy to work out. If the kernel client broke directly, there will be
output in ``dmesg``. Collect it and any inappropriate kernel state.

Slow requests
-------------

Unfortunately the kernel client does not support the admin socket, but it has
similar (if limited) interfaces if your kernel has debugfs enabled. There
will be a folder in ``sys/kernel/debug/ceph/``, and that folder (whose name will
look something like ``28f7427e-5558-4ffd-ae1a-51ec3042759a.client25386880``)
will contain a variety of files that output interesting output when you ``cat``
them. These files are described below; the most interesting when debugging
slow requests are probably the ``mdsc`` and ``osdc`` files.

* bdi: BDI info about the Ceph system (blocks dirtied, written, etc)
* caps: counts of file "caps" structures in-memory and used
* client_options: dumps the options provided to the CephFS mount
* dentry_lru: Dumps the CephFS dentries currently in-memory
* mdsc: Dumps current requests to the MDS
* mdsmap: Dumps the current MDSMap epoch and MDSes
* mds_sessions: Dumps the current sessions to MDSes
* monc: Dumps the current maps from the monitor, and any "subscriptions" held
* monmap: Dumps the current monitor map epoch and monitors
* osdc: Dumps the current ops in-flight to OSDs (ie, file data IO)
* osdmap: Dumps the current OSDMap epoch, pools, and OSDs

If there are no stuck requests but you have file IO which is not progressing,
you might have a...

Disconnected+Remounted FS
=========================
Because CephFS has a "consistent cache", if your network connection is
disrupted for a long enough time, the client will be forcibly
disconnected from the system. At this point, the kernel client is in
a bind: it cannot safely write back dirty data, and many applications
do not handle IO errors correctly on close().
At the moment, the kernel client will remount the FS, but outstanding file system
IO may or may not be satisfied. In these cases, you may need to reboot your
client system.

You can identify you are in this situation if dmesg/kern.log report something like::

   Jul 20 08:14:38 teuthology kernel: [3677601.123718] ceph: mds0 closed our session
   Jul 20 08:14:38 teuthology kernel: [3677601.128019] ceph: mds0 reconnect start
   Jul 20 08:14:39 teuthology kernel: [3677602.093378] ceph: mds0 reconnect denied
   Jul 20 08:14:39 teuthology kernel: [3677602.098525] ceph:  dropping dirty+flushing Fw state for ffff8802dc150518 1099935956631
   Jul 20 08:14:39 teuthology kernel: [3677602.107145] ceph:  dropping dirty+flushing Fw state for ffff8801008e8518 1099935946707
   Jul 20 08:14:39 teuthology kernel: [3677602.196747] libceph: mds0 172.21.5.114:6812 socket closed (con state OPEN)
   Jul 20 08:14:40 teuthology kernel: [3677603.126214] libceph: mds0 172.21.5.114:6812 connection reset
   Jul 20 08:14:40 teuthology kernel: [3677603.132176] libceph: reset on mds0

This is an area of ongoing work to improve the behavior. Kernels will soon
be reliably issuing error codes to in-progress IO, although your application(s)
may not deal with them well. In the longer-term, we hope to allow reconnect
and reclaim of data in cases where it won't violate POSIX semantics (generally,
data which hasn't been accessed or modified by other clients).

Mounting
========

Mount 5 Error
-------------

A mount 5 error typically occurs if a MDS server is laggy or if it crashed.
Ensure at least one MDS is up and running, and the cluster is ``active +
healthy``. 

Mount 12 Error
--------------

A mount 12 error with ``cannot allocate memory`` usually occurs if you  have a
version mismatch between the :term:`Ceph Client` version and the :term:`Ceph
Storage Cluster` version. Check the versions using::

	ceph -v
	
If the Ceph Client is behind the Ceph cluster, try to upgrade it::

	sudo apt-get update && sudo apt-get install ceph-common 

You may need to uninstall, autoclean and autoremove ``ceph-common`` 
and then reinstall it so that you have the latest version.

Dynamic Debugging
=================

You can enable dynamic debug against the CephFS module.

Please see: https://github.com/ceph/ceph/blob/master/src/script/kcon_all.sh