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

======================
 Troubleshooting OSDs
======================

Before troubleshooting your OSDs, check your monitors and network first. If
you execute ``ceph health`` or ``ceph -s`` on the command line and Ceph returns
a health status, the return of a status means that the monitors have a quorum.
If you don't have a monitor quorum or if there are errors with the monitor
status, address the monitor issues first. Check your networks to ensure they
are running properly, because networks may have a significant impact on OSD
operation and performance.



Obtaining Data About OSDs
=========================

A good first step in troubleshooting your OSDs is to obtain information in
addition to the information you collected while `monitoring your OSDs`_
(e.g., ``ceph osd tree``).


Ceph Logs
---------

If you haven't changed the default path, you can find Ceph log files at
``/var/log/ceph``::

	ls /var/log/ceph

If you don't get enough log detail, you can change your logging level.  See
`Logging and Debugging`_ for details to ensure that Ceph performs adequately
under high logging volume.


Admin Socket
------------

Use the admin socket tool to retrieve runtime information. For details, list
the sockets for your Ceph processes::

	ls /var/run/ceph

Then, execute the following, replacing ``{socket-name}`` with an actual
socket name to show the list of available options::

	ceph --admin-daemon /var/run/ceph/{socket-name} help

The admin socket, among other things, allows you to:

- List your configuration at runtime
- Dump historic operations
- Dump the operation priority queue state
- Dump operations in flight
- Dump perfcounters


Display Freespace
-----------------

Filesystem issues may arise. To display your filesystem's free space, execute
``df``. ::

	df -h

Execute ``df --help`` for additional usage.


I/O Statistics
--------------

Use `iostat`_ to identify I/O-related issues. ::

	iostat -x


Diagnostic Messages
-------------------

To retrieve diagnostic messages, use ``dmesg`` with ``less``, ``more``, ``grep``
or ``tail``.  For example::

	dmesg | grep scsi


Stopping w/out Rebalancing
==========================

Periodically, you may need to perform maintenance on a subset of your cluster,
or resolve a problem that affects a failure domain (e.g., a rack). If you do not
want CRUSH to automatically rebalance the cluster as you stop OSDs for
maintenance, set the cluster to ``noout`` first::

	ceph osd set noout

Once the cluster is set to ``noout``, you can begin stopping the OSDs within the
failure domain that requires maintenance work. ::

	ceph osd stop osd.{num}

.. note:: Placement groups within the OSDs you stop will become ``degraded``
   while you are addressing issues with within the failure domain.

Once you have completed your maintenance, restart the OSDs. ::

	ceph osd start osd.{num}

Finally, you must unset the cluster from ``noout``. ::

	ceph osd unset noout



.. _osd-not-running:

OSD Not Running
===============

Under normal circumstances, simply restarting the ``ceph-osd`` daemon will
allow it to rejoin the cluster and recover.

An OSD Won't Start
------------------

If you start your cluster and an OSD won't start, check the following:

- **Configuration File:** If you were not able to get OSDs running from
  a new installation, check your configuration file to ensure it conforms
  (e.g., ``host`` not ``hostname``, etc.).

- **Check Paths:** Check the paths in your configuration, and the actual
  paths themselves for data and journals. If you separate the OSD data from
  the journal data and there are errors in your configuration file or in the
  actual mounts, you may have trouble starting OSDs. If you want to store the
  journal on a block device, you should partition your journal disk and assign
  one partition per OSD.

- **Kernel Version:** Identify the kernel version and distribution you
  are using. Ceph uses some third party tools by default, which may be
  buggy or may conflict with certain distributions and/or kernel
  versions (e.g., Google perftools). Check the `OS recommendations`_
  to ensure you have addressed any issues related to your kernel.

- **Segment Fault:** If there is a segment fault, turn your logging up
  (if it isn't already), and try again. If it segment faults again,
  contact the ceph-devel email list and provide your Ceph configuration
  file, your monitor output and the contents of your log file(s).

If you cannot resolve the issue and the email list isn't helpful, you may
contact `Inktank`_ for support.


An OSD Failed
-------------

When a ``ceph-osd`` process dies, the monitor will learn about the failure
from surviving ``ceph-osd`` daemons and report it via the ``ceph health``
command::

	ceph health
	HEALTH_WARN 1/3 in osds are down

Specifically, you will get a warning whenever there are ``ceph-osd``
processes that are marked ``in`` and ``down``.  You can identify which
``ceph-osds`` are ``down`` with::

	ceph health detail
	HEALTH_WARN 1/3 in osds are down
	osd.0 is down since epoch 23, last address 192.168.106.220:6800/11080

If there is a disk
failure or other fault preventing ``ceph-osd`` from functioning or
restarting, an error message should be present in its log file in
``/var/log/ceph``.

If the daemon stopped because of a heartbeat failure, the underlying
kernel file system may be unresponsive. Check ``dmesg`` output for disk
or other kernel errors.

If the problem is a software error (failed assertion or other
unexpected error), it should be reported to the `ceph-devel`_ email list.


No Free Drive Space
-------------------

Ceph prevents you from writing to a full OSD so that you don't lose data.
In an operational cluster, you should receive a warning when your cluster
is getting near its full ratio. The ``mon osd full ratio`` defaults to
``0.95``, or 95% of capacity before it stops clients from writing data.
The ``mon osd nearfull ratio`` defaults to ``0.85``, or 85% of capacity
when it generates a health warning.

Full cluster issues usually arise when testing how Ceph handles an OSD
failure on a small cluster. When one node has a high percentage of the
cluster's data, the cluster can easily eclipse its nearfull and full ratio
immediately. If you are testing how Ceph reacts to OSD failures on a small
cluster, you should leave ample free disk space and consider temporarily
lowering the ``mon osd full ratio`` and ``mon osd nearfull ratio``.

Full ``ceph-osds`` will be reported by ``ceph health``::

	ceph health
	HEALTH_WARN 1 nearfull osds
	osd.2 is near full at 85%

Or::

	ceph health
	HEALTH_ERR 1 nearfull osds, 1 full osds
	osd.2 is near full at 85%
	osd.3 is full at 97%

The best way to deal with a full cluster is to add new ``ceph-osds``, allowing
the cluster to redistribute data to the newly available storage.

If you cannot start an OSD because it is full, you may delete some data by deleting
some placement group directories in the full OSD.

.. important:: If you choose to delete a placement group directory on a full OSD,
   **DO NOT** delete the same placement group directory on another full OSD, or
   **YOU MAY LOSE DATA**. You **MUST** maintain at least one copy of your data on
   at least one OSD.

See `Monitor Config Reference`_ for additional details.


OSDs are Slow/Unresponsive
==========================

A commonly recurring issue involves slow or unresponsive OSDs. Ensure that you
have eliminated other troubleshooting possibilities before delving into OSD
performance issues. For example, ensure that your network(s) is working properly
and your OSDs are running. Check to see if OSDs are throttling recovery traffic.

.. tip:: Newer versions of Ceph provide better recovery handling by preventing
   recovering OSDs from using up system resources so that ``up`` and ``in``
   OSDs aren't available or are otherwise slow.


Networking Issues
-----------------

Ceph is a distributed storage system, so it  depends upon networks to peer with
OSDs, replicate objects, recover from faults and check heartbeats. Networking
issues can cause OSD latency and flapping OSDs. See `Flapping OSDs`_ for
details.

Ensure that Ceph processes and Ceph-dependent processes are connected and/or
listening. ::

	netstat -a | grep ceph
	netstat -l | grep ceph
	sudo netstat -p | grep ceph

Check network statistics. ::

	netstat -s


Drive Configuration
-------------------

A storage drive should only support one OSD. Sequential read and sequential
write throughput can bottleneck if other processes share the drive, including
journals, operating systems, monitors, other OSDs and non-Ceph processes.

Ceph acknowledges writes *after* journaling, so fast SSDs are an attractive
option to accelerate the response time--particularly when using the ``ext4`` or
XFS filesystems. By contrast, the ``btrfs`` filesystem can write and journal
simultaneously.

.. note:: Partitioning a drive does not change its total throughput or
   sequential read/write limits. Running a journal in a separate partition
   may help, but you should prefer a separate physical drive.


Bad Sectors / Fragmented Disk
-----------------------------

Check your disks for bad sectors and fragmentation. This can cause total throughput
to drop substantially.


Co-resident Monitors/OSDs
-------------------------

Monitors are generally light-weight processes, but they do lots of ``fsync()``,
which can interfere with other workloads, particularly if monitors run on the
same drive as your OSDs. Additionally, if you run monitors on the same host as
the OSDs, you may incur performance issues related to:

- Running an older kernel (pre-3.0)
- Running Argonaut with an old ``glibc``
- Running a kernel with no syncfs(2) syscall.

In these cases, multiple OSDs running on the same host can drag each other down
by doing lots of commits. That often leads to the bursty writes.


Co-resident Processes
---------------------

Spinning up co-resident processes such as a cloud-based solution, virtual
machines and other applications that write data to Ceph while operating on the
same hardware as OSDs can introduce significant OSD latency. Generally, we
recommend optimizing a host for use with Ceph and using other hosts for other
processes. The practice of separating Ceph operations from other applications
may help improve performance and may streamline troubleshooting and maintenance.


Logging Levels
--------------

If you turned logging levels up to track an issue and then forgot to turn
logging levels back down, the OSD may be putting a lot of logs onto the disk. If
you intend to keep logging levels high, you may consider mounting a drive to the
default path for logging (i.e., ``/var/log/ceph/$cluster-$name.log``).


Recovery Throttling
-------------------

Depending upon your configuration, Ceph may reduce recovery rates to maintain
performance or it may increase recovery rates to the point that recovery
impacts OSD performance. Check to see if the OSD is recovering.


Kernel Version
--------------

Check the kernel version you are running. Older kernels may not receive
new backports that Ceph depends upon for better performance.


Kernel Issues with SyncFS
-------------------------

Try running one OSD per host to see if performance improves. Old kernels
might not have a recent enough version of ``glibc`` to support ``syncfs(2)``.


Filesystem Issues
-----------------

Currently, we recommend deploying clusters with XFS or ext4. The btrfs
filesystem has many attractive features, but bugs in the filesystem may
lead to performance issues.


Insufficient RAM
----------------

We recommend 1GB of RAM per OSD daemon. You may notice that during normal
operations, the OSD only uses a fraction of that amount (e.g., 100-200MB).
Unused RAM makes it tempting to use the excess RAM for co-resident applications,
VMs and so forth. However, when OSDs go into recovery mode, their memory
utilization spikes. If there is no RAM available, the OSD performance will slow
considerably.


Old Requests or Slow Requests
-----------------------------

If a ``ceph-osd`` daemon is slow to respond to a request, it will generate log messages
complaining about requests that are taking too long.  The warning threshold
defaults to 30 seconds, and is configurable via the ``osd op complaint time``
option.  When this happens, the cluster log will receive messages.

Legacy versions of Ceph complain about 'old requests`::

	osd.0 192.168.106.220:6800/18813 312 : [WRN] old request osd_op(client.5099.0:790 fatty_26485_object789 [write 0~4096] 2.5e54f643) v4 received at 2012-03-06 15:42:56.054801 currently waiting for sub ops

New versions of Ceph complain about 'slow requests`::

	{date} {osd.num} [WRN] 1 slow requests, 1 included below; oldest blocked for > 30.005692 secs
	{date} {osd.num}  [WRN] slow request 30.005692 seconds old, received at {date-time}: osd_op(client.4240.0:8 benchmark_data_ceph-1_39426_object7 [write 0~4194304] 0.69848840) v4 currently waiting for subops from [610]


Possible causes include:

- A bad drive (check ``dmesg`` output)
- A bug in the kernel file system bug (check ``dmesg`` output)
- An overloaded cluster (check system load, iostat, etc.)
- A bug in the ``ceph-osd`` daemon.

Possible solutions

- Remove VMs Cloud Solutions from Ceph Hosts
- Upgrade Kernel
- Upgrade Ceph
- Restart OSDs



Flapping OSDs
=============

We recommend using both a public (front-end) network and a cluster (back-end)
network so that you can better meet the capacity requirements of object
replication. Another advantage is that you can run a cluster network such that
it isn't connected to the internet, thereby preventing some denial of service
attacks. When OSDs peer and check heartbeats, they use the cluster (back-end)
network when it's available. See `Monitor/OSD Interaction`_ for details.

However, if the cluster (back-end) network fails or develops significant latency
while the public (front-end) network operates optimally, OSDs currently do not
handle this situation well. What happens is that OSDs mark each other ``down``
on the monitor, while marking themselves ``up``. We call this scenario
'flapping`.

If something is causing OSDs to 'flap' (repeatedly getting marked ``down`` and
then ``up`` again), you can force the monitors to stop the flapping with::

	ceph osd set noup      # prevent OSDs from getting marked up
	ceph osd set nodown    # prevent OSDs from getting marked down

These flags are recorded in the osdmap structure::

	ceph osd dump | grep flags
	flags no-up,no-down

You can clear the flags with::

	ceph osd unset noup
	ceph osd unset nodown

Two other flags are supported, ``noin`` and ``noout``, which prevent
booting OSDs from being marked ``in`` (allocated data) or protect OSDs
from eventually being marked ``out`` (regardless of what the current value for 
``mon osd down out interval`` is).

.. note:: ``noup``, ``noout``, and ``nodown`` are temporary in the
   sense that once the flags are cleared, the action they were blocking
   should occur shortly after.  The ``noin`` flag, on the other hand,
   prevents OSDs from being marked ``in`` on boot, and any daemons that
   started while the flag was set will remain that way.






.. _iostat: http://en.wikipedia.org/wiki/Iostat
.. _Ceph Logging and Debugging: ../../configuration/ceph-conf#ceph-logging-and-debugging
.. _Logging and Debugging: ../log-and-debug
.. _Debugging and Logging: ../debug
.. _Monitor/OSD Interaction: ../../configuration/mon-osd-interaction
.. _Monitor Config Reference: ../../configuration/mon-config-ref
.. _monitoring your OSDs: ../../operations/monitoring-osd-pg
.. _subscribe to the ceph-devel email list: mailto:majordomo@vger.kernel.org?body=subscribe+ceph-devel
.. _unsubscribe from the ceph-devel email list: mailto:majordomo@vger.kernel.org?body=unsubscribe+ceph-devel
.. _subscribe to the ceph-users email list: mailto:ceph-users-join@lists.ceph.com
.. _unsubscribe from the ceph-users email list: mailto:ceph-users-leave@lists.ceph.com
.. _Inktank: http://inktank.com
.. _OS recommendations: ../../../install/os-recommendations
.. _ceph-devel: ceph-devel@vger.kernel.org