Adding upstream version 18.2.2.upstream/18.2.2

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

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+======================
+ Troubleshooting OSDs
+======================
+
+Before troubleshooting the cluster's OSDs, check the monitors
+and the network. 
+
+First, determine whether the monitors have a quorum. Run the ``ceph health``
+command or the ``ceph -s`` command and if Ceph shows ``HEALTH_OK`` then there
+is a monitor quorum. 
+
+If the monitors don't have a quorum or if there are errors with the monitor
+status, address the monitor issues before proceeding by consulting the material
+in `Troubleshooting Monitors <../troubleshooting-mon>`_.
+
+Next, check your networks to make sure that they are running properly. Networks
+can have a significant impact on OSD operation and performance. Look for
+dropped packets on the host side and CRC errors on the switch side.
+
+
+Obtaining Data About OSDs
+=========================
+
+When troubleshooting OSDs, it is useful to collect different kinds of
+information about the OSDs. Some information comes from the practice of
+`monitoring OSDs`_ (for example, by running the ``ceph osd tree`` command).
+Additional information concerns the topology of your cluster, and is discussed
+in the following sections.
+
+
+Ceph Logs
+---------
+
+Ceph log files are stored under ``/var/log/ceph``. Unless the path has been
+changed (or you are in a containerized environment that stores logs in a
+different location), the log files can be listed by running the following
+command:
+
+.. prompt:: bash
+
+   ls /var/log/ceph
+
+If there is not enough log detail, change the logging level. To ensure that
+Ceph performs adequately under high logging volume, see `Logging and
+Debugging`_.
+
+
+
+Admin Socket
+------------
+
+Use the admin socket tool to retrieve runtime information. First, list the
+sockets of Ceph's daemons by running the following command:
+
+.. prompt:: bash
+
+   ls /var/run/ceph
+
+Next, run a command of the following form (replacing ``{daemon-name}`` with the
+name of a specific daemon: for example, ``osd.0``):
+
+.. prompt:: bash
+
+   ceph daemon {daemon-name} help
+
+Alternatively, run the command with a ``{socket-file}`` specified (a "socket
+file" is a specific file in ``/var/run/ceph``):
+
+.. prompt:: bash
+
+   ceph daemon {socket-file} help
+
+The admin socket makes many tasks possible, including:
+
+- Listing Ceph configuration at runtime
+- Dumping historic operations
+- Dumping the operation priority queue state
+- Dumping operations in flight
+- Dumping perfcounters
+
+Display Free Space
+------------------
+
+Filesystem issues may arise. To display your filesystems' free space, run the
+following command:
+
+.. prompt:: bash
+
+   df -h
+
+To see this command's supported syntax and options, run ``df --help``.
+
+I/O Statistics
+--------------
+
+The `iostat`_ tool can be used to identify I/O-related issues. Run the
+following command:
+
+.. prompt:: bash
+
+   iostat -x
+
+
+Diagnostic Messages
+-------------------
+
+To retrieve diagnostic messages from the kernel, run the ``dmesg`` command and
+specify the output with ``less``, ``more``, ``grep``, or ``tail``. For
+example: 
+
+.. prompt:: bash
+
+    dmesg | grep scsi
+
+Stopping without Rebalancing
+============================
+
+It might be occasionally necessary to perform maintenance on a subset of your
+cluster or to resolve a problem that affects a failure domain (for example, a
+rack).  However, when you stop OSDs for maintenance, you might want to prevent
+CRUSH from automatically rebalancing the cluster. To avert this rebalancing
+behavior, set the cluster to ``noout`` by running the following command:
+
+.. prompt:: bash
+
+   ceph osd set noout
+
+.. warning:: This is more a thought exercise offered for the purpose of giving
+   the reader a sense of failure domains and CRUSH behavior than a suggestion
+   that anyone in the post-Luminous world run ``ceph osd set noout``. When the
+   OSDs return to an ``up`` state, rebalancing will resume and the change
+   introduced by the ``ceph osd set noout`` command will be reverted.
+
+In Luminous and later releases, however, it is a safer approach to flag only
+affected OSDs.  To add or remove a ``noout`` flag to a specific OSD, run a
+command like the following:
+
+.. prompt:: bash
+
+   ceph osd add-noout osd.0
+   ceph osd rm-noout  osd.0
+
+It is also possible to flag an entire CRUSH bucket. For example, if you plan to
+take down ``prod-ceph-data1701`` in order to add RAM, you might run the
+following command:
+
+.. prompt:: bash
+
+   ceph osd set-group noout prod-ceph-data1701
+
+After the flag is set, stop the OSDs and any other colocated
+Ceph services within the failure domain that requires maintenance work::
+
+   systemctl stop ceph\*.service ceph\*.target
+
+.. note:: When an OSD is stopped, any placement groups within the OSD are
+   marked as ``degraded``.
+
+After the maintenance is complete, it will be necessary to restart the OSDs
+and any other daemons that have stopped. However, if the host was rebooted as
+part of the maintenance, they do not need to be restarted and will come back up
+automatically. To restart OSDs or other daemons, use a command of the following
+form:
+
+.. prompt:: bash
+
+   sudo systemctl start ceph.target
+
+Finally, unset the ``noout`` flag as needed by running commands like the
+following:
+
+.. prompt:: bash
+
+   ceph osd unset noout
+   ceph osd unset-group noout prod-ceph-data1701
+
+Many contemporary Linux distributions employ ``systemd`` for service
+management.  However, for certain operating systems (especially older ones) it
+might be necessary to issue equivalent ``service`` or ``start``/``stop``
+commands.
+
+
+.. _osd-not-running:
+
+OSD Not Running
+===============
+
+Under normal conditions, restarting a ``ceph-osd`` daemon will allow it to
+rejoin the cluster and recover.
+
+
+An OSD Won't Start
+------------------
+
+If the cluster has started but an OSD isn't starting, 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 to the
+  standard (for example, make sure that it says ``host`` and not ``hostname``,
+  etc.).
+
+- **Check Paths:** Ensure that the paths specified in the configuration
+  correspond to the paths for data and metadata that actually exist (for
+  example, the paths to the journals, the WAL, and the DB). Separate the OSD
+  data from the metadata in order to see whether there are errors in the
+  configuration file and in the actual mounts. If so, these errors might
+  explain why OSDs are not starting. To store the metadata on a separate block
+  device, partition or LVM the drive and assign one partition per OSD.
+
+- **Check Max Threadcount:** If the cluster has a node with an especially high
+  number of OSDs, it might be hitting the default maximum number of threads
+  (usually 32,000).  This is especially likely to happen during recovery.
+  Increasing the maximum number of threads to the maximum possible number of
+  threads allowed (4194303) might help with the problem. To increase the number
+  of threads to the maximum, run the following command:
+
+  .. prompt:: bash
+
+     sysctl -w kernel.pid_max=4194303
+
+  If this increase resolves the issue, you must make the increase permanent by
+  including a ``kernel.pid_max`` setting either in a file under
+  ``/etc/sysctl.d`` or within the master ``/etc/sysctl.conf`` file. For
+  example::
+
+     kernel.pid_max = 4194303
+
+- **Check ``nf_conntrack``:** This connection-tracking and connection-limiting
+  system causes problems for many production Ceph clusters. The problems often
+  emerge slowly and subtly. As cluster topology and client workload grow,
+  mysterious and intermittent connection failures and performance glitches
+  occur more and more, especially at certain times of the day. To begin taking
+  the measure of your problem, check the ``syslog`` history for "table full"
+  events. One way to address this kind of problem is as follows: First, use the
+  ``sysctl`` utility to assign ``nf_conntrack_max`` a much higher value. Next,
+  raise the value of ``nf_conntrack_buckets`` so that ``nf_conntrack_buckets``
+  × 8 = ``nf_conntrack_max``; this action might require running commands
+  outside of ``sysctl`` (for example, ``"echo 131072 >
+  /sys/module/nf_conntrack/parameters/hashsize``). Another way to address the
+  problem is to blacklist the associated kernel modules in order to disable
+  processing altogether. This approach is powerful, but fragile. The modules
+  and the order in which the modules must be listed can vary among kernel
+  versions. Even when blacklisted, ``iptables`` and ``docker`` might sometimes
+  activate connection tracking anyway, so we advise a "set and forget" strategy
+  for the tunables. On modern systems, this approach will not consume
+  appreciable resources.
+
+- **Kernel Version:** Identify the kernel version and distribution that are in
+  use. By default, Ceph uses third-party tools that might be buggy or come into
+  conflict with certain distributions or kernel versions (for example, Google's
+  ``gperftools`` and ``TCMalloc``). Check the `OS recommendations`_ and the
+  release notes for each Ceph version in order to make sure that you have
+  addressed any issues related to your kernel.
+
+- **Segment Fault:** If there is a segment fault, increase log levels and
+  restart the problematic daemon(s). If segment faults recur, search the Ceph
+  bug tracker `https://tracker.ceph/com/projects/ceph
+  <https://tracker.ceph.com/projects/ceph/>`_ and the ``dev`` and
+  ``ceph-users`` mailing list archives `https://ceph.io/resources
+  <https://ceph.io/resources>`_ to see if others have experienced and reported
+  these issues. If this truly is a new and unique failure, post to the ``dev``
+  email list and provide the following information: the specific Ceph release
+  being run, ``ceph.conf`` (with secrets XXX'd out), your monitor status
+  output, and excerpts from your log file(s).
+
+
+An OSD Failed
+-------------
+
+When an OSD fails, this means that a ``ceph-osd`` process is unresponsive or
+has died and that the corresponding OSD has been marked ``down``. Surviving
+``ceph-osd`` daemons will report to the monitors that the OSD appears to be
+down, and a new status will be visible in the output of the ``ceph health``
+command, as in the following example:
+
+.. prompt:: bash
+
+   ceph health
+
+::
+
+   HEALTH_WARN 1/3 in osds are down
+
+This health alert is raised whenever there are one or more OSDs marked ``in``
+and ``down``. To see which OSDs are ``down``, add ``detail`` to the command as in
+the following example:
+
+.. prompt:: bash
+
+   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
+
+Alternatively, run the following command:
+
+.. prompt:: bash
+
+    ceph osd tree down
+
+If there is a drive failure or another fault that is preventing a given
+``ceph-osd`` daemon from functioning or restarting, then there should be an
+error message present in its log file under ``/var/log/ceph``.
+
+If the ``ceph-osd`` daemon stopped because of a heartbeat failure or a
+``suicide timeout`` error, then the underlying drive or filesystem might be
+unresponsive. Check ``dmesg`` output and `syslog`  output for drive errors or
+kernel errors. It might be necessary to specify certain flags (for example,
+``dmesg -T`` to see human-readable timestamps) in order to avoid mistaking old
+errors for new errors.
+
+If an entire host's OSDs are ``down``, check to see if there is a network
+error or a hardware issue with the host.
+
+If the OSD problem is the result of a software error (for example, a failed
+assertion or another unexpected error), search for reports of the issue in the
+`bug tracker <https://tracker.ceph/com/projects/ceph>`_ , the `dev mailing list
+archives <https://lists.ceph.io/hyperkitty/list/dev@ceph.io/>`_, and the
+`ceph-users mailing list archives
+<https://lists.ceph.io/hyperkitty/list/ceph-users@ceph.io/>`_.  If there is no
+clear fix or existing bug, then :ref:`report the problem to the ceph-devel
+email list <Get Involved>`.
+
+
+.. _no-free-drive-space:
+
+No Free Drive Space
+-------------------
+
+If an OSD is full, Ceph prevents data loss by ensuring that no new data is
+written to the OSD. In an properly running cluster, health checks are raised
+when the cluster's OSDs and pools approach certain "fullness" ratios. The
+``mon_osd_full_ratio`` threshold defaults to ``0.95`` (or 95% of capacity):
+this is the point above which clients are prevented from writing data. The
+``mon_osd_backfillfull_ratio`` threshold defaults to ``0.90`` (or 90% of
+capacity): this is the point above which backfills will not start. The
+``mon_osd_nearfull_ratio`` threshold defaults to ``0.85`` (or 85% of capacity):
+this is the point at which it raises the ``OSD_NEARFULL`` health check.
+
+OSDs within a cluster will vary in how much data is allocated to them by Ceph.
+To check "fullness" by displaying data utilization for every OSD, run the
+following command:
+
+.. prompt:: bash
+
+   ceph osd df
+
+To check "fullness" by displaying a cluster’s overall data usage and data
+distribution among pools, run the following command:
+
+.. prompt:: bash
+
+   ceph df 
+
+When examining the output of the ``ceph df`` command, pay special attention to
+the **most full** OSDs, as opposed to the percentage of raw space used. If a
+single outlier OSD becomes full, all writes to this OSD's pool might fail as a
+result. When ``ceph df`` reports the space available to a pool, it considers
+the ratio settings relative to the *most full* OSD that is part of the pool. To
+flatten the distribution, two approaches are available: (1) Using the
+``reweight-by-utilization`` command to progressively move data from excessively
+full OSDs or move data to insufficiently full OSDs, and (2) in later revisions
+of Luminous and subsequent releases, exploiting the ``ceph-mgr`` ``balancer``
+module to perform the same task automatically.
+
+To adjust the "fullness" ratios, run a command or commands of the following
+form:
+
+.. prompt:: bash
+
+   ceph osd set-nearfull-ratio <float[0.0-1.0]>
+   ceph osd set-full-ratio <float[0.0-1.0]>
+   ceph osd set-backfillfull-ratio <float[0.0-1.0]>
+
+Sometimes full cluster issues arise because an OSD has failed. This can happen
+either because of a test or because the cluster is small, very full, or
+unbalanced. When an OSD or node holds an excessive percentage of the cluster's
+data, component failures or natural growth can result in the ``nearfull`` and
+``full`` ratios being exceeded.  When testing Ceph's resilience to OSD failures
+on a small cluster, it is advised to leave ample free disk space and to
+consider temporarily lowering the OSD ``full ratio``, OSD ``backfillfull
+ratio``, and OSD ``nearfull ratio``.
+
+The "fullness" status of OSDs is visible in the output of the ``ceph health``
+command, as in the following example:
+
+.. prompt:: bash
+
+   ceph health
+
+::
+
+  HEALTH_WARN 1 nearfull osd(s)
+
+For details, add the ``detail`` command as in the following example:
+
+.. prompt:: bash
+
+    ceph health detail
+
+::
+
+    HEALTH_ERR 1 full osd(s); 1 backfillfull osd(s); 1 nearfull osd(s)
+    osd.3 is full at 97%
+    osd.4 is backfill full at 91%
+    osd.2 is near full at 87%
+
+To address full cluster issues, it is recommended to add capacity by adding
+OSDs. Adding new OSDs allows the cluster to redistribute data to newly
+available storage. Search for ``rados bench`` orphans that are wasting space.
+
+If a legacy Filestore OSD cannot be started because it is full, it is possible
+to reclaim space by deleting a small number of 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. **OTHERWISE YOU WILL LOSE DATA**. You **MUST** maintain at least one
+   copy of your data on at least one OSD. Deleting placement group directories
+   is a rare and extreme intervention. It is not to be undertaken lightly.
+
+See `Monitor Config Reference`_ for more information.
+
+
+OSDs are Slow/Unresponsive
+==========================
+
+OSDs are sometimes slow or unresponsive. When troubleshooting this common
+problem, it is advised to eliminate other possibilities before investigating
+OSD performance issues. For example, be sure to confirm that your network(s)
+are working properly, to verify that your OSDs are running, and to check
+whether OSDs are throttling recovery traffic.
+
+.. tip:: In pre-Luminous releases of Ceph, ``up`` and ``in`` OSDs were
+   sometimes not available or were otherwise slow because recovering OSDs were
+   consuming system resources. Newer releases provide better recovery handling
+   by preventing this phenomenon.
+
+
+Networking Issues
+-----------------
+
+As a distributed storage system, Ceph relies upon networks for OSD peering and
+replication, recovery from faults, and periodic heartbeats. Networking issues
+can cause OSD latency and flapping OSDs. For more information, see `Flapping
+OSDs`_.
+
+To make sure that Ceph processes and Ceph-dependent processes are connected and
+listening, run the following commands:
+
+.. prompt:: bash
+
+   netstat -a | grep ceph
+   netstat -l | grep ceph
+   sudo netstat -p | grep ceph
+
+To check network statistics, run the following command:
+
+.. prompt:: bash
+
+   netstat -s
+
+Drive Configuration
+-------------------
+
+An SAS or SATA storage drive should house only one OSD, but a NVMe drive can
+easily house two or more. However, it is possible for read and write throughput
+to bottleneck if other processes share the drive. Such processes include:
+journals / metadata, operating systems, Ceph monitors, ``syslog`` logs, other
+OSDs, and non-Ceph processes.
+
+Because Ceph acknowledges writes *after* journaling, fast SSDs are an
+attractive option for accelerating response time -- particularly when using the
+``XFS`` or ``ext4`` filesystems for legacy FileStore OSDs.  By contrast, the
+``Btrfs`` file system can write and journal simultaneously. (However, use of
+``Btrfs`` is not recommended for production deployments.)
+
+.. note:: Partitioning a drive does not change its total throughput or
+   sequential read/write limits. Throughput might be improved somewhat by
+   running a journal in a separate partition, but it is better still to run
+   such a journal in a separate physical drive.
+   
+.. warning:: Reef does not support FileStore. Releases after Reef do not
+   support FileStore. Any information that mentions FileStore is pertinent only
+   to the Quincy release of Ceph and to releases prior to Quincy.
+
+
+Bad Sectors / Fragmented Disk
+-----------------------------
+
+Check your drives for bad blocks, fragmentation, and other errors that can
+cause significantly degraded performance. Tools that are useful in checking for
+drive errors include ``dmesg``, ``syslog`` logs, and ``smartctl`` (found in the
+``smartmontools`` package).
+
+.. note:: ``smartmontools`` 7.0 and late provides NVMe stat passthrough and
+   JSON output.
+
+
+Co-resident Monitors/OSDs
+-------------------------
+
+Although monitors are relatively lightweight processes, performance issues can
+result when monitors are run on the same host machine as an OSD. Monitors issue
+many ``fsync()`` calls and this can interfere with other workloads. The danger
+of performance issues is especially acute when the monitors are co-resident on
+the same storage drive as an OSD. In addition, if the monitors are running an
+older kernel (pre-3.0) or a kernel with no ``syncfs(2)`` syscall, then multiple
+OSDs running on the same host might make so many commits as to undermine each
+other's performance.  This problem sometimes results in what is called "the
+bursty writes".
+
+
+Co-resident Processes
+---------------------
+
+Significant OSD latency can result from processes that write data to Ceph (for
+example, cloud-based solutions and virtual machines) while operating on the
+same hardware as OSDs. For this reason, making such processes co-resident with
+OSDs is not generally recommended. Instead, the recommended practice is to
+optimize certain hosts for use with Ceph and use other hosts for other
+processes. This practice of separating Ceph operations from other applications
+might help improve performance and might also streamline troubleshooting and
+maintenance.
+
+Running co-resident processes on the same hardware is sometimes called
+"convergence". When using Ceph, engage in convergence only with expertise and
+after consideration.
+
+
+Logging Levels
+--------------
+
+Performance issues can result from high logging levels. Operators sometimes
+raise logging levels in order to track an issue and then forget to lower them
+afterwards. In such a situation, OSDs might consume valuable system resources to
+write needlessly verbose logs onto the disk. Anyone who does want to use high logging
+levels is advised to consider mounting a drive to the default path for logging
+(for example, ``/var/log/ceph/$cluster-$name.log``).
+
+Recovery Throttling
+-------------------
+
+Depending upon your configuration, Ceph may reduce recovery rates to maintain
+client or OSD performance, or it may increase recovery rates to the point that
+recovery impacts client or OSD performance. Check to see if the client or OSD
+is recovering.
+
+
+Kernel Version
+--------------
+
+Check the kernel version that you are running. Older kernels may lack updates
+that improve Ceph performance. 
+
+
+Kernel Issues with SyncFS
+-------------------------
+
+If you have 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
+-----------------
+
+In post-Luminous releases, we recommend deploying clusters with the BlueStore
+back end.  When running a pre-Luminous release, or if you have a specific
+reason to deploy OSDs with the previous Filestore backend, we recommend
+``XFS``.
+
+We recommend against using ``Btrfs`` or ``ext4``.  The ``Btrfs`` filesystem has
+many attractive features, but bugs may lead to performance issues and spurious
+ENOSPC errors.  We do not recommend ``ext4`` for Filestore OSDs because
+``xattr`` limitations break support for long object names, which are needed for
+RGW.
+
+For more information, see `Filesystem Recommendations`_.
+
+.. _Filesystem Recommendations: ../configuration/filesystem-recommendations
+
+Insufficient RAM
+----------------
+
+We recommend a *minimum* of 4GB of RAM per OSD daemon and we suggest rounding
+up from 6GB to 8GB. During normal operations, you may notice that ``ceph-osd``
+processes use only a fraction of that amount.  You might be tempted to use the
+excess RAM for co-resident applications or to skimp on each node's memory
+capacity. However, when OSDs experience recovery their memory utilization
+spikes. If there is insufficient RAM available during recovery, OSD performance
+will slow considerably and the daemons may even crash or be killed by the Linux
+``OOM Killer``.
+
+
+Blocked Requests or Slow Requests
+---------------------------------
+
+When a ``ceph-osd`` daemon is slow to respond to a request, the cluster log
+receives messages reporting ops that are taking too long. The warning threshold
+defaults to 30 seconds and is configurable via the ``osd_op_complaint_time``
+setting.
+
+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
+
+Newer 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 failing drive (check ``dmesg`` output)
+- A bug in the kernel file system (check ``dmesg`` output)
+- An overloaded cluster (check system load, iostat, etc.)
+- A bug in the ``ceph-osd`` daemon.
+
+Possible solutions:
+
+- Remove VMs from Ceph hosts
+- Upgrade kernel
+- Upgrade Ceph
+- Restart OSDs
+- Replace failed or failing components
+
+Debugging Slow Requests
+-----------------------
+
+If you run ``ceph daemon osd.<id> dump_historic_ops`` or ``ceph daemon osd.<id>
+dump_ops_in_flight``, you will see a set of operations and a list of events
+each operation went through. These are briefly described below.
+
+Events from the Messenger layer:
+
+- ``header_read``: The time that the messenger first started reading the message off the wire.
+- ``throttled``: The time that the messenger tried to acquire memory throttle space to read
+  the message into memory.
+- ``all_read``: The time that the messenger finished reading the message off the wire.
+- ``dispatched``: The time that the messenger gave the message to the OSD.
+- ``initiated``: This is identical to ``header_read``. The existence of both is a
+  historical oddity.
+
+Events from the OSD as it processes ops:
+
+- ``queued_for_pg``: The op has been put into the queue for processing by its PG.
+- ``reached_pg``: The PG has started performing the op.
+- ``waiting for \*``: The op is waiting for some other work to complete before
+  it can proceed (for example, a new OSDMap; the scrubbing of its object
+  target; the completion of a PG's peering; all as specified in the message).
+- ``started``: The op has been accepted as something the OSD should do and 
+  is now being performed.
+- ``waiting for subops from``: The op has been sent to replica OSDs.
+
+Events from ```Filestore```:
+
+- ``commit_queued_for_journal_write``: The op has been given to the FileStore.
+- ``write_thread_in_journal_buffer``: The op is in the journal's buffer and is waiting
+  to be persisted (as the next disk write).
+- ``journaled_completion_queued``: The op was journaled to disk and its callback
+  has been queued for invocation.
+
+Events from the OSD after data has been given to underlying storage:
+
+- ``op_commit``: The op has been committed (that is, written to journal) by the
+  primary OSD.
+- ``op_applied``: The op has been `write()'en
+  <https://www.freebsd.org/cgi/man.cgi?write(2)>`_ to the backing FS (that is,
+  applied in memory but not flushed out to disk) on the primary.
+- ``sub_op_applied``: ``op_applied``, but for a replica's "subop".
+- ``sub_op_committed``: ``op_commit``, but for a replica's subop (only for EC pools).
+- ``sub_op_commit_rec/sub_op_apply_rec from <X>``: The primary marks this when it
+  hears about the above, but for a particular replica (i.e. ``<X>``).
+- ``commit_sent``: We sent a reply back to the client (or primary OSD, for sub ops).
+
+Some of these events may appear redundant, but they cross important boundaries
+in the internal code (such as passing data across locks into new threads).
+
+
+Flapping OSDs
+=============
+
+"Flapping" is the term for the phenomenon of an OSD being repeatedly marked
+``up`` and then ``down`` in rapid succession.  This section explains how to
+recognize flapping, and how to mitigate it.
+
+When OSDs peer and check heartbeats, they use the cluster (back-end) network
+when it is available. See `Monitor/OSD Interaction`_ for details.
+
+The upstream Ceph community has traditionally recommended separate *public*
+(front-end) and *private* (cluster / back-end / replication) networks. This
+provides the following benefits:
+
+#. Segregation of (1) heartbeat traffic and replication/recovery traffic
+   (private) from (2) traffic from clients and between OSDs and monitors
+   (public). This helps keep one stream of traffic from DoS-ing the other,
+   which could in turn result in a cascading failure.
+
+#. Additional throughput for both public and private traffic.
+
+In the past, when common networking technologies were measured in a range
+encompassing 100Mb/s and 1Gb/s, this separation was often critical. But with
+today's 10Gb/s, 40Gb/s, and 25/50/100Gb/s networks, the above capacity concerns
+are often diminished or even obviated.  For example, if your OSD nodes have two
+network ports, dedicating one to the public and the other to the private
+network means that you have no path redundancy.  This degrades your ability to
+endure network maintenance and network failures without significant cluster or
+client impact. In situations like this, consider instead using both links for
+only a public network: with bonding (LACP) or equal-cost routing (for example,
+FRR) you reap the benefits of increased throughput headroom, fault tolerance,
+and reduced OSD flapping.
+
+When a private network (or even a single host link) fails or degrades while the
+public network continues operating normally, OSDs may not handle this situation
+well. In such situations, OSDs use the public network to report each other
+``down`` to the monitors, while marking themselves ``up``. The monitors then
+send out-- again on the public network--an updated cluster map with the
+affected OSDs marked `down`. These OSDs reply to the monitors "I'm not dead
+yet!", and the cycle repeats. We call this scenario 'flapping`, and it can be
+difficult to isolate and remediate. Without a private network, this irksome
+dynamic is avoided: OSDs are generally either ``up`` or ``down`` without
+flapping.
+
+If something does cause OSDs to 'flap' (repeatedly being marked ``down`` and
+then ``up`` again), you can force the monitors to halt the flapping by
+temporarily freezing their states:
+
+.. prompt:: bash
+
+   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:
+
+.. prompt:: bash
+
+   ceph osd dump | grep flags
+
+::
+
+   flags no-up,no-down
+
+You can clear these flags with:
+
+.. prompt:: bash
+
+   ceph osd unset noup
+   ceph osd unset nodown
+
+Two other flags are available, ``noin`` and ``noout``, which prevent booting
+OSDs from being marked ``in`` (allocated data) or protect OSDs from eventually
+being marked ``out`` (regardless of the current value of
+``mon_osd_down_out_interval``).
+
+.. note:: ``noup``, ``noout``, and ``nodown`` are temporary in the sense that
+   after the flags are cleared, the action that they were blocking should be
+   possible shortly thereafter. But the ``noin`` flag prevents OSDs from being
+   marked ``in`` on boot, and any daemons that started while the flag was set
+   will remain that way.
+
+.. note:: The causes and effects of flapping can be mitigated somewhat by
+   making careful adjustments to ``mon_osd_down_out_subtree_limit``,
+   ``mon_osd_reporter_subtree_level``, and ``mon_osd_min_down_reporters``.
+   Derivation of optimal settings depends on cluster size, topology, and the
+   Ceph release in use. The interaction of all of these factors is subtle and
+   is beyond the scope of this document.
+
+
+.. _iostat: https://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
+
+.. _monitoring OSDs: ../../operations/monitoring-osd-pg/#monitoring-osds
+
+.. _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
+.. _OS recommendations: ../../../start/os-recommendations
+.. _ceph-devel: ceph-devel@vger.kernel.org