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278 lines
9.8 KiB
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=====================
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Troubleshooting PGs
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=====================
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Placement Groups Never Get Clean
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================================
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There are a few cases where Ceph placement groups never get clean:
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#. **One OSD:** If you deviate from the quick start and use only one OSD, you
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will likely run into problems. OSDs report other OSDs to the monitor, and
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also interact with other OSDs when replicating data. If you have only one
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OSD, a second OSD cannot check its heartbeat. Also, if you remove an OSD
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and have only one OSD remaining, you may encounter problems. An secondary
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or tertiary OSD expects another OSD to tell it which placement groups it
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should have. The lack of another OSD prevents this from occurring. So a
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placement group can remain stuck “stale” forever.
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#. **Pool Size = 1**: If you have only one copy of an object, no other OSD will
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tell the OSD which objects it should have. For each placement group mapped
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to the remaining OSD (see ``ceph pg dump``), you can force the OSD to notice
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the placement groups it needs by running::
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ceph pg force_create_pg <pgid>
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#. **CRUSH Rules:** Another candidate for placement groups remaining
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unclean involves errors in your CRUSH map.
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As a general rule, you should run your cluster with more than one OSD and a
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pool size greater than 1 object replica.
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Stuck Placement Groups
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======================
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It is normal for placement groups to enter states like "degraded" or "peering"
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following a failure. Normally these states indicate the normal progression
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through the failure recovery process. However, if a placement group stays in one
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of these states for a long time this may be an indication of a larger problem.
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For this reason, the monitor will warn when placement groups get "stuck" in a
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non-optimal state. Specifically, we check for:
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* ``inactive`` - The placement group has not been ``active`` for too long
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(i.e., it hasn't been able to service read/write requests).
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* ``unclean`` - The placement group has not been ``clean`` for too long
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(i.e., it hasn't been able to completely recover from a previous failure).
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* ``stale`` - The placement group status has not been updated by a ``ceph-osd``,
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indicating that all nodes storing this placement group may be ``down``.
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You can explicitly list stuck placement groups with one of::
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ceph pg dump_stuck stale
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ceph pg dump_stuck inactive
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ceph pg dump_stuck unclean
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For stuck ``stale`` placement groups, it is normally a matter of getting the
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right ``ceph-osd`` daemons running again. For stuck ``inactive`` placement
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groups, it is usually a peering problem (see :ref:`failures-osd-peering`). For
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stuck ``unclean`` placement groups, there is usually something preventing
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recovery from completing, like unfound objects (see
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:ref:`failures-osd-unfound`);
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.. _failures-osd-peering:
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Placement Group Down - Peering Failure
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======================================
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In certain cases, the ``ceph-osd`` `Peering` process can run into
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problems, preventing a PG from becoming active and usable. For
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example, ``ceph health`` might report::
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ceph health detail
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HEALTH_ERR 7 pgs degraded; 12 pgs down; 12 pgs peering; 1 pgs recovering; 6 pgs stuck unclean; 114/3300 degraded (3.455%); 1/3 in osds are down
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...
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pg 0.5 is down+peering
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pg 1.4 is down+peering
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...
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osd.1 is down since epoch 69, last address 192.168.106.220:6801/8651
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We can query the cluster to determine exactly why the PG is marked ``down`` with::
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ceph pg 0.5 query
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.. code-block:: javascript
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{ "state": "down+peering",
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...
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"recovery_state": [
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{ "name": "Started\/Primary\/Peering\/GetInfo",
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"enter_time": "2012-03-06 14:40:16.169679",
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"requested_info_from": []},
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{ "name": "Started\/Primary\/Peering",
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"enter_time": "2012-03-06 14:40:16.169659",
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"probing_osds": [
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0,
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1],
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"blocked": "peering is blocked due to down osds",
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"down_osds_we_would_probe": [
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1],
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"peering_blocked_by": [
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{ "osd": 1,
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"current_lost_at": 0,
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"comment": "starting or marking this osd lost may let us proceed"}]},
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{ "name": "Started",
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"enter_time": "2012-03-06 14:40:16.169513"}
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]
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}
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The ``recovery_state`` section tells us that peering is blocked due to
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down ``ceph-osd`` daemons, specifically ``osd.1``. In this case, we can start that ``ceph-osd``
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and things will recover.
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Alternatively, if there is a catastrophic failure of ``osd.1`` (e.g., disk
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failure), we can tell the cluster that it is ``lost`` and to cope as
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best it can.
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.. important:: This is dangerous in that the cluster cannot
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guarantee that the other copies of the data are consistent
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and up to date.
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To instruct Ceph to continue anyway::
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ceph osd lost 1
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Recovery will proceed.
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.. _failures-osd-unfound:
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Unfound Objects
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===============
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Under certain combinations of failures Ceph may complain about
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``unfound`` objects::
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ceph health detail
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HEALTH_WARN 1 pgs degraded; 78/3778 unfound (2.065%)
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pg 2.4 is active+degraded, 78 unfound
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This means that the storage cluster knows that some objects (or newer
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copies of existing objects) exist, but it hasn't found copies of them.
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One example of how this might come about for a PG whose data is on ceph-osds
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1 and 2:
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* 1 goes down
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* 2 handles some writes, alone
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* 1 comes up
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* 1 and 2 repeer, and the objects missing on 1 are queued for recovery.
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* Before the new objects are copied, 2 goes down.
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Now 1 knows that these object exist, but there is no live ``ceph-osd`` who
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has a copy. In this case, IO to those objects will block, and the
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cluster will hope that the failed node comes back soon; this is
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assumed to be preferable to returning an IO error to the user.
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First, you can identify which objects are unfound with::
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ceph pg 2.4 list_missing [starting offset, in json]
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.. code-block:: javascript
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{ "offset": { "oid": "",
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"key": "",
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"snapid": 0,
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"hash": 0,
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"max": 0},
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"num_missing": 0,
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"num_unfound": 0,
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"objects": [
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{ "oid": "object 1",
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"key": "",
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"hash": 0,
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"max": 0 },
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...
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],
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"more": 0}
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If there are too many objects to list in a single result, the ``more``
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field will be true and you can query for more. (Eventually the
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command line tool will hide this from you, but not yet.)
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Second, you can identify which OSDs have been probed or might contain
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data::
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ceph pg 2.4 query
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.. code-block:: javascript
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"recovery_state": [
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{ "name": "Started\/Primary\/Active",
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"enter_time": "2012-03-06 15:15:46.713212",
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"might_have_unfound": [
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{ "osd": 1,
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"status": "osd is down"}]},
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In this case, for example, the cluster knows that ``osd.1`` might have
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data, but it is ``down``. The full range of possible states include::
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* already probed
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* querying
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* osd is down
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* not queried (yet)
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Sometimes it simply takes some time for the cluster to query possible
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locations.
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It is possible that there are other locations where the object can
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exist that are not listed. For example, if a ceph-osd is stopped and
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taken out of the cluster, the cluster fully recovers, and due to some
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future set of failures ends up with an unfound object, it won't
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consider the long-departed ceph-osd as a potential location to
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consider. (This scenario, however, is unlikely.)
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If all possible locations have been queried and objects are still
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lost, you may have to give up on the lost objects. This, again, is
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possible given unusual combinations of failures that allow the cluster
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to learn about writes that were performed before the writes themselves
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are recovered. To mark the "unfound" objects as "lost"::
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ceph pg 2.5 mark_unfound_lost revert
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This the final argument specifies how the cluster should deal with
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lost objects. Currently the only supported option is "revert", which
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will either roll back to a previous version of the object or (if it
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was a new object) forget about it entirely. Use this with caution, as
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it may confuse applications that expected the object to exist.
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Homeless Placement Groups
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=========================
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It is possible for all OSDs that had copies of a given placement groups to fail.
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If that's the case, that subset of the object store is unavailable, and the
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monitor will receive no status updates for those placement groups. To detect
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this situation, the monitor marks any placement group whose primary OSD has
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failed as ``stale``. For example::
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ceph health
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HEALTH_WARN 24 pgs stale; 3/300 in osds are down
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You can identify which placement groups are ``stale``, and what the last OSDs to
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store them were, with::
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ceph health detail
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HEALTH_WARN 24 pgs stale; 3/300 in osds are down
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...
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pg 2.5 is stuck stale+active+remapped, last acting [2,0]
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...
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osd.10 is down since epoch 23, last address 192.168.106.220:6800/11080
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osd.11 is down since epoch 13, last address 192.168.106.220:6803/11539
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osd.12 is down since epoch 24, last address 192.168.106.220:6806/11861
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If we want to get placement group 2.5 back online, for example, this tells us that
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it was last managed by ``osd.0`` and ``osd.2``. Restarting those ``ceph-osd``
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daemons will allow the cluster to recover that placement group (and, presumably,
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many others).
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Only a Few OSDs Receive Data
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============================
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If you have many nodes in your cluster and only a few of them receive data,
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`check`_ the number of placement groups in your pool. Since placement groups get
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mapped to OSDs, a small number of placement groups will not distribute across
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your cluster. Try creating a pool with a placement group count that is a
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multiple of the number of OSDs. See `Placement Groups`_ for details. The default
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placement group count for pools isn't useful, but you can change it `here`_.
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.. _check: ../../operations/placement-groups#get-the-number-of-placement-groups
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.. _here: ../../configuration/pool-pg-config-ref
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.. _Placement Groups: ../../operations/placement-groups
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