
================================
Ceph file system client eviction
================================

When a file system client is unresponsive or otherwise misbehaving, it
may be necessary to forcibly terminate its access to the file system.  This
process is called *eviction*.

Evicting a CephFS client prevents it from communicating further with MDS
daemons and OSD daemons.  If a client was doing buffered IO to the file system,
any un-flushed data will be lost.

Clients may either be evicted automatically (if they fail to communicate
promptly with the MDS), or manually (by the system administrator).

The client eviction process applies to clients of all kinds, this includes
FUSE mounts, kernel mounts, nfs-ganesha gateways, and any process using
libcephfs.

Automatic client eviction
=========================

There are three situations in which a client may be evicted automatically.

#. On an active MDS daemon, if a client has not communicated with the MDS for over
   ``session_autoclose`` (a file system variable) seconds (300 seconds by
   default), then it will be evicted automatically.

#. On an active MDS daemon, if a client has not responded to cap revoke messages
   for over ``mds_cap_revoke_eviction_timeout`` (configuration option) seconds.
   This is disabled by default.

#. During MDS startup (including on failover), the MDS passes through a
   state called ``reconnect``.  During this state, it waits for all the
   clients to connect to the new MDS daemon.  If any clients fail to do
   so within the time window (``mds_reconnect_timeout``, 45 seconds by default)
   then they will be evicted.

A warning message is sent to the cluster log if either of these situations
arises.

Manual client eviction
======================

Sometimes, the administrator may want to evict a client manually.  This
could happen if a client has died and the administrator does not
want to wait for its session to time out, or it could happen if
a client is misbehaving and the administrator does not have access to
the client node to unmount it.

It is useful to inspect the list of clients first:

::

    ceph tell mds.0 client ls

    [
        {
            "id": 4305,
            "num_leases": 0,
            "num_caps": 3,
            "state": "open",
            "replay_requests": 0,
            "completed_requests": 0,
            "reconnecting": false,
            "inst": "client.4305 172.21.9.34:0/422650892",
            "client_metadata": {
                "ceph_sha1": "ae81e49d369875ac8b569ff3e3c456a31b8f3af5",
                "ceph_version": "ceph version 12.0.0-1934-gae81e49 (ae81e49d369875ac8b569ff3e3c456a31b8f3af5)",
                "entity_id": "0",
                "hostname": "senta04",
                "mount_point": "/tmp/tmpcMpF1b/mnt.0",
                "pid": "29377",
                "root": "/"
            }
        }
    ]
    


Once you have identified the client you want to evict, you can
do that using its unique ID, or various other attributes to identify it:

::
    
    # These all work
    ceph tell mds.0 client evict id=4305
    ceph tell mds.0 client evict client_metadata.=4305


Advanced: Un-blocklisting a client
==================================

Ordinarily, a blocklisted client may not reconnect to the servers: it
must be unmounted and then mounted anew.

However, in some situations it may be useful to permit a client that
was evicted to attempt to reconnect.

Because CephFS uses the RADOS OSD blocklist to control client eviction,
CephFS clients can be permitted to reconnect by removing them from
the blocklist:

::

    $ ceph osd blocklist ls
    listed 1 entries
    127.0.0.1:0/3710147553 2018-03-19 11:32:24.716146
    $ ceph osd blocklist rm 127.0.0.1:0/3710147553
    un-blocklisting 127.0.0.1:0/3710147553


Doing this may put data integrity at risk if other clients have accessed
files that the blocklisted client was doing buffered IO to.  It is also not
guaranteed to result in a fully functional client -- the best way to get
a fully healthy client back after an eviction is to unmount the client
and do a fresh mount.

If you are trying to reconnect clients in this way, you may also
find it useful to set ``client_reconnect_stale`` to true in the
FUSE client, to prompt the client to try to reconnect.

Advanced: Configuring blocklisting
==================================

If you are experiencing frequent client evictions, due to slow
client hosts or an unreliable network, and you cannot fix the underlying
issue, then you may want to ask the MDS to be less strict.

It is possible to respond to slow clients by simply dropping their
MDS sessions, but permit them to re-open sessions and permit them
to continue talking to OSDs.  To enable this mode, set
``mds_session_blocklist_on_timeout`` to false on your MDS nodes.

For the equivalent behaviour on manual evictions, set
``mds_session_blocklist_on_evict`` to false.

Note that if blocklisting is disabled, then evicting a client will
only have an effect on the MDS you send the command to.  On a system
with multiple active MDS daemons, you would need to send an
eviction command to each active daemon.  When blocklisting is enabled 
(the default), sending an eviction command to just a single
MDS is sufficient, because the blocklist propagates it to the others.

.. _background_blocklisting_and_osd_epoch_barrier:

Background: Blocklisting and OSD epoch barrier
==============================================

After a client is blocklisted, it is necessary to make sure that
other clients and MDS daemons have the latest OSDMap (including
the blocklist entry) before they try to access any data objects
that the blocklisted client might have been accessing.

This is ensured using an internal "osdmap epoch barrier" mechanism.

The purpose of the barrier is to ensure that when we hand out any
capabilities which might allow touching the same RADOS objects, the
clients we hand out the capabilities to must have a sufficiently recent
OSD map to not race with cancelled operations (from ENOSPC) or
blocklisted clients (from evictions).

More specifically, the cases where an epoch barrier is set are:

 * Client eviction (where the client is blocklisted and other clients
   must wait for a post-blocklist epoch to touch the same objects).
 * OSD map full flag handling in the client (where the client may
   cancel some OSD ops from a pre-full epoch, so other clients must
   wait until the full epoch or later before touching the same objects).
 * MDS startup, because we don't persist the barrier epoch, so must
   assume that latest OSD map is always required after a restart.

Note that this is a global value for simplicity. We could maintain this on
a per-inode basis. But we don't, because:

 * It would be more complicated.
 * It would use an extra 4 bytes of memory for every inode.
 * It would not be much more efficient as, almost always, everyone has
   the latest OSD map. And, in most cases everyone will breeze through this
   barrier rather than waiting.
 * This barrier is done in very rare cases, so any benefit from per-inode
   granularity would only very rarely be seen.

The epoch barrier is transmitted along with all capability messages, and
instructs the receiver of the message to avoid sending any more RADOS
operations to OSDs until it has seen this OSD epoch.  This mainly applies
to clients (doing their data writes directly to files), but also applies
to the MDS because things like file size probing and file deletion are
done directly from the MDS.
