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191 lines
7.4 KiB
ReStructuredText
191 lines
7.4 KiB
ReStructuredText
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================================
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Ceph file system client eviction
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================================
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When a file system client is unresponsive or otherwise misbehaving, it
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may be necessary to forcibly terminate its access to the file system. This
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process is called *eviction*.
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Evicting a CephFS client prevents it from communicating further with MDS
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daemons and OSD daemons. If a client was doing buffered IO to the file system,
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any un-flushed data will be lost.
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Clients may either be evicted automatically (if they fail to communicate
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promptly with the MDS), or manually (by the system administrator).
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The client eviction process applies to clients of all kinds, this includes
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FUSE mounts, kernel mounts, nfs-ganesha gateways, and any process using
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libcephfs.
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Automatic client eviction
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=========================
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There are three situations in which a client may be evicted automatically.
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#. On an active MDS daemon, if a client has not communicated with the MDS for over
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``session_autoclose`` (a file system variable) seconds (300 seconds by
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default), then it will be evicted automatically.
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#. On an active MDS daemon, if a client has not responded to cap revoke messages
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for over ``mds_cap_revoke_eviction_timeout`` (configuration option) seconds.
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This is disabled by default.
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#. During MDS startup (including on failover), the MDS passes through a
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state called ``reconnect``. During this state, it waits for all the
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clients to connect to the new MDS daemon. If any clients fail to do
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so within the time window (``mds_reconnect_timeout``, 45 seconds by default)
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then they will be evicted.
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A warning message is sent to the cluster log if either of these situations
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arises.
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Manual client eviction
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======================
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Sometimes, the administrator may want to evict a client manually. This
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could happen if a client has died and the administrator does not
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want to wait for its session to time out, or it could happen if
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a client is misbehaving and the administrator does not have access to
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the client node to unmount it.
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It is useful to inspect the list of clients first:
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::
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ceph tell mds.0 client ls
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[
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{
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"id": 4305,
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"num_leases": 0,
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"num_caps": 3,
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"state": "open",
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"replay_requests": 0,
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"completed_requests": 0,
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"reconnecting": false,
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"inst": "client.4305 172.21.9.34:0/422650892",
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"client_metadata": {
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"ceph_sha1": "ae81e49d369875ac8b569ff3e3c456a31b8f3af5",
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"ceph_version": "ceph version 12.0.0-1934-gae81e49 (ae81e49d369875ac8b569ff3e3c456a31b8f3af5)",
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"entity_id": "0",
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"hostname": "senta04",
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"mount_point": "/tmp/tmpcMpF1b/mnt.0",
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"pid": "29377",
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"root": "/"
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}
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}
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]
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Once you have identified the client you want to evict, you can
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do that using its unique ID, or various other attributes to identify it:
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::
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# These all work
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ceph tell mds.0 client evict id=4305
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ceph tell mds.0 client evict client_metadata.=4305
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Advanced: Un-blocklisting a client
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==================================
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Ordinarily, a blocklisted client may not reconnect to the servers: it
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must be unmounted and then mounted anew.
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However, in some situations it may be useful to permit a client that
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was evicted to attempt to reconnect.
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Because CephFS uses the RADOS OSD blocklist to control client eviction,
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CephFS clients can be permitted to reconnect by removing them from
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the blocklist:
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::
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$ ceph osd blocklist ls
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listed 1 entries
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127.0.0.1:0/3710147553 2018-03-19 11:32:24.716146
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$ ceph osd blocklist rm 127.0.0.1:0/3710147553
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un-blocklisting 127.0.0.1:0/3710147553
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Doing this may put data integrity at risk if other clients have accessed
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files that the blocklisted client was doing buffered IO to. It is also not
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guaranteed to result in a fully functional client -- the best way to get
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a fully healthy client back after an eviction is to unmount the client
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and do a fresh mount.
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If you are trying to reconnect clients in this way, you may also
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find it useful to set ``client_reconnect_stale`` to true in the
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FUSE client, to prompt the client to try to reconnect.
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Advanced: Configuring blocklisting
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==================================
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If you are experiencing frequent client evictions, due to slow
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client hosts or an unreliable network, and you cannot fix the underlying
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issue, then you may want to ask the MDS to be less strict.
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It is possible to respond to slow clients by simply dropping their
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MDS sessions, but permit them to re-open sessions and permit them
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to continue talking to OSDs. To enable this mode, set
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``mds_session_blocklist_on_timeout`` to false on your MDS nodes.
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For the equivalent behaviour on manual evictions, set
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``mds_session_blocklist_on_evict`` to false.
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Note that if blocklisting is disabled, then evicting a client will
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only have an effect on the MDS you send the command to. On a system
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with multiple active MDS daemons, you would need to send an
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eviction command to each active daemon. When blocklisting is enabled
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(the default), sending an eviction command to just a single
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MDS is sufficient, because the blocklist propagates it to the others.
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.. _background_blocklisting_and_osd_epoch_barrier:
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Background: Blocklisting and OSD epoch barrier
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==============================================
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After a client is blocklisted, it is necessary to make sure that
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other clients and MDS daemons have the latest OSDMap (including
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the blocklist entry) before they try to access any data objects
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that the blocklisted client might have been accessing.
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This is ensured using an internal "osdmap epoch barrier" mechanism.
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The purpose of the barrier is to ensure that when we hand out any
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capabilities which might allow touching the same RADOS objects, the
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clients we hand out the capabilities to must have a sufficiently recent
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OSD map to not race with cancelled operations (from ENOSPC) or
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blocklisted clients (from evictions).
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More specifically, the cases where an epoch barrier is set are:
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* Client eviction (where the client is blocklisted and other clients
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must wait for a post-blocklist epoch to touch the same objects).
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* OSD map full flag handling in the client (where the client may
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cancel some OSD ops from a pre-full epoch, so other clients must
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wait until the full epoch or later before touching the same objects).
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* MDS startup, because we don't persist the barrier epoch, so must
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assume that latest OSD map is always required after a restart.
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Note that this is a global value for simplicity. We could maintain this on
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a per-inode basis. But we don't, because:
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* It would be more complicated.
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* It would use an extra 4 bytes of memory for every inode.
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* It would not be much more efficient as, almost always, everyone has
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the latest OSD map. And, in most cases everyone will breeze through this
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barrier rather than waiting.
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* This barrier is done in very rare cases, so any benefit from per-inode
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granularity would only very rarely be seen.
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The epoch barrier is transmitted along with all capability messages, and
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instructs the receiver of the message to avoid sending any more RADOS
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operations to OSDs until it has seen this OSD epoch. This mainly applies
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to clients (doing their data writes directly to files), but also applies
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to the MDS because things like file size probing and file deletion are
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done directly from the MDS.
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