mirror of
https://github.com/ceph/ceph
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9f53eeb88d
Adding this at this time to give us a sensible place to talk about the epoch barrier stuff. The eviction stuff will probably get simplified once we add a mon-side eviction command that handles blacklisting and MDS session eviction in one go. Signed-off-by: John Spray <john.spray@redhat.com>
120 lines
4.1 KiB
ReStructuredText
120 lines
4.1 KiB
ReStructuredText
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Ceph filesystem client eviction
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===============================
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When a filesystem client is unresponsive or otherwise misbehaving, it
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may be necessary to forcibly terminate its access to the filesystem. This
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process is called *eviction*.
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This process is somewhat thorough in order to protect against data inconsistency
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resulting from misbehaving clients.
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OSD blacklisting
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----------------
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First, prevent the client from performing any more data operations by *blacklisting*
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it at the RADOS level. You may be familiar with this concept as *fencing* in other
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storage systems.
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Identify the client to evict from the MDS session list:
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::
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# ceph daemon mds.a session ls
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[
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{ "id": 4117,
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"num_leases": 0,
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"num_caps": 1,
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"state": "open",
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"replay_requests": 0,
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"reconnecting": false,
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"inst": "client.4117 172.16.79.251:0\/3271",
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"client_metadata": { "entity_id": "admin",
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"hostname": "fedoravm.localdomain",
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"mount_point": "\/home\/user\/mnt"}}]
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In this case the 'fedoravm' client has address ``172.16.79.251:0/3271``, so we blacklist
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it as follows:
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::
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# ceph osd blacklist add 172.16.79.251:0/3271
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blacklisting 172.16.79.251:0/3271 until 2014-12-09 13:09:56.569368 (3600 sec)
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OSD epoch barrier
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-----------------
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While the evicted client is now marked as blacklisted in the central (mon) copy of the OSD
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map, it is now necessary to ensure that this OSD map update has propagated to all daemons
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involved in subsequent filesystem I/O. To do this, use the ``osdmap barrier`` MDS admin
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socket command.
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First read the latest OSD epoch:
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::
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# ceph osd dump
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epoch 12
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fsid fd61ca96-53ff-4311-826c-f36b176d69ea
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created 2014-12-09 12:03:38.595844
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modified 2014-12-09 12:09:56.619957
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...
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In this case it is 12. Now request the MDS to barrier on this epoch:
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::
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# ceph daemon mds.a osdmap barrier 12
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MDS session eviction
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--------------------
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Finally, it is safe to evict the client's MDS session, such that any capabilities it held
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may be issued to other clients. The ID here is the ``id`` attribute from the ``session ls``
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output:
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::
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# ceph daemon mds.a session evict 4117
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That's it! The client has now been evicted, and any resources it had locked will
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now be available for other clients.
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Background: OSD epoch barrier
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-----------------------------
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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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blacklisted clients (from evictions)
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More specifically, the cases where we set an epoch barrier are:
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* Client eviction (where the client is blacklisted and other clients
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must wait for a post-blacklist 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. 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 the latest
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OSD map anyway, in most cases everyone will breeze through this barrier
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rather than waiting.
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* We only do this barrier 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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