mirror of
https://github.com/ceph/ceph
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8ebd0fb74a
Fixes: http://tracker.ceph.com/issues/38729 Signed-off-by: "Yan, Zheng" <zyan@redhat.com>
112 lines
4.7 KiB
ReStructuredText
112 lines
4.7 KiB
ReStructuredText
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Experimental Features
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=====================
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CephFS includes a number of experimental features which are not fully stabilized
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or qualified for users to turn on in real deployments. We generally do our best
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to clearly demarcate these and fence them off so they cannot be used by mistake.
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Some of these features are closer to being done than others, though. We describe
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each of them with an approximation of how risky they are and briefly describe
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what is required to enable them. Note that doing so will *irrevocably* flag maps
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in the monitor as having once enabled this flag to improve debugging and
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support processes.
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Inline data
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-----------
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By default, all CephFS file data is stored in RADOS objects. The inline data
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feature enables small files (generally <2KB) to be stored in the inode
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and served out of the MDS. This may improve small-file performance but increases
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load on the MDS. It is not sufficiently tested to support at this time, although
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failures within it are unlikely to make non-inlined data inaccessible
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Inline data has always been off by default and requires setting
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the ``inline_data`` flag.
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Mantle: Programmable Metadata Load Balancer
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-------------------------------------------
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Mantle is a programmable metadata balancer built into the MDS. The idea is to
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protect the mechanisms for balancing load (migration, replication,
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fragmentation) but stub out the balancing policies using Lua. For details, see
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:doc:`/cephfs/mantle`.
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Snapshots
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---------
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Like multiple active MDSes, CephFS is designed from the ground up to support
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snapshotting of arbitrary directories. There are no known bugs at the time of
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writing, but there is insufficient testing to provide stability guarantees and
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every expansion of testing has generally revealed new issues. If you do enable
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snapshots and experience failure, manual intervention will be needed.
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Snapshots are known not to work properly with multiple filesystems (below) in
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some cases. Specifically, if you share a pool for multiple FSes and delete
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a snapshot in one FS, expect to lose snapshotted file data in any other FS using
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snapshots. See the :doc:`/dev/cephfs-snapshots` page for more information.
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For somewhat obscure implementation reasons, the kernel client only supports up
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to 400 snapshots (http://tracker.ceph.com/issues/21420).
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Snapshotting was blocked off with the ``allow_new_snaps`` flag prior to Mimic.
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Multiple filesystems within a Ceph cluster
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------------------------------------------
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Code was merged prior to the Jewel release which enables administrators
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to create multiple independent CephFS filesystems within a single Ceph cluster.
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These independent filesystems have their own set of active MDSes, cluster maps,
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and data. But the feature required extensive changes to data structures which
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are not yet fully qualified, and has security implications which are not all
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apparent nor resolved.
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There are no known bugs, but any failures which do result from having multiple
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active filesystems in your cluster will require manual intervention and, so far,
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will not have been experienced by anybody else -- knowledgeable help will be
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extremely limited. You also probably do not have the security or isolation
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guarantees you want or think you have upon doing so.
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Note that snapshots and multiple filesystems are *not* tested in combination
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and may not work together; see above.
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Multiple filesystems were available starting in the Jewel release candidates
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but must be turned on via the ``enable_multiple`` flag until declared stable.
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LazyIO
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------
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LazyIO relaxes POSIX semantics. Buffered reads/writes are allowed even when a
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file is opened by multiple applications on multiple clients. Applications are
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responsible for managing cache coherency themselves.
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Previously experimental features
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================================
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Directory Fragmentation
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-----------------------
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Directory fragmentation was considered experimental prior to the *Luminous*
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(12.2.x). It is now enabled by default on new filesystems. To enable directory
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fragmentation on filesystems created with older versions of Ceph, set
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the ``allow_dirfrags`` flag on the filesystem:
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::
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ceph fs set <filesystem name> allow_dirfrags 1
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Multiple active metadata servers
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--------------------------------
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Prior to the *Luminous* (12.2.x) release, running multiple active metadata
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servers within a single filesystem was considered experimental. Creating
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multiple active metadata servers is now permitted by default on new
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filesystems.
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Filesystems created with older versions of Ceph still require explicitly
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enabling multiple active metadata servers as follows:
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::
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ceph fs set <filesystem name> allow_multimds 1
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Note that the default size of the active mds cluster (``max_mds``) is
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still set to 1 initially.
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