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diff --git a/doc/cephfs/experimental-features.rst b/doc/cephfs/experimental-features.rst new file mode 100644 index 00000000..3344a34d --- /dev/null +++ b/doc/cephfs/experimental-features.rst @@ -0,0 +1,111 @@ + +Experimental Features +===================== + +CephFS includes a number of experimental features which are not fully stabilized +or qualified for users to turn on in real deployments. We generally do our best +to clearly demarcate these and fence them off so they cannot be used by mistake. + +Some of these features are closer to being done than others, though. We describe +each of them with an approximation of how risky they are and briefly describe +what is required to enable them. Note that doing so will *irrevocably* flag maps +in the monitor as having once enabled this flag to improve debugging and +support processes. + +Inline data +----------- +By default, all CephFS file data is stored in RADOS objects. The inline data +feature enables small files (generally <2KB) to be stored in the inode +and served out of the MDS. This may improve small-file performance but increases +load on the MDS. It is not sufficiently tested to support at this time, although +failures within it are unlikely to make non-inlined data inaccessible + +Inline data has always been off by default and requires setting +the ``inline_data`` flag. + +Mantle: Programmable Metadata Load Balancer +------------------------------------------- + +Mantle is a programmable metadata balancer built into the MDS. The idea is to +protect the mechanisms for balancing load (migration, replication, +fragmentation) but stub out the balancing policies using Lua. For details, see +:doc:`/cephfs/mantle`. + +Snapshots +--------- +Like multiple active MDSes, CephFS is designed from the ground up to support +snapshotting of arbitrary directories. There are no known bugs at the time of +writing, but there is insufficient testing to provide stability guarantees and +every expansion of testing has generally revealed new issues. If you do enable +snapshots and experience failure, manual intervention will be needed. + +Snapshots are known not to work properly with multiple filesystems (below) in +some cases. Specifically, if you share a pool for multiple FSes and delete +a snapshot in one FS, expect to lose snapshotted file data in any other FS using +snapshots. See the :doc:`/dev/cephfs-snapshots` page for more information. + +For somewhat obscure implementation reasons, the kernel client only supports up +to 400 snapshots (http://tracker.ceph.com/issues/21420). + +Snapshotting was blocked off with the ``allow_new_snaps`` flag prior to Mimic. + +Multiple filesystems within a Ceph cluster +------------------------------------------ +Code was merged prior to the Jewel release which enables administrators +to create multiple independent CephFS filesystems within a single Ceph cluster. +These independent filesystems have their own set of active MDSes, cluster maps, +and data. But the feature required extensive changes to data structures which +are not yet fully qualified, and has security implications which are not all +apparent nor resolved. + +There are no known bugs, but any failures which do result from having multiple +active filesystems in your cluster will require manual intervention and, so far, +will not have been experienced by anybody else -- knowledgeable help will be +extremely limited. You also probably do not have the security or isolation +guarantees you want or think you have upon doing so. + +Note that snapshots and multiple filesystems are *not* tested in combination +and may not work together; see above. + +Multiple filesystems were available starting in the Jewel release candidates +but must be turned on via the ``enable_multiple`` flag until declared stable. + +LazyIO +------ +LazyIO relaxes POSIX semantics. Buffered reads/writes are allowed even when a +file is opened by multiple applications on multiple clients. Applications are +responsible for managing cache coherency themselves. + +Previously experimental features +================================ + +Directory Fragmentation +----------------------- + +Directory fragmentation was considered experimental prior to the *Luminous* +(12.2.x). It is now enabled by default on new filesystems. To enable directory +fragmentation on filesystems created with older versions of Ceph, set +the ``allow_dirfrags`` flag on the filesystem: + +:: + + ceph fs set <filesystem name> allow_dirfrags 1 + +Multiple active metadata servers +-------------------------------- + +Prior to the *Luminous* (12.2.x) release, running multiple active metadata +servers within a single filesystem was considered experimental. Creating +multiple active metadata servers is now permitted by default on new +filesystems. + +Filesystems created with older versions of Ceph still require explicitly +enabling multiple active metadata servers as follows: + +:: + + ceph fs set <filesystem name> allow_multimds 1 + +Note that the default size of the active mds cluster (``max_mds``) is +still set to 1 initially. + |