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-
-===================================
-Configuring Directory fragmentation
-===================================
-
-In CephFS, directories are *fragmented* when they become very large
-or very busy.  This splits up the metadata so that it can be shared
-between multiple MDS daemons, and between multiple objects in the
-metadata pool.
-
-In normal operation, directory fragmentation is invisbible to
-users and administrators, and all the configuration settings mentioned
-here should be left at their default values.
-
-While directory fragmentation enables CephFS to handle very large
-numbers of entries in a single directory, application programmers should
-remain conservative about creating very large directories, as they still
-have a resource cost in situations such as a CephFS client listing
-the directory, where all the fragments must be loaded at once.
-
-All directories are initially created as a single fragment.  This fragment
-may be *split* to divide up the directory into more fragments, and these
-fragments may be *merged* to reduce the number of fragments in the directory.
-
-Splitting and merging
-=====================
-
-An MDS will only consider doing splits and merges if the ``mds_bal_frag``
-setting is true in the MDS's configuration file, and the allow_dirfrags
-setting is true in the filesystem map (set on the mons).  These settings
-are both true by default since the *Luminous* (12.2.x) release of Ceph.
-
-When an MDS identifies a directory fragment to be split, it does not
-do the split immediately.  Because splitting interrupts metadata IO,
-a short delay is used to allow short bursts of client IO to complete
-before the split begins.  This delay is configured with
-``mds_bal_fragment_interval``, which defaults to 5 seconds.
-
-When the split is done, the directory fragment is broken up into
-a power of two number of new fragments.  The number of new
-fragments is given by two to the power ``mds_bal_split_bits``, i.e.
-if ``mds_bal_split_bits`` is 2, then four new fragments will be
-created.  The default setting is 3, i.e. splits create 8 new fragments.
-
-The criteria for initiating a split or a merge are described in the
-following sections.
-
-Size thresholds
-===============
-
-A directory fragment is elegible for splitting when its size exceeds
-``mds_bal_split_size`` (default 10000).  Ordinarily this split is
-delayed by ``mds_bal_fragment_interval``, but if the fragment size
-exceeds a factor of ``mds_bal_fragment_fast_factor`` the split size,
-the split will happen immediately (holding up any client metadata
-IO on the directory).
-
-``mds_bal_fragment_size_max`` is the hard limit on the size of
-directory fragments.  If it is reached, clients will receive
-ENOSPC errors if they try to create files in the fragment.  On
-a properly configured system, this limit should never be reached on
-ordinary directories, as they will have split long before.  By default,
-this is set to 10 times the split size, giving a dirfrag size limit of
-100000.  Increasing this limit may lead to oversized directory fragment
-objects in the metadata pool, which the OSDs may not be able to handle.
-
-A directory fragment is elegible for merging when its size is less
-than ``mds_bal_merge_size``.  There is no merge equivalent of the
-"fast splitting" explained above: fast splitting exists to avoid
-creating oversized directory fragments, there is no equivalent issue
-to avoid when merging.  The default merge size is 50.
-
-Activity thresholds
-===================
-
-In addition to splitting fragments based
-on their size, the MDS may split directory fragments if their
-activity exceeds a threshold.
-
-The MDS maintains separate time-decaying load counters for read and write
-operations on directory fragments.  The decaying load counters have an
-exponential decay based on the ``mds_decay_halflife`` setting.
-
-On writes, the write counter is
-incremented, and compared with ``mds_bal_split_wr``, triggering a 
-split if the threshold is exceeded.  Write operations include metadata IO
-such as renames, unlinks and creations. 
-
-The ``mds_bal_split_rd`` threshold is applied based on the read operation
-load counter, which tracks readdir operations.
-
-By the default, the read threshold is 25000 and the write threshold is
-10000, i.e. 2.5x as many reads as writes would be required to trigger
-a split.
-
-After fragments are split due to the activity thresholds, they are only
-merged based on the size threshold (``mds_bal_merge_size``), so 
-a spike in activity may cause a directory to stay fragmented
-forever unless some entries are unlinked.
-