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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.
+