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Merge pull request #46950 from dparmar18/fix_dirfrags_doc

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doc/cephfs/dirfrags: clarify the unit of threshold limits

Reviewed-by: Anthony D'Atri <anthony.datri@gmail.com>
Reviewed-by: Venky Shankar <vshankar@redhat.com>
Reviewed-by: Rishabh Dave <ridave@redhat.com>
Reviewed-by: Kotresh HR <khiremat@redhat.com>
Reviewed-by: Milind Changire <mchangir@redhat.com>
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Venky Shankar 2022-07-12 12:26:45 +05:30 committed by GitHub
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doc/cephfs/dirfrags.rst
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@ -46,8 +46,8 @@ Size thresholds
===============
A directory fragment is eligible 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
``mds_bal_split_size`` (default 10000 directory entries). 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).
@ -58,14 +58,15 @@ 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.
100000 directory entries. 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 eligible 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.
to avoid when merging. The default merge size is 50 directory entries.
Activity thresholds
===================
@ -79,16 +80,19 @@ 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
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.
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.
The ``mds_bal_split_rd`` and ``mds_bal_split_wr`` configs represent the
popularity threshold. In the MDS these are measured as "read/write temperatures"
which is closely related to the number of respective read/write operations.
By default, the read threshold is 25000 operations and the write
threshold is 10000 operations, 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