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 invisible 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.
The root directory cannot be fragmented.
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
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 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.
A directory fragment is eligible for splitting when its size exceeds
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).
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 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
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 directory entries.
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
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.
mds_bal_split_rd threshold is applied based on the read operation
load counter, which tracks readdir operations.
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 (
a spike in activity may cause a directory to stay fragmented
forever unless some entries are unlinked.