Explanation/Reference:
Fragmentation is unused disk space. Fragmentation is created when Essbase writes a data block to a new location on disk and leaves unused space in the former location of the data block. Block size increases because data from a data load or calculation is appended to the blocks; the blocks must therefore be written to the end of a data file."
Fragmentation is likely to occur with the following:
Read/write databases that users are constantly updating with data (E)

Databases that execute calculations around the clock

Databases that frequently update and recalculate dense members (B)

Data loads that are poorly designed

Databases that contain a significant number of Dynamic Calc and Store members

Databases that use an isolation level of uncommitted access with commit block set to zero

You can prevent and remove fragmentation:
To prevent fragmentation, optimize data loads by sorting load records based upon sparse dimension

members
Perform an export of the database, delete all data in the database with CLEARDATA, and reload the

export file
Force a dense restructure of the database

Reference Improving Essbase Performance, Eliminating Fragmentation