Are you getting the best results from DBMT? The answer to that question will depend on the state of the freespace of the Domino Data Directory, as seen by the server OS. In an attempt to avoid fragmenting nsf files during it's compact process DBMT had a pre-allocation feature included, BUT.... this requires clean contiguous freespace. Recently while helping a customer deal with some performance and fragmentation issues we encountered a perfect example of how DBMT does not work well if contiguous freespace is limited or non-existent (unfortunately a very common scenario).
Although we are discussing a Domino V10 server the situation had become very similar to that described in John Paganetti's excellent presentation from Connect 2014 "ID104 - Best Practices for Upgrading IBM Notes and Domino 9.x" which referenced our R&D in this area and included a link to our blog.
Before:- this was Domino's Data directory (D:\). (DBMT was trying, but it was clearly not delivering it's full potential). Notice the largest chunk of contiguous freespace is just 1040 MB, this is all DBMT has to work with and cripples it's potential effectiveness, leading to even more fragmented nsf files.
The Golden Rule Is This:
If the freespace is scrambled then DBMT preallocation is going to produce fragmented files, It can only deliver optimum results if contiguous freespace is available, and that's only going to happen if something is actively defragmenting it. And for that we recommend Defrag.NSF or DDO (shameless product plug).