As previously discussed and referenced by others and IBM, DBMT does a much better job. It tries to create a contiguous NSF file, but it can ONLY do that it the freespace on the server is not highly fragmented, and the only way that is going to happen is if you have taken steps to keep it in shape (for example by using DDO or Defrag.NSF). We have covered this before, but here is a recent real world example.
It starts off with a 600MB NSF that is in one piece (thanks to DDO)
DBMT runs on the database (this server has over 1TB of fragmented freespace)
And now we see the results. What was once a single fragment is now 567 fragments.
So in summary DBMT does a MUCH better job than compact -c, but it can still make a mess of things and that is why in DDO we have wrapped DBMT with a bunch of smarts and then incorporated Defrag.NSF in the deal. The results are and will always be much much better.
DDO - Domino Optimized.
Note: Example server was Windows Domino 9.01 FP7 Server, 2.5TB of data. It was a VM using Enterprise SAN storage.
Thanks for reading.