Over time (and it doesn't take long) every Domino Server I have used on Windows begins to have a heavily fragmented file system. Most defrag vendors have solutions for Exchange and SQL Server, none for Domino.
Shutting down Domino to defrag its drives isn't a good solution in ...
In relation to the defragmentation idea, allow for compact -c to actually ADD white space. This can help to ensure that the database can have some "room for growth" and alleviate some disk concerns.