From other comments, it sounds like it is defragmenting a database file which is normally only a few hundred MB max. However, defragmenting this file means it essentially writes a duplicate of that file, switches over to the new file, and deletes the old one. So, it uses a lot of writes, but its writing the same couple hundred MB over and over again, with the total usage only being 2X the max size of the file, assuming no fragmentation. If there is a lot of fragmentation, that multiplier would be less than 2.