In this paper I have described DMFS, a layered file system developed for NetBSD to support a tertiary storage system. I have briefly described NAStore 3, the storage system of which it is a part, and described DMFS in more detail. I have described the changes to NetBSD needed to support this work. I have given an overview of how a system process interacts with the DMFS layer to archive and restore files, and I have touched on how some typical operations are affected by the DMFS layer. Finally, I presented some simple performance measurements which indicate that while DMFS might impose a performance degradation, it is not significant and that the rule of thumb value of 1 to 2 percent is probably a reasonable upper bound for the performance penalty.