Disk optimiser extensions
Disk optimisers, or "defraggers", typically work by putting the partition off-line so that it can carry out its job uninterrupted. The problem with this is that you cannot access your data until the optimiser has finished. You cannot even stop the process part way through.
PFS-2 is different. There is a defined interface so that PFS-2 and the optimiser can share the disk.
This means that you can access your data while the disk is being optimised. You can even create, update or delete files. I think it is wonderful to think of a file being optimised while it is being written. It is possible.