(4th par.) A messaging interface is preferable for many reasons, not the least of which being that it makes stacking actually work the way it should work, as independant and opaque elements which stack together to form a whole. For example, with the new API a capability layer could be slapped onto a filesystem that otherwise doesn't implement one of its own, and the enduser would not know the difference. Filesytems are almost universally self-contained entities. A message-based API would allow these entities to run in userspace for debugging or even in a deployment when one absolutely cannot afford a crash. Why run msdosfs or cd9660 in the kernel and risk a crash when it would operate just as well in userland?
