Ever try playing with plan9? It persuaded me that, yes, the modern tree filesystem sucks, but not because organizing persistent data into heirarchies is bad, but because you don't always want the physical translation presented to you. Plan9 was pervasively virtual, with an applications filesystem view being radically different than anothers, and with everything accessable through file read / writes (including OS system calls besides open, read, write, and close).
I've never tried Plan9. Doing searches over deep source code hierarchies has been my most recent experience with the inefficiency of the directory tree (nevermind the inefficiency of overly-complex source code). Really, there's no fundamental reason why `find /usr -name foo` has to take all day. While installing software, also, I was struck by how long it took, while it was apparently updating all the filesystem metadata. For the problem of long installs, I had a brainstorm: what if userspace could hand the filesystem an entire archive in a single transaction, and let the filesystem handle materializing it into a directory tree at its leisure? Just give the filesystem a tarball, rather than the application having to piece together the subtree step by tiny step? It would certainly make it more ACID, and it's not like tarfiles are bleeding-edge technology. Maybe that's a good project for someone who wants to hack filesystems.
