What makes a good filesystem?

I've got a vague itch to write a file system, for an academic exercise
if nothing else, and I'm curious about what good filesystem design is
all about. Particularly any properties that a filesystem design
must have and/or should have to pass muster with the fs guys, and doubly
particular how important these properties are.

Besides the usual preemption safety/locking/smp awareness and all that
stuff that's a given for kernel code in general, what are good
principles to code for when you're designing a filesystem?

In particular what I'm curious about includes but is by no means limited
to the following:
1. POSIX compliance (given, probably trivial to research)
2. Integrity in the presence of catastrophic interruptions (crashes,
kernel panics, power outages, random reboots) (I'm guessing a Good
Thing)
3. Performance (given)
4. Robustness in the presence of flaky media (unsure how mandatory or
required this is)
5. Anything I missed

I'm intrigued by btrfs and tux3, and also a tad envious of the brains
behind them.


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