AFAIK, in NILFS, which is another log structured file system, it comes for free - the garbage collector will reclaim those snapshots over time, though, but there's an easy way to access those snapshots, as long as they are there.
For "rotating rust", I'm far from sure that current file systems are optimized for disks at all. First of all, the "best" block size for a random access medium is one where transfer time=access time. A contemporary hard disk can read about 1MB or seek to another location, so block size should be 1MB (increasing to 2MB next year or so ;-). This should certainly contain a whole bunch of small files when used for small files, but still, access granularity below 1MB doesn't make sense. Second, metadata: For all "normal" use, main memory is big enough to keep all metadata in main memory. Keep a log on the disk where you write your metadata transactions sequentially, and on unmount (and when the log exceeds a certain size), dump the whole metadata on disk. Would make live a lot easier for the spinning rust.
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