Dave Chinner [off-list ref] wrote:

quoted

What I'd like to do is remove the fanout directories, so that for each logical
"volume"[*] I have a single directory with all the files in it.  But that
means sticking massive amounts of entries into a single directory and hoping
it (a) isn't too slow and (b) doesn't hit the capacity limit.

Note that if you use a single directory, you are effectively single
threading modifications to your file index. You still need to use
fanout directories if you want concurrency during modification for
the cachefiles index, but that's a different design criteria
compared to directory capacity and modification/lookup scalability.

I knew there was something I was overlooking.  This might be a more important
criterion.  I should try benchmarking this, see what difference it makes
eliminating the extra lookup step (which is probably cheap) versus the
concurrency.

David