[Linux-cachefs] How capacious and well-indexed are ext4, xfs and btrfs directories?

[David Howells]

Dave Chinner <david at fromorbit.com> wrote:

> > 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