Widening ext4's readdir() cookie

E.g. why not make the order a ring? Store last created ID with the directory. Use an index into that order that is easily capable of answering "What is the next value in the order?" - tree indices typically are capable of this.

To create an entry:

- locate the ID in your ordered index (B-tree, whatever)
- Determine the next free ID by walking the index to the next existing ID
- Create your entry with that free ID, update your index
- Update the directory's last created ID

Insertions have to be done atomically wrt directory and index and any readers. Removals do not need to modify the last-created ID in the directory though. But it does not matter a jot how the internal structure of the index changes.

No collisions, no hashes, no encoding the structure of indices into cookies. No nasty hacks. Just work with the grain by making the underlying order you're indexing have the properties you want.

You are limited to # of directory entries being the size of the ring, but it seems NFS compatibility would allow that ring to be (2^64)-1.

Why wouldn't that work?