bug#46905: Add a note about ext4's dir_index to the manual

["Ludovic Courtès" <ludo@gnu.org>]

Hi!

Tobias Geerinckx-Rice <me@tobias.gr> skribis:

> Mainly a reminder to myself to do so.
>
> Guix can break ext4, and more than once has, with a inscrutable error
> message.  We should document it explicitly.
>
> See <http://logs.guix.gnu.org/guix/2021-03-03.log#220037>.

You wrote:

--8<---------------cut here---------------start------------->8---
<nckx>OK, that's the culprit (it's common: ext4 has a design flaw in that it hashes directory entries and... just
dies when the hash table gets ‘full’). Try ‘tune2fs -E "hash_alg=tea" /dev/foo’ to select a different hash algo, or
‘tune2fs -O "^dir_index" /dev/foo’ to disable it completely if that doesn't help.
--8<---------------cut here---------------end--------------->8---

Really?  I’ve almost always used ‘dir_index’ in my Guix + NixOS days,
and IIRC store accesses were much slower before that was a thing.

On berlin, /gnu is a 37 TiB file system with ‘dir_index’ turned on and
it works well.

I don’t think we can claim there’s a design flaw.  WDYT?

Ludo’.