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Subject: bug#46905: Add a note about ext4's dir_index to the manual
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From: Ludovic =?UTF-8?Q?Court=C3=A8s?= <ludo@gnu.org>
References: <87v9a89c8v.fsf@nckx>
Date: Thu, 04 Mar 2021 23:01:26 +0100
In-Reply-To: <87v9a89c8v.fsf@nckx> (Tobias Geerinckx-Rice's message of "Wed,
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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 i=
t hashes directory entries and... just
dies when the hash table gets =E2=80=98full=E2=80=99). Try =E2=80=98tune2fs=
 -E "hash_alg=3Dtea" /dev/foo=E2=80=99 to select a different hash algo, or
=E2=80=98tune2fs -O "^dir_index" /dev/foo=E2=80=99 to disable it completely=
 if that doesn't help.
--8<---------------cut here---------------end--------------->8---

Really?  I=E2=80=99ve almost always used =E2=80=98dir_index=E2=80=99 in my =
Guix + NixOS days,
and IIRC store accesses were much slower before that was a thing.

On berlin, /gnu is a 37=C2=A0TiB file system with =E2=80=98dir_index=E2=80=
=99 turned on and
it works well.

I don=E2=80=99t think we can claim there=E2=80=99s a design flaw.  WDYT?

Ludo=E2=80=99.