From mboxrd@z Thu Jan  1 00:00:00 1970
From: Hendrik Levsen <hendrik-j5CO6tLloWodnm+yROfE0A@public.gmane.org>
Subject: Re: couldn't mount because of unsupported optional features
 (477e7ad1e859f753)
Date: Fri, 31 Dec 2021 09:51:23 +0000
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To: linux-nilfs-u79uwXL29TY76Z2rM5mHXA@public.gmane.org

On 30/12/21 11:00 pm, Peter Grandi wrote:
>> Trying to mount a NILFS partition
> To be pedantic, but it matters, it is "block device" in UNIX-like
> systems, and more "NILFS2 instance", as here could be multiple
> NILFS2 instances even in a single block device (but that is a
> very rare setup usually requiring 'losetup' mounts).

Correct, being sloppy here in my terminology, it's a block device which
is a RAID-1 (+ dmcrypt/luks) which gives me confidence that the
underlying hardware is ok.

>> fails with "couldn't mount because of unsupported optional
>> features (477e7ad1e859f753)". [...]
> That does not look a lucky situation. You can use 'lscp
> /dev/...'  to list the checkpoints and try to mount an older
> checkpoint with 'mount -t nilfs2 -o cp=... /dev/... ...' to
> mount it and resume work from that. In theory older checkpoints
> will be fully consistent even if the latest one is corrupted.

Thanks Peter, it seems both lscp and mount -o cp need a functioning
super block though.

> Unless that  message means that the NILFS2 instance is corrupted
> because of "issues" (usually hardware, most common with block
> devices on USB storage devices).

I might dig into this a little deeper, the data isn't that important but
gaining a correct understanding of NILFS working principles is. My
understanding so far was that it's quite hard for data to become
entirely inaccessible.

This looks like a good idea, linear scan for segment nodes:
https://www.spinics.net/lists/linux-nilfs/msg02198.html Could be the
start of the fsck that never happened.

-- h.