chrootable root fs dirs, one mounted at / during boot

Question

Let's say I have a HDD with one partition at /dev/sdb1 with the following contents:

% sudo mount /dev/sdb1 /mnt
% cd /mnt
% ls
osA/ osB/
% ls *

osA:
bin  boot/  dev/  etc/  home/  lib  lib64  lost+found/  mnt/
opt/ proc/  root/ run/  sbin   srv/ sys/   tmp/         usr/  var/

osB:
bin  boot/  dev/  etc/  home/  lib  lib64  lost+found/  mnt/
opt/ proc/  root/ run/  sbin   srv/ sys/   tmp/         usr/  var/

I'd like to somehow have the directory osA on /dev/sdb1 mounted as / during boot through /etc/fstab (possibly) and to be able to change that later to be osB if I need that. I'd like this done on ext4 fs (if possible). Is this doable?

Another way of wording:

I'd like a partition to host multiple "root file system directories" and to be able to mount either directory to be / at boot time.

I always dedicated a whole partition to /. Not sure how to go about it when it's a subdirectory on a partition.

My current /etc/fstab entry (whole partition is dedicated to /):

UUID=0a7baeae-857d-11eb-8dcd-0242ac130003 / ext4 rw,relatime 0 1

I'm running Archlinux.

Motivation:

• Sometimes I have to reinstall the system because of problems with the package manager. Thought that perhaps this approach could be quicker than setting everything up from scratch, with a way to go back to previous setup.
• Seeing this as a possible way to try distributions and package managers by just switching what / is mounted to
• By having only one partition on /dev/sdb I would not have to resize OS and data partitions, could just have data as a "data" folder alongside "osA" and "osB"

Edit1:

If you have suggestions on how I could ask the question more clearly - please let me know.

These questions seem somewhat related although the first one seems to delve into some unrelated things, second received no answer, checking third: 39423 374806 206493 613179

I think I'm looking to see if I can bind-mount / in /etc/fstab once /dev/sdb1 is already mounted somewhere (never done this before, don't know if it will work - can try this and post results - insights welcome).

Edit2:

Realised that to bind-mount I need a mount point first, and can't have one without the / itself.

Answer 1

I think you've dismissed the first question's answer too quickly. I was about to writeup pretty much the same answer. So for the discussion of exactly how to do this I would follow that answer.

Why you need to do this with initramfs...

Most mount points are defined in /etc/fstab. However this is [virtually always] stored on the root file system (/). You can't configure which partition is root in a file stored on root. You need something to configure which partition is mounted as root before /etc/fstab can be read. It's the job of initramfs to mount root (/) and then chroot into it.

It's important to understand your system is already mounting a different file system as it's first root file system, then mounting your chosen root filesystem elsewhere, then calling chroot into that file system.

As the other answer says, there's not [commonly] any configuration option in initramfs to make it chroot into a subdirectory. But it's simple enough to modify the scripts /init runs and thus modify where the script will chroot into.