Hi Didier,
I actually have a jffs2 filesystem on another part of the flash
memory, but I decided not to use it for the purpose, for one reason: A
single stupid bug might make the device non-bootable. Because, even if
the jffs2 or squashfs is initially mounted read-only, the main reason to
use it is the possibility to remount it read-write and modify it. You
see the point? Better work hard in the beginning to work out a good
stable initramfs which nobody can change/corrupt.
Having an unmodifiable rootfs is indeed a very important guarantee to
have. But having it on the flash device doesn't mean it's a good idea
to remount it read-write ! As you say, your rootfs should remain
read-only all the time - that's why I traditionally used cramfs or
squashfs. You can partition the flash to contain both your read-only
rootfs and some jffs2 containing your read-write data.
With the initramfs, you restart always from a clean situation (once
debugged of course). And the userland in my initramfs has a possibility
to escape the normal sequence and start an interactive session, which
allows to debug the environment.
Same thing with a read-only rootfs stored in flash, with the additional
benefit that your debug environment doesn't have to be loaded into RAM
if you don't need it. Put a full recovery system in your rootfs, make it
as big and friendly as will fit in half your flash (to leave room for
firmware upgrades!), don't get constrained by RAM.
By the way, you wrote that initramfs is a trap. What is the trap,
is it initramfs or the need for swhtch_root? Is it different if I
switch_root from another filesystem?
initramfs is not a lethal trap, it's just that in my experience, using
it will end up making you work more than not using it, for several
reasons. switch_root is one of those reasons; if you're booting from
another filesystem, nothing forces you to switch_root. The simplest,
easiest design is to directly boot on your real rootfs - and whenever
you need to change roots, pivot_root is a lot friendlier than
switch_root, letting you keep working, open fds on the old root and
clean up later.
--
Laurent
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