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List:       busybox
Subject:    Re: [OT] initramfs use cases (was: Why '-c DEV' option for switch_root?)
From:       James B <jamesbond3142 () gmail ! com>
Date:       2013-12-16 15:14:38
Message-ID: 20131216221438.fd1f06c5cb75bb9b8d883862 () gmail ! com
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On Mon, 16 Dec 2013 11:00:34 +0000
Laurent Bercot <ska-dietlibc@skarnet.org> wrote:

> 
> > You are making a very *big* assumption that the kernel can find
> > the real root filesystem without userspace help. In cases where
> > the kernel can't - initramfs/initmpfs is in fact very useful.
> 
> Show me a precise, real-life example, and I'll tell you how I would
> proceed. Or agree with you.

Lauri and Didier have given you good examples - I thought their use cases (PXE / NFS \
root) are immediately obvious as the "classic" need for initramfs. I will add another \
one - running aufs or overlayfs as root filesystem. 

> 
> Actually, Lauri has a point: if the root filesystem has to be in RAM
> (storage-less machines) then yes, initramfs is useful - as a real
> root filesystem; you probably don't even need to switch_root, which
> simplifies things a lot.

Yes. Whether to switch_root or not, it depends on the needs, though - if one wants to \
have persistent changes saved across sessions, it's probably best to switch root to \
network-based filesystem or use some kind of overlayfs I hinted above to do it.

> But as far as machines with some kind of storage go, I've seen my
> fair share of them, and I've never encountered a case where initramfs
> was the best solution. I don't claim to have seen it all, though -
> there might be use cases I'm not aware of.

In this case there are always alternative solutions. In devices with storage where \
the initial root filesystem is always accessible (e.g. in ROM) you can always do your \
initialisation straight in the real root and then pivot_root to do something else if \
need be (this effectively makes the your real root's init script does the work that \
is normally done in initramfs). Whether this is better than doing it in initramfs is \
debatable; but as I said earlier, the  initramfs/initmpfs is indisposable if you need \
to use rootfs which can't be mounted directly by the kernel without userspace help.

-- 
James B <jamesbond3142@gmail.com>
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