linux-kernel - Re: Linux mdadm superblock question.

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Message-Id: <201002172229.07511.elendil@planet.nl>
Date:	Wed, 17 Feb 2010 22:29:05 +0100
From:	Frans Pop <elendil@...net.nl>
To:	Gabor Gombas <gombasg@...ikabel.hu>
Cc:	Rudy Zijlstra <rudy@...mpydevil.homelinux.org>,
	kyle@...fetthome.net, neilb@...e.de, babydr@...y-dragons.com,
	davidsen@....com, volkerarmin@...glemail.com,
	mjevans1983@...il.com, linux-kernel@...r.kernel.org,
	linux-raid@...r.kernel.org
Subject: Re: Linux mdadm superblock question.

On Wednesday 17 February 2010, Gabor Gombas wrote:
> On Wed, Feb 17, 2010 at 02:26:46PM +0100, Frans Pop wrote:
> > That's simply not true, at least not for Debian. If you actually use
> > the distro tools [1] the only assumptions are made at kernel
> > *installation* time, not at kernel build time.
>
> And that's why network-booted diskless clients and virtual guests have
> all sort of useless modules loaded; the HW where the kernel package was
> installed in this case is very different from the HW where the kernel
> will run.

Interesting use case. But also a use case for which initramfs-tools 
probably very simply was never intended.

I agree that in this case you probably want to either
- have a very generic initrd that supports anything (Debian default) [1]
or
- provide a list of modules to include in the initrd based on knowing the
  hardware you want to support (e.g. using /etc/initramfs-tools/modules)
  and prevent including anything based on the host system.

I've never really done that so I don't know if initramfs-tools has any 
features to support that.

> If only there were a switch to prohibit ever looking at the 
> current machine's configuration when generating the initramfs...

Did you ever file a wishlist bug report for that?

> > I've been using initramfs-tools generated initrds for years without
> > problems, and that includes "root on LVM on LUKS encrypted partition"
> > and "root on LVM on RAID" setups.
>
> I've tried a couple of times to use a Debian-built initramfs with a
> custom built kernel. The kernel worked fine without an initramfs (it had
> everything built in), but it did not boot with the initramfs.

It's obviously hard to comment on something like this without more details 
(which would be off-topic for this list).


[1] Could still leave you with problems if the clients use something fancy 
for the root fs that uses info copied from /etc.
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