Re: Dracut -- Cross distribution initramfs infrastructure

[Jeremy Katz]

On Dec 19, 2008, at 10:27 AM, Theodore Tso wrote:
> On Fri, Dec 19, 2008 at 02:55:26PM +0100, Hannes Reinecke wrote:
> > The goal of the initrd is to activate and mount the root fs.
> > And the root fs _only_. Every other system should be configured
> > once the main system is running.
>
> Don't forget resuming from hibernation....

I haven't, although I haven't sat down to implement it yet.

> And of course, activating and mounting the root filesystem can be
> quite complicated --- it can involve loading driver modules,
> activiating md and/or lvm, prompting for a password, setting up
> networking (dhcp, routing, dns) for iSCSI and/or NFS/AFS/Lustre/et.al,
> the equivalent setup for Fiber Channel attached disks, etc.  If
> there's any cryptography involved, the user may need to be prompted
> for a password and/or key and/or fingerprint scan to unlock TPM unit
> to access the key, etc.

Well, driver modules should be being loaded by udev.  Period.  If  
something requires a manual modprobe, that's a bug IMHO.  The other  
stuff, while non-trivial, is surprisingly doable from udev rules.

> There may also be times when it is useful to operate on the root
> filesystem in some way before it is mounted; in most cases the
> operation can bedone on a filesystem mounted read-only, yes --- but at
> the cost of needing to reboot afterwards if the root filesystem needs
> to be modified by said userspace tool.

I think that once you start getting into this realm, though, you end  
up with an incredibly over-complicated and slow initramfs.  If we  
instead focus on keeping things "fast", the reboot afterwards isn't  
that costly.

> Finally, note that part the discussion at the Kernel Summit, and also
> what David Jones was looking to work at, was to do something that
> could included as part of the kernel sources.  The idea is that as
> responsibility for early boot is moved from the kernel, an mkinitramfs
> which is fixed and distributed by the distribution might not work with
> a newer kernel.org kernel.  So the idea that was explored was adding a
> common mkinitramfs with basic functionality into kernel sources, with
> the ability for distributions to add various "value add" enhancements
> if they like.  This way if the kernel wants to move more functionality
> (for example, in the area of resuming from hibernation) out of the
> kernel into initramfs, it can do so without breaking the ability of
> older distributions from being able to use kernel.org kernels.
>
> So IMHO, it's important not only that the distributions standardize on
> a single initramfs framework, but that framework get integrated into
> the kernel sources.

Yeah, Dave and I have talked a fair bit about that.  It's just  
significantly easier to get something going _outside_ of the kernel  
sources and then work towards integrating it.  The plus side of  
integrating it is that the existing bits to generate a built-in  
initramfs can go away.

Jeremy
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