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Re: Allow Using unionfs Early at Boot

From: olafBuddenhagen
Subject: Re: Allow Using unionfs Early at Boot
Date: Fri, 3 Apr 2009 08:11:58 +0200
User-agent: Mutt/1.5.18 (2008-05-17)


> Date: Wed, 01 Apr 2009 19:11:24 +0200 From: "Michael M. Behman"
> <michael.behman@gmail.com>

> I wanted to subscribe to GNU/Hurd in GSOC2009, so I had some questions
> about the "Allow Using unionfs Early at Boot
> <http://www.gnu.org/software/hurd/community/gsoc/project_ideas/unionfs_boot.html>"
> project.

Unfortunately, among the tasks on the ideas list, this is one of those I
know least about... When I added it last year, I had hoped that the
person who worked on this last would add some details; but he didn't :-(

> first of all, what seems to be the obstacles?

I can't really tell what the obstacle is, because I don't really know
how this should be done in the first place...

I guess one possible approach would be running unionfs itself as the
bootstrap filesystem. I can see at least two problems with that: One is
that only libdiskfs contains the special handling necessary for
bootstrap filesystems, while libnetfs-based translators like unionfs
presently don't have it. This shouldn't be too hard to fix, though.

The other problem is that unionfs itself needs access to the "real"
ext2fs where the files actually live... So that would have to be
launched somehow along with the unionfs serving as bootstrap filesystem.

Another possible approach might be to use ext2fs as the bootstrap
filesystem, but then launch unionfs very early in the boot process
somehow, so the rest of the system sees the unioned filesystem only.
This requires finding some way to launch unionfs very early, before most
of the usual system initialisation; and also making sure that a
translator like unionfs can actually be started at such an early stage,
when the normal environment is not in place yet.

I don't know enough about the bootstrap process to go into any more

> How much do I have to know to work on this project?

You will definitely need to gain a very good understanding of the
bootstrap process, and about translator startup in general.

Note that you should have enough time for that during GSoC; but you
definitely need to start digging into it *now*, so that before the end
of the student selection phase you already will have made some progress,
and we can be confident that you will be able to figure it all out in

> If there are any documentations, can anyone tell me where to find
> them?

IIRC there is a rough description of the boot process somewhere on the
net, but it is rather superficial. (Don't know the URL off-hand.)

I guess there is no other option really than reading the relevant code,
and asking some smart questions where necessary...


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