Unionfs (was: Re: [gentoo-catalyst] Keeping a test environment using catalyst)

[Paul Smith]

%% Chris Gianelloni <wolf31o2@g.o> writes:

  cg> That being said, I would love to see some work being done by
  cg> somebody on poking around with unionfs.  I think it would be very
  cg> good for the upcoming LiveCD/Installer disk, as it would allow
  cg> people to configure their settings, then transfer them to the
  cg> running system once the installation is complete.

So, I've compared the Gentoo initrd with the Knoppix 3.8.2
(unionfs-based) initrd.  The Knoppix init is different in some
interesting ways: for one thing they never pivot_root, nor chroot or
anything.  They basically use the same / directory, that was created by
the kernel during initrd, for the entire lifetime of the system.

What they do, basically, is this (I've stripped out a lot of detail of
course):

    mkdir /ramdisk /UNIONFS

    mount -o tmpfs /ramdisk /ramdisk
    mount -o unionfs -o dirs=/ramdisk=rw:/cdrom=ro /UNIONFS /UNIONFS

Now they have a unionfs filesystem at /UNIONFS, which has at the top a
ramdisk (really located in /ramdisk) and underneath that the cdrom
contents (really located in /cdrom).

They then proceed to, essentially, make everything in / be a symlink
pointing into /UNIONFS (obviously there are various complexities
involved here).

Then they just exec the real init.

I don't know why they don't use pivot_root; that seems cleaner to me.
But, maybe something about unionfs doesn't allow pivot_root to work.
I'll keep poking at this.


I _thought_ that Knoppix allowed you to make the top of the unionfs an
actual device, like a USB keydisk or something, instead of a ramdisk,
but offhand I don't see anything in their initrd that does anything like
that: it seems hardcoded to go to a ramdisk as far as I can tell.  I'll
make sure of this; to me that's the real benefit of unionfs: to use it
to customize/personalize a generic livecd, and make those customizations
persistent.  If you don't have this I'm not sure what the advantage of
unionfs is over the more traditional methods, like Gentoo's.  I guess
the only real advantage is that the entire disk is modifiable rather
than just specific areas.

-- 
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 Paul D. Smith <psmith@××××××.com>           HASMAT--HA Software Mthds & Tools
 "Please remain calm...I may be mad, but I am a professional." --Mad Scientist
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   These are my opinions---Nortel Networks takes no responsibility for them.
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