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

[Chris Gianelloni]

On Thu, 2005-05-19 at 11:37 -0400, Paul Smith wrote:
> 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'm just curious why this needs to be done within the initrd, at all.  I
guess it could be, but I could also see it working outside the initrd.

Anyway, if we were to do it, we would allow for mounting a USB key or
other such device.  Most likely, it would be via a unionfs=/dev/$blah
option, that would mount that device, along with the CD, as a unionfs.
One thing I probably would *not* do is make the entire CD a unionfs, as
I just think it would be a waste of energy and would make
troubleshooting much harder.  Things like /var, /etc, /root, and /home
would definitely be cool to not only be writable, but also survivable
from a reboot.

> 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.

Having the entire disk modifiable, in many cases, isn't a good idea.

I would prefer that only certain parts be writable.  After all, who
really needs to write to /usr or /bin on a CD?

-- 
Chris Gianelloni
Release Engineering - Strategic Lead/QA Manager
Games - Developer
Gentoo Linux