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

[Chris Gianelloni]

On Wed, 2005-05-25 at 13:54 -0400, Paul Smith wrote:
> %% Chris Gianelloni <wolf31o2@g.o> writes:
>
>   cg> I'm just curious why this needs to be done within the initrd, at
>   cg> all.  I guess it could be, but I could also see it working outside
>   cg> the initrd.
>
> If you do it after initrd, you'll have to be very, very careful.
> "Outside" the initrd you've invoked the real init, and it's starting to
> bring up the system.  If you haven't already set up the unionfs before
> that, you better be darn sure you do it immediately... if not you will
> have applications coming up with the wrong configs, failing because they
> can't write to /etc and/or /var, etc.

That's a good point.  I was just thinking that by holding off until a
bit later, it allows us to have our full module list, rather than the
limited subset provided in the initrd.

Essentially, anything that we can use to boot a CD from, we can use as a
unionfs device.  This would include IDE/SCSI/SATA/USB/Firewire devices.

>   cg> Anyway, if we were to do it, we would allow for mounting a USB key
>   cg> or other such device.  Most likely, it would be via a
>   cg> unionfs=/dev/$blah option, that would mount that device, along
>   cg> with the CD, as a unionfs.
>
> Yes.  I have this working right now, although it just uses a ramdisk
> today.  Obviously using my current method, in order to use a keydisk or
> maybe even a file on a FAT partition which contains an ext2 partition
> and is mounted as a loop FS, you would need to add smarts to the initrd
> (linuxrc) to detect the hardware and mount those partitions.

Right.  This would take more logic in the initrd.

>   cg> One thing I probably would *not* do is make the entire CD a
>   cg> unionfs, as I just think it would be a waste of energy and would
>   cg> make troubleshooting much harder.
>
> I'm not sure what you mean by "not make the entire CD a unionfs"...?  Do
> you just mean that some root directories would not link to the unionfs
> but instead would go to the cdrom?  Or...?

Correct.  I don't really see a point in having the entire CD be
writable.  I can understand it for configuration directories and such,
but nobody really needs to write to /bin or /usr on a CD.

>   cg> Things like /var, /etc, /root, and /home would definitely be cool
>   cg> to not only be writable, but also survivable from a reboot.
>
> Yes, that's the ultimate goal of course.

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