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List:       gentoo-osx
Subject:    Re: [gentoo-osx] New document: Project targets
From:       Dirk_Schönberger <dirk.schoenberger () sz-online ! de>
Date:       2005-12-16 22:36:28
Message-ID: 00f501c60291$2d02a7a0$14b2a8c0 () rincewind
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> How does the absense of a read-only file-system affect the ability to
> have a union-mount only 'visible' for a specific user or user-process?
> Or is this read-only thing necessary to solve another problem?
> Essential for the union-mount solution to work, is that it can at
> least be *only* visible/available for a given (user) process.  Otherwise
> your system is less different from a progressive system.  Still in that
> case, the union-mount solution might have some advantages, like simple
> repair, and a backup procedure (unmount the union-mount, or restart the
> machine -- assuming you didn't add the union-mount to fstab).

The idea seems to be that you have a "live" version of the host system (real
"/", which is mounted read-only and unioned with
a copy on write system).
Current Darwin only allows .dmg or mounting from an unmounted device. In
both cases the underlying system is
intrinsically read-only, i.e. the union fs is not really needed.
I don't think it is a good idea to unomount / remount read-only your root
file system.
If you clone a existing installation and mount this, I think you have
effectively a progressive system. This approach is mentioned in the
entoo-macos bootstrap howto.

That the union file system is visible to all processes becomes a secondary
problem (I think it just means that you could have only one parallel
Gentoo union / chroot)

The basic problem with the Darwin unionfs implementation is that you have to
have a (read-only) file system in the first place, which you can union to.
As far as I understand the Linux version (which may be only a wishlist entry
resp. a specification, you can do things like

mount folder1 read-only U folder2 read-only U folder 3 read-write (where U
is a concatenation operator)
The FreeBSD soltuins divides these usecases into the actual union (unionfs),
and the mount from a subfolder of a mounted deice (nullfs)



Regards
Dirk



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