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SubjectRe: Overlay writable filesystem over r/o partition [long]
Bryn Paul Arnold Jones asks:

> Hmm, wasn't ifs abandonded as too hard to do in kernelspace?

Here are some messages from earlier times it was discussed on linux-
kernel.
___
Trevor Johnson

Date: Mon, 11 Mar 96 12:09:01
From: Ray_Van_Tassle-CRV004@email.mot.com
To: jhenders@bogon.com
Cc: linux-kernel@vger.rutgers.edu
Subject: Union fs

>
> >
> > "union fs"??? What's a union fs?
> > One that costs too much and stops working at the drop of a hat, perhaps?
> >
> A union file system can be mounted over another read 0only file system.
> The files underneath show through until you modify one, at which time
> the midifyed file is written to the union fs. This allows you to, for
> instance, compile a kernel on a mounted cdrom drive, where the object
> files would be written onto the union fs over layed on it.

Ah-----This is how clearcase works (it's the source control utility we use
at work on a Sun).
Date: Thu, 14 Mar 96 15:33:27
From: kjahds@kjahds.com
To: linus.torvalds@cs.helsinki.fi
Cc: bpaj@gytha.demon.co.uk,jhenders@bogon.com, linux-kernel@vger.rutgers.edu
Subject: Re: ext2 in a dos file/partition

On Sat, 9 Mar 1996, Linus Torvalds wrote:

> While at USENIX, I saw the _correct_ way to do a union FS. It was done
> as a pre-loaded shared library, and because of that it was a lot more
> flexible than any kernel implementation would ever be (it was not
> limited to the unix mount global state: it was used more like Plan-9's
> dynamic per-process mounting).

You mean by (effectively) wrapping the libc file I/O functions? This seems
feasible, but I wonder about coherency and race conditions with each process
doing it's own "inheritance", and how the information on inheriting mounts
would be disseminated. (I suppose a deamon using IPC could resolve a lot
of this.)

> After having seen that, I don't think I necessarily would even want a
> kernel implementation. It simply was so much better done in user
> space..

If my guess is right, it sounds like it has it's own problems, but even so it
must be less painful then doing it in kernel space.

> The stuff had been done by Dave Korn (or Korn shell fame (and don't
> flame me if his first name isn't "Dave", my mind is going)) at AT&T.
> Damn, but I can't remember his email address..
>
> Linus

--
Kenneth Albanowski (kjahds@kjahds.com, CIS: 70705,126)

Date: Wed, 27 Nov 1996 23:15:28 +0000 (GMT)
From: Alan Cox <alan@lxorguk.ukuu.org.uk>
To: "Steven N. Hirsch" <shirsch@ibm.net>
Cc: mjr@iki.fi, djc@microwave.com, kaos@edison.dialix.com.au,
linux-kernel@vger.rutgers.edu
Subject: Re: Union file system

> For any interested folks, there is a comprehensive discussion of Union
> file systems in the book "Unix Internals" by Uresh Vahalia
> (Prentice-Hall, 1996. ISBN 0-13-101908-2). This book is well worth
> owning as a reference for some of the deeper technical topics that pop

A union FS btw is _horrible_ to do well. Consider a process with an mmap
of a union FS object that is replaced by a file elsewhere and keeping
the mmap coherency correct. Now truncate the file at the same instant
and enjoy

Alan

Date: 27 Nov 1996 18:00:08 -0600
From: "Eric W. Biederman" <ebiederm@cse.unl.edu>
To: Mikko Rauhala <mjr@iki.fi>
Cc: "D. Chiodo" <djc@microwave.com>, Keith Owens <kaos@edison.dialix.com.au>,
linux-kernel@vger.rutgers.edu
Subject: Re: Union file system


>>>>> "Mikko" == Mikko J Rauhala <mjrauhal@cc.helsinki.fi> writes:

Mikko> On Wed, 27 Nov 1996, D. Chiodo wrote:

>> Symlinks? Mount all the cd, network, disk, etc, then make symlinks from
>> whereyou want you everything to appear to wherever the appropriate files
>> are...

Mikko> Won't cut it. We're talking about a system that would allow you to eg.
Mikko> mount a CD-ROM drive and then mount another filesystem "over" that, so
Mikko> that if you write to the file system, the CD-ROM files are hidden behind
Mikko> the new files on the writable media. Would be rather handy in some
cases...

The cyrus people messing with guile have a hacked nfs server that
(unless the reporting is all wrong) will allow this capability. You
might have to configure it some yourself though. There was a posting
on linux-announce a while ago about this. The claim was that it could
do everything the hurd could do only slower.

Eric
Date: Wed, 27 Nov 1996 18:33:19 -0500 (EST)
From: Greg Alexander <galexand@sietch.bloomington.in.us>
To: linux-kernel@vger.rutgers.edu
Subject: Re: Union file system


I guess no one else really knows about this: Inherited File System. Pitch
in and help fix the bugs, I guess.. :)
ftp://tsx-11.mit.edu/pub/linux/ALPHA/ifs/

Greg Alexander
http://www.cia-g.com/~sietch/

Date: Wed, 27 Nov 1996 14:25:00 -0800 (PST)
From: Michael Neuffer <neuffer@nomis.i-connect.net>
To: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: mjr@iki.fi, djc@microwave.com, kaos@edison.dialix.com.au,
linux-kernel@vger.rutgers.edu
Subject: Re: Union file system


On Wed, 27 Nov 1996, Alan Cox wrote:

>
> > Won't cut it. We're talking about a system that would allow you to eg.
> > mount a CD-ROM drive and then mount another filesystem "over" that, so
> > that if you write to the file system, the CD-ROM files are hidden behind
> > the new files on the writable media. Would be rather handy in some cases...
>
> Sounds like IFS. Im not sure what happened to IFS however.

The original author stopped maintaining it way back after it didn't make
it into the kernel.

Mike

Michael Neuffer i-Connect.Net, a Division of iConnect Corp.
mike@i-Connect.Net 14355 SW Allen Blvd., Suite 140
503.641.8774 Beaverton, OR 97005

Date: 28 Nov 1996 00:31:05 +0100
From: Matthias Urlichs <smurf@noris.de>
To: linux-kernel@vger.rutgers.edu
Subject: Re: Union file system


In linux.dev.kernel, article <m0vSnkq-0004omC@pcpaul.wau.mis.ah.nl>,
paul@wau.mis.ah.nl (Paul Slootman) writes:
>
> When I tried Yggdrasil's stuff a couple of years back, that distribution
> has something called IFS ("Werner's Inherited File System", IIRC). This
>
> I recall that it worked quite well. Anyone know why it didn't get into
> the mainstream kernel?
>
Because it was a total hack (just look at the source code), because it had
unresolved bugs (like failing to free inodes under some unfathomable
circumstances, which meant you were unable to unmount the file systems you
used as source for your IFS mount at last startup), and because Linus
thinks, quite rightly IMHO, that anything you can do with such a beast in
the kernel, you can do even better with a preloaded shared library.

--
Competitive fury is not always anger. It is the true missionary's
courage and zeal in facing the possibility that one's best may not be enough.
--Gene Scott
--
Matthias Urlichs \ noris network GmbH / Xlink-POP N
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