linux-kernel - Re: [RFC 0/7] [RFC] cramfs: fake write support

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Date:	Mon, 2 Jun 2008 09:17:59 +0200 (CEST)
From:	Jan Engelhardt <jengelh@...ozas.de>
To:	Erez Zadok <ezk@...sunysb.edu>
cc:	Arnd Bergmann <arnd@...db.de>, Jamie Lokier <jamie@...reable.org>,
	Phillip Lougher <phillip@...gher.demon.co.uk>,
	David Newall <davidn@...idnewall.com>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	hch@....de
Subject: Re: [RFC 0/7] [RFC] cramfs: fake write support 

On Monday 2008-06-02 06:37, Erez Zadok wrote:
>> Jan Engelhardt wrote:
>> > On Sunday 2008-06-01 08:02, David Newall wrote:
>> >>   
>> >>> I prefer the technique of union of a tmpfs over some other fs
>> >>
>> >> You're right in principle, but unfortunately there is to date no working
>> >> implementation of union mounts. Giving users the option of using an
>> >> existing file system with a few tweaks can only be better than than
>> >> forcing them to use hacks like unionfs.

>Folks, I've said it before: unioning is a deceptively simple idea in
>principle, and &^@...&^@ hard in practice.  And anyone who thinks otherwise
>is welcome to write a *versatile* unioning implementation on their own. Once
>you get through all corner cases and satisfy all the features which users
>want, you have a complex large file system.
>[...]

To the original posters:

I urge those who do believe {au,union}fs is too fat to go and build
their unioning into their on-disk filesystems, then let users run it
(remark: iff you can convince (or force) them why they should not be
using existing fs), let users report issues and iron it out for
perhaps 2-3 years, and then see how much your implementation has
grown. That is, if you actually added code (see remark 1).

About last year (June 2007), SLAX sought a solution that enhances
VFAT with UNIX permissions -- much like the old umsdosfs. A kernel
solution was initially preferred by Tomas (SLAX developer), yet I
(who got to write posixovl then) went for FUSE. It was about 20 KB
when it was moderately usable. The end result? Posixovl is a 46 KB C
file today. For userspace code. I bet it would be much more if it was
in-kernel.

Take that as a hint when developing your fs-specific unioning.
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