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

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Message-Id: <200806020437.m524bWiW027508@agora.fsl.cs.sunysb.edu>
Date:	Mon, 2 Jun 2008 00:37:32 -0400
From:	Erez Zadok <ezk@...sunysb.edu>
To:	Arnd Bergmann <arnd@...db.de>, Jamie Lokier <jamie@...reable.org>,
	Phillip Lougher <phillip@...gher.demon.co.uk>,
	Jan Engelhardt <jengelh@...putergmbh.de>,
	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 

> 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.
> >
> >I've not used unionfs (nor aufs) so I'm not aware of its foibles, but I
> >can say that it's the right kind of solution.  Rather than spend effort
> >implementing write support for read-only filesystems, why not put your
> >time into fixing whatever you see wrong with one or both of those?
> 
> I have to join in. Unionfs and AUFS may be bigger in bytes than the
> embedded developer wants to sacrifice, but that is what it takes for
> a solid implementation that has to deal with things like NFS and
> mmap. Even so, there is a fs called mini_fo you can try using if
> you disagree with the size of unionfs/aufs, at the cost of not having
> support for all corner cases.

I agree w/ Jan E.

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.

I believe that implementing unioning inside actual filesystems is totally the
wrong direction: going to lower layers is wrong, instead of going up to a
VFS-based solution.  Unioning is a namespace operation that should not be
done deep inside a lower f/s.

People often wonder why FScache is (reportedly) so complex and big.  It's
b/c in some part it has to deal with similar issues: unioning is
copy-on-write, whereas caching is copy-on-read.

Nevertheless, I can understand if the embedded community wants lightweight
unioning.  Union Mounts initially may not support everything that unionfs
does, but it should be smaller, and it should be enough I believe for the
basic unioning uses --- perhaps even for the embedded community.  If so,
then I suggest people offer to help Bharata and Jan Blunk's efforts, rather
than [sic] cramming unioning into a single file system.

Erez.
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