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

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Date:	Sun, 1 Jun 2008 23:25:52 -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>,
	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 

Arnd Bergmann:
> Besides, there are a many more problems with unionfs, which have
> all been mentioned in the previous review cycles. Aufs doesn't
> address those either AFAIK, with the exception of at least
> not making additional copies in the page cache when writing to
> a file.

Correction: Unionfs doesn't make additional copies in the page cache.

Arnd, I favor a more generic approach, one that will work with the vast
majority of file systems that people use w/ unioning, preferably all of
them.  Supporting copy-on-write in cramfs will only help a small subset of
users.  Yes, it might be simple, but I fear it won't be useful enough to
convince existing users of unioning to switch over.  And I don't think we
should add CoW support in every file system -- the complexity will be much
more than using unionfs or some other VFS-based solution.

I can see some advantages (re: cache coherency) by hacking CoW support
directly into a f/s.  If you want to use a filesystem-specific solution,
then I suggest you don't modify a file system used as a source in a union,
but one used as a destination.  You'll have better overage that way.  The
vast majority of times, unionfs users will either write to tmpfs or ext2;
but the source readonly f/s can be a lot of different ones (most popular are
ext*, nfs*, isofs, and cramfs/squashfs).

I find it somewhat ironic to hear the argument that "union mounts isn't
stable yet, so lets come up with a new solution inside cramfs."  Why should
your solution become stable much faster than union mounts (which also had
patches floating around for a long time already).

If you have cycles to spare, why not help Bharata and Jan?

Cheers,
Erez.
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