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

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Message-ID: <20080601122818.GA3687@shareable.org>
Date:	Sun, 1 Jun 2008 13:28:18 +0100
From:	Jamie Lokier <jamie@...reable.org>
To:	Phillip Lougher <phillip@...gher.demon.co.uk>
Cc:	Arnd Bergmann <arnd@...db.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

Phillip Lougher wrote:
> If I read the patches correctly, when a file page is written to, only 
> that page gets copied into the page cache and locked, the other pages 
> continue to be read off disk from cramfs?  With Unionfs a page write 
> causes the entire file to be copied up to the r/w tmpfs and locked into 
> the page cache causing unnecessary RAM overhead.

Ok, so why not fix that in unionfs?  An option so that holes in the
overlay file let through data from the underlying file sounds like it
would be generally useful, and quite easy to implement.

If not unionfs, a "union-tmpfs" combination would be good.  Many
filesystems aren't well suited to being the overlay filesystem -
adding to the implementation's complexity - but a modified tmpfs could
be very well suited.

-- Jamie
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