linux-kernel - Re: [PATCH 05/24] Unionfs: Copyup Functionality

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Date:	Mon, 08 Jan 2007 17:00:15 -0500
From:	Shaya Potter <spotter@...columbia.edu>
To:	Andrew Morton <akpm@...l.org>
Cc:	"Josef 'Jeff' Sipek" <jsipek@...sunysb.edu>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	hch@...radead.org, viro@....linux.org.uk, torvalds@...l.org,
	mhalcrow@...ibm.com, David Quigley <dquigley@....cs.sunysb.edu>,
	Erez Zadok <ezk@...sunysb.edu>
Subject: Re: [PATCH 05/24] Unionfs: Copyup Functionality

On Mon, 2007-01-08 at 13:29 -0800, Andrew Morton wrote:
> On Sun,  7 Jan 2007 23:12:57 -0500
> "Josef 'Jeff' Sipek" <jsipek@...sunysb.edu> wrote:
> 
> > From: Josef "Jeff" Sipek <jsipek@...sunysb.edu>
> > 
> > This patch contains the functions used to perform copyup operations in unionfs.
> 
> What is a copyup operation and why does it exist?
> 
> It seems to be copying the entire contents of certain files.  That's not a
> thing I'd have expected to see in a union filesystem.  Explain it all,
> please?  (Somewhere where the info will be retained for posterity - a
> random email is good, but not sufficient...)

to do the random e-mail, it's because it just doesn't union
"directories", but lets you assign read-write, read-only properties to
them.

If you try to modify a file on a read-only layer, it will be copied to
the top most layer (has to be read-write) and then modified.  It can be
slow though (imagine modifying a 1GB file).

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