linux-kernel - Re: [PATCH 00/32] VFS based Union Mount (V3)

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Message-Id: <200905191421.21026.arnd@arndb.de>
Date:	Tue, 19 May 2009 14:21:20 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Jan Blunck <jblunck@...e.de>
Cc:	Miklos Szeredi <miklos@...redi.hu>, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, viro@...iv.linux.org.uk,
	bharata@...ibm.com, dwmw2@...radead.org, mszeredi@...e.cz,
	vaurora@...hat.com
Subject: Re: [PATCH 00/32] VFS based Union Mount (V3)

On Tuesday 19 May 2009, Jan Blunck wrote:
> On Tue, May 19, Arnd Bergmann wrote:
> > This would work, but you'd have to do this for each file system if you want
> > to be able to use it as the top of the union while backed by a read-only
> > block device or when you don't want it to be written.
> 
> I know that the requirement for the topmost filesystem to be able to create
> directories and fill them with fallthrus is an unattractive one. On the other
> hand this is the cost that you have to pay at the moment to get this kind of
> functionality. This implementation will not help with all use-cases. Its focus
> is to get certain use-cases right.

So what would go wrong if you only made them persistent for writable file
systems, but allowed fallthrough dentries to be discarded for read-only
file systems? As long as the lower layers don't change, you should still
be able to reconstruct the same dentries every time you do a readdir, right?

	Arnd <><
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