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

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Message-ID: <20090519103924.GL16526@bolzano.suse.de>
Date:	Tue, 19 May 2009 12:39:24 +0200
From:	Jan Blunck <jblunck@...e.de>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	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 Tue, May 19, Miklos Szeredi wrote:

> On Tue, 19 May 2009, Jan Blunck wrote:
> > The directory in the topmost filesystem is created during
> > lookup. The contents of the directory isn't copied up presistently
> > at that point in time. Therefore you have an empty directory in the
> > topmost filesystem after the lookup. This was necessary to get rid
> > of the union_relookup_topmost() calls during create, mknod, mkdir
> > etc.
> > 
> > When readdir is called, the topmost directory is filed up with
> > fallthru entries which are persistently stored. This is only
> > necessary to get readdir right wrt POSIX. During lookup the fallthru
> > dentry, which is in fact a special negative dentry, is ignored and
> > therefore the lookup continues on the lower filesystem.
> 
> So this means that the topmost branch always needs to be writable,
> right?  It isn't possible to make a union of two iso9660 filesystems,
> for example?

Exactly. Although, you can do that with the help of tmpfs on top of the two
iso9660 filesystems. Or by adding fake write support to iso9660 ...

I know that this seems to be suboptimal but it is the cost that the POSIX
correct readdir comes with.
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