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

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Message-Id: <200905191354.08701.arnd@arndb.de>
Date:	Tue, 19 May 2009 13:54:08 +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:
> > 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.

But how do you get there? You can mount the tmpfs on top of two iso9660
file systems, but it seems that you wouldn't be able to get the two
stacked on top of each other in the first place.

Also, by mounting a tmpfs on top, wouldn't you you violate the requirement
for persistent inode numbers again?

> Or by adding fake write support to iso9660 ... 

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.

	Arnd <><
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