linux-kernel - Re: [PATCH 01/24] Unionfs: Documentation

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Date:	Tue, 9 Jan 2007 09:49:35 +0000
From:	Christoph Hellwig <hch@...radead.org>
To:	Josef Sipek <jsipek@....cs.sunysb.edu>
Cc:	Andrew Morton <akpm@...l.org>,
	Shaya Potter <spotter@...columbia.edu>,
	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 01/24] Unionfs: Documentation

On Mon, Jan 08, 2007 at 06:25:16PM -0500, Josef Sipek wrote:
> > There's no such problem with bind mounts.  It's surprising to see such a
> > restriction with union mounts.
> 
> Bind mounts are a purely VFS level construct. Unionfs is, as the name
> implies, a filesystem. Last year at OLS, it seemed that a lot of people
> agreed that unioning is neither purely a fs construct, nor purely a vfs
> construct.
> 
> I'm using Unionfs (and ecryptfs) as guinea pigs to make linux fs stacking
> friendly - a topic to be discussed at LSF in about a month.

And unionfs is the wrong thing do use for this.  Unioning is a complex
namespace operation and needs to be implemented in the VFS or at least
needs a lot of help from the VFS.  Getting namespace cache coherency
and especially locking right is imposisble with out that.

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