From mboxrd@z Thu Jan 1 00:00:00 1970 From: Christoph Hellwig Subject: Re: [PATCH 01/24] Unionfs: Documentation Date: Tue, 9 Jan 2007 09:49:35 +0000 Message-ID: <20070109094935.GA12406@infradead.org> References: <1168229596580-git-send-email-jsipek@cs.sunysb.edu> <1168229596875-git-send-email-jsipek@cs.sunysb.edu> <20070108111852.ee156a90.akpm@osdl.org> <20070108131957.cbaf6736.akpm@osdl.org> <20070108232516.GB1269@filer.fsl.cs.sunysb.edu> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Andrew Morton , Shaya Potter , Josef 'Jeff' Sipek , linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, hch@infradead.org, viro@ftp.linux.org.uk, torvalds@osdl.org, mhalcrow@us.ibm.com, David Quigley , Erez Zadok Return-path: To: Josef Sipek Content-Disposition: inline In-Reply-To: <20070108232516.GB1269@filer.fsl.cs.sunysb.edu> Sender: linux-kernel-owner@vger.kernel.org List-Id: linux-fsdevel.vger.kernel.org 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.