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

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Message-ID: <20070109103659.GB25438@filer.fsl.cs.sunysb.edu>
Date:	Tue, 9 Jan 2007 05:36:59 -0500
From:	Josef Sipek <jsipek@....cs.sunysb.edu>
To:	Christoph Hellwig <hch@...radead.org>,
	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,
	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 Tue, Jan 09, 2007 at 09:49:35AM +0000, Christoph Hellwig wrote:
> 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.

What I meant was that I use them as an example for a linear and fanout
stacking examples. While unioning itself is a complex operation, the general
idea of one set of vfs objects (dentry, inode, etc.) pointing to several
lower ones is very generic and applies to all fan-out stackable fs.

Josef "Jeff" Sipek.

-- 
Linux, n.:
  Generous programmers from around the world all join forces to help
  you shoot yourself in the foot for free. 
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