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

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Message-ID: <20070108233737.GC1269@filer.fsl.cs.sunysb.edu>
Date:	Mon, 8 Jan 2007 18:37:37 -0500
From:	Josef Sipek <jsipek@....cs.sunysb.edu>
To:	Andrew Morton <akpm@...l.org>
Cc:	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 02:02:24PM -0800, Andrew Morton wrote:
> On Mon, 08 Jan 2007 16:30:48 -0500
> Shaya Potter <spotter@...columbia.edu> wrote:
> 
> > On Mon, 2007-01-08 at 13:19 -0800, Andrew Morton wrote:
> > > On Mon, 8 Jan 2007 14:43:39 -0500 (EST) Shaya Potter <spotter@...columbia.edu> wrote:
> > > >  It's the same thing as modifying a block 
> > > > device while a file system is using it.  Now, when unionfs gets confused, 
> > > > it shouldn't oops, but would one expect ext3 to allow one to modify its 
> > > > backing store while its using it?
> > > 
> > > There's no such problem with bind mounts.  It's surprising to see such a
> > > restriction with union mounts.
> > 
> > the difference is bind mounts are a vfs construct, while unionfs is a
> > file system.
> 
> Well yes.  So the top-level question is "is this the correct way of doing
> unionisation?".
 
Namespace unification doesn't seem to fit into neither vfs-only nor fs-only
category. My guess is that some of the code that's currently in unionfs
could be replaced by some vfs-level magic.
 
> I suspect not, in which case unionfs is at best a stopgap until someone
> comes along and implements unionisation at the VFS level, at which time
> unionfs goes away.
> 
> That could take a long time.  The questions we're left to ponder over are
> things like
> 
> a) is unionfs a sufficiently useful stopgap to justify a merge and

We (unionfs team) think so :)

> b) would an interim merge of unionfs increase or decrease the motivation
>    for someone to do a VFS implementation?

And is a VFS implementation the right way to do it?

> I suspect the answer to b) is "increase": if unionfs proves to be useful
> then people will be motivated to produce more robust implementations of the
> same functionality.

I think it would "increase" the chance of people doing the right thing -
whatever it may be.

> Is there vendor interest in unionfs?

Many people are currently using the full - lkml-unsafe :) version of the
code which has a considerable amount of bells and whistles - at least
compared to the minimal version submitted. I want to take the good things
"port" them over and make sure everything is good.

Josef "Jeff" Sipek.

-- 
The box said "Windows XP or better required". So I installed Linux.
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