Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756778AbXFTR2h (ORCPT ); Wed, 20 Jun 2007 13:28:37 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753141AbXFTR2b (ORCPT ); Wed, 20 Jun 2007 13:28:31 -0400 Received: from filer.fsl.cs.sunysb.edu ([130.245.126.2]:47845 "EHLO filer.fsl.cs.sunysb.edu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752250AbXFTR2a (ORCPT ); Wed, 20 Jun 2007 13:28:30 -0400 Date: Wed, 20 Jun 2007 13:28:09 -0400 Message-Id: <200706201728.l5KHS9hj029372@agora.fsl.cs.sunysb.edu> From: Erez Zadok To: Jan Blunck Cc: linux-kernel@vger.kernel.org, jsipek@ic.sunysb.edu Subject: Re: [RFC PATCH 1/4] Union mount documentation. In-reply-to: Your message of "Wed, 20 Jun 2007 07:29:55 -0000." X-MailKey: Erez_Zadok Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2042 Lines: 39 In message , Jan Blunck writes: > On Tue, 19 Jun 2007 22:59:51 -0700, Arjan van de Ven wrote: > > > first of all I'm happy to see that people are still working on unionfs; > > I'd love to have functionality like this show up in Linux. > > This has nothing to do with unionfs. This is about doing a VFS based > approach to union mounts. Unification is a name-based construct so it > belongs into VFS and not into a separate file system. Jan, while I agree with you in principle that unification is a VFS-level namespace construct, I disagree with you that unioning doesn't belong in a separate f/s. As someone whose group developed three generations of the stackable file system Unionfs (see http://unionfs.filesystems.org/), I can tell you from my experience and the experience of numerous users, that the devil is the details -- or the so-called orthogonal issues. To get a fully working unioning implementation, one that the many current users of Unionfs could use, you'll have to deal with many issues and corner cases: cache coherency, inode persistency (and network f/s exports), copyups, whiteouts and opaque dirs, how to deal with "odd" file systems which don't support native whiteouts and such, directory reading (seekdir), and more. Our third generation Unionfs, the one with On-Disk Format (ODF), handles all of these. Rather than reproduce all that discussion here, I'll point people to read more info here: So, to have a fully usable union mounts implementation, you're going to have to support a lot of existing features; but if you were to support them all at the VFS level, you will have bloated the VFS considerably with stuff that many would argue does not belong in the VFS. Sincerely, Erez Zadok. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/