From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932120AbWGKTz7 (ORCPT ); Tue, 11 Jul 2006 15:55:59 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S932119AbWGKTz6 (ORCPT ); Tue, 11 Jul 2006 15:55:58 -0400 Received: from [212.33.164.213] ([212.33.164.213]:57874 "EHLO raad.intranet") by vger.kernel.org with ESMTP id S932114AbWGKTz5 (ORCPT ); Tue, 11 Jul 2006 15:55:57 -0400 From: Al Boldi To: linux-fsdevel@vger.kernel.org Subject: Re: [RFC] VFS: FS CoW using redirection Date: Tue, 11 Jul 2006 22:57:08 +0300 User-Agent: KMail/1.5 Cc: linux-kernel@vger.kernel.org References: <200607082041.54489.a1426z@gawab.com> <200607091550.36407.a1426z@gawab.com> In-Reply-To: <200607091550.36407.a1426z@gawab.com> MIME-Version: 1.0 Content-Disposition: inline Content-Type: text/plain; charset="windows-1256" Content-Transfer-Encoding: 7bit Message-Id: <200607112257.08071.a1426z@gawab.com> Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Al Boldi wrote: > Eric Van Hensbergen wrote: > > On 7/8/06, Al Boldi wrote: > > > Copy on Write is a neat way to quickly achieve a semi-clustered > > > system, by mounting any shared FS read-only and redirecting writes to > > > some perClient FS. > > > > > > Would this redirection be easy to implement into the VFS? > > > > There are a variety of solutions that have been proposed or are > > available to do this sort of thing. You may want to start by looking > > at unionfs and mapfs. There are also folks looking at doing this from > > the block layer (look at the dm-userspace + cowd as well as evms and > > lvm snapshots). > > The idea here, is to do things completely transparent while being as > unintrusive as possible, by relying on the current FS infrastructure, w/o > introducing another VFS. > > Consider something simple like this: > > VFS - anyFS1 (r/w) used normally, unless ENotFound, then redirect read to > \ anyFS2, or CoW from anyFS2 to anyFS1. > anyFS2 (r/o) used normally. > > i.e: Does the VFS provide hooks for perMount Handlers? Silence is probably an indication that the VFS does not provide hooks. The question then: 1. Would it be difficult to introduce perMount Handler hooks into the VFS? 2. Would it be easier to replace the current VFS with one that offers a more flexible/open configuration? 3. Would these changes be welcome, or is the VFS considered to be a forbidden zone? Thanks! -- Al