From mboxrd@z Thu Jan 1 00:00:00 1970 From: Goswin von Brederlow Subject: Re: [fuse-devel] delta filesystem prototype Date: Thu, 05 Mar 2009 20:58:20 +0100 Message-ID: <87mybzd9nn.fsf@frosties.localdomain> References: <87sklyh3wu.fsf@frosties.localdomain> <200903010138.39329.bs_lists@aakef.fastmail.fm> <87eixhfsyi.fsf@frosties.localdomain> <87y6vlcr6p.fsf@frosties.localdomain> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: goswin-v-b@web.de, bs_lists@aakef.fastmail.fm, fuse-devel@lists.sourceforge.net, linux-fsdevel@vger.kernel.org To: Miklos Szeredi Return-path: Received: from fmmailgate03.web.de ([217.72.192.234]:55340 "EHLO fmmailgate03.web.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754590AbZCET6q (ORCPT ); Thu, 5 Mar 2009 14:58:46 -0500 In-Reply-To: (Miklos Szeredi's message of "Thu, 05 Mar 2009 14:06:15 +0100") Sender: linux-fsdevel-owner@vger.kernel.org List-ID: Miklos Szeredi writes: > On Wed, 04 Mar 2009, Goswin von Brederlow wrote: >> Bernd and I ment the following scenario: >> >> /dev/sda1 /union/read-only >> tmpfs /union/read-write >> >> with a delta-fs merging the two. Then running "echo foo > >> /union/read-only/path/file" could be desasterous to your data. > > Well, if the writable branch is really meant to be a clone of the > underlying fs, then yes. But writable unions are _not_ clones either, > very far from that. The problem is that /delta-fs/path/file would suddenly be a composite of the new file /union/read-only/path/file and any stored delta information in /union/read-write/path/file of the old file. In unionfs-fuse files are currently always completly copy-up-ed when modified. There a change of /union/read-only/path/file will give the new file if it wasn't modified or the old modified file if it was. But never mix the two. >> The underlying branch must not change on its own. The delta-fs could >> change it on command but that wasn't what we where thinking of. > > Delta-fs could deal with changes in the underlying filesystem, in some > cases much better than unionfs (for example if the file was renamed in > the delta-fs, and it was later modified in the underlying fs). > > Thanks, > Miklos Ugh, no. The delta-fs has no way of knowing when a file in a branch will be changed outside of delta-fs. And it would have to know that before the change so it can copy-up relevant data before they change. MfG Goswin