From owner-freebsd-questions@FreeBSD.ORG Mon Nov 5 03:38:04 2012 Return-Path: Delivered-To: freebsd-questions@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [69.147.83.52]) by hub.freebsd.org (Postfix) with ESMTP id 57FA6E8E for ; Mon, 5 Nov 2012 03:38:04 +0000 (UTC) (envelope-from rfg@tristatelogic.com) Received: from outgoing.tristatelogic.com (segfault.tristatelogic.com [69.62.255.118]) by mx1.freebsd.org (Postfix) with ESMTP id 2DD158FC0C for ; Mon, 5 Nov 2012 03:38:03 +0000 (UTC) Received: from segfault-nmh-helo.tristatelogic.com (localhost [127.0.0.1]) by segfault.tristatelogic.com (Postfix) with ESMTP id 8B10C5081B for ; Sun, 4 Nov 2012 19:38:02 -0800 (PST) To: freebsd-questions@freebsd.org Subject: Re: Questions about dump/restore to/from DVD media In-Reply-To: <50971B88.40303@herveybayaustralia.com.au> Date: Sun, 04 Nov 2012 19:38:02 -0800 Message-ID: <22041.1352086682@tristatelogic.com> From: "Ronald F. Guilmette" X-BeenThere: freebsd-questions@freebsd.org X-Mailman-Version: 2.1.14 Precedence: list List-Id: User questions List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Mon, 05 Nov 2012 03:38:04 -0000 In message <50971B88.40303@herveybayaustralia.com.au>, Da Rock wrote: >Also, you may have considered this already (or not :) ), but you are >using a direct write to backup your system, and then considering >compression on top of that. CD/DVD filesystems incorporate some parity >to allow for defects and scratches, so growisofs might be best to use to >ensure some integrity to your data. > >Minimising your space may be good, but a single bit could render all >your efforts for nought- especially given the compression leaves no room >for error ;) I'm not sure if the error detection/correction on DVDs... either -Rs or +Rs... is a function of the _filesystem_. In fact I don't believe that it is, but I could be wrong. Google for this: DVD+R error correction and there are plenty of references. The ones that I read in the past seemed to suggest that the error detection/correction is a fundamental aspect of how data gets written to both -R and +R disks, totally independent of whether the data being written was organized into any type of filesystem or none at all. In fact, part of the reason that I only use DVD+Rs these days is because I read something that said that something like 1/4 of every block of data on DVD-R disks is not even covered by any error correction code AT ALL. Ah, yes... here is one such reference: http://adterrasperaspera.com/blog/2006/10/30/how-to-choose-cddvd-archival-media "The DVD-R specification states that for every 192 bits, 64 of them are not protected under any scheme, 24 of them are protected by 24 bits of parity, and the last 56 bits are protected by another 24 bits of parity. This weird (to put it mildly) scheme allows you to easily scramble or lose 25% of the data that is required to read your disk! This information is almost more important than the actual data burned on the disc itself. The DVD+R specification, however, states that for every 204 bits of information, it is split into four blocks of 52 bits containing 1 sync bit to prevent misreading because of phase changes, 31 bits of data, and a 20 bit parity (that protects all 32 bits of data)..." Regards, rfg