From owner-freebsd-questions@FreeBSD.ORG  Mon Nov  5 06:58:39 2012
Return-Path: <owner-freebsd-questions@FreeBSD.ORG>
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 0B9FF958
 for <freebsd-questions@freebsd.org>; Mon,  5 Nov 2012 06:58:39 +0000 (UTC)
 (envelope-from bonomi@mail.r-bonomi.com)
Received: from mail.r-bonomi.com (mx-out.r-bonomi.com [204.87.227.120])
 by mx1.freebsd.org (Postfix) with ESMTP id 977F78FC08
 for <freebsd-questions@freebsd.org>; Mon,  5 Nov 2012 06:58:38 +0000 (UTC)
Received: (from bonomi@localhost)
 by mail.r-bonomi.com (8.14.4/rdb1) id qA56wo7J042603;
 Mon, 5 Nov 2012 00:58:50 -0600 (CST)
Date: Mon, 5 Nov 2012 00:58:50 -0600 (CST)
From: Robert Bonomi <bonomi@mail.r-bonomi.com>
Message-Id: <201211050658.qA56wo7J042603@mail.r-bonomi.com>
To: freebsd-questions@freebsd.org, freebsd-questions@herveybayaustralia.com.au
Subject: Re: Questions about dump/restore to/from DVD media
In-Reply-To: <509751D5.7060902@herveybayaustralia.com.au>
X-BeenThere: freebsd-questions@freebsd.org
X-Mailman-Version: 2.1.14
Precedence: list
List-Id: User questions <freebsd-questions.freebsd.org>
List-Unsubscribe: <http://lists.freebsd.org/mailman/options/freebsd-questions>, 
 <mailto:freebsd-questions-request@freebsd.org?subject=unsubscribe>
List-Archive: <http://lists.freebsd.org/pipermail/freebsd-questions>
List-Post: <mailto:freebsd-questions@freebsd.org>
List-Help: <mailto:freebsd-questions-request@freebsd.org?subject=help>
List-Subscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-questions>, 
 <mailto:freebsd-questions-request@freebsd.org?subject=subscribe>
X-List-Received-Date: Mon, 05 Nov 2012 06:58:39 -0000


> Date: Mon, 05 Nov 2012 15:42:45 +1000
> From: Da Rock <freebsd-questions@herveybayaustralia.com.au>
> Subject: Re: Questions about dump/restore to/from DVD media
>
> On 11/05/12 14:14, Polytropon wrote:
> For reference, if one did backup the whole slice/disk using dd and then 
> compressed the data, would that effectively compress all those
> 'unallocated' nodes?

NO.  The unallocated' blocks still have whatever data was in them.

*IF* you copy /dev/zero to a new file, to fill the disk, then rm
-that- file, the compression will be higher.  'How much' depends on
how empty the disk is.