From owner-freebsd-fs@FreeBSD.ORG  Sat Jul 14 11:18:49 2007
Return-Path: <owner-freebsd-fs@FreeBSD.ORG>
X-Original-To: freebsd-fs@freebsd.org
Delivered-To: freebsd-fs@freebsd.org
Received: from mx1.freebsd.org (mx1.freebsd.org [69.147.83.52])
	by hub.freebsd.org (Postfix) with ESMTP id F00A616A402;
	Sat, 14 Jul 2007 11:18:49 +0000 (UTC)
	(envelope-from brde@optusnet.com.au)
Received: from mail03.syd.optusnet.com.au (mail03.syd.optusnet.com.au
	[211.29.132.184])
	by mx1.freebsd.org (Postfix) with ESMTP id 75DFC13C4A6;
	Sat, 14 Jul 2007 11:18:49 +0000 (UTC)
	(envelope-from brde@optusnet.com.au)
Received: from besplex.bde.org (c220-239-235-248.carlnfd3.nsw.optusnet.com.au
	[220.239.235.248])
	by mail03.syd.optusnet.com.au (8.13.1/8.13.1) with ESMTP id
	l6EBIjFT002407
	(version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-SHA bits=256 verify=NO);
	Sat, 14 Jul 2007 21:18:47 +1000
Date: Sat, 14 Jul 2007 21:18:45 +1000 (EST)
From: Bruce Evans <brde@optusnet.com.au>
X-X-Sender: bde@besplex.bde.org
To: Bruce Evans <brde@optusnet.com.au>
In-Reply-To: <20070713223111.P2242@besplex.bde.org>
Message-ID: <20070714211354.I1555@besplex.bde.org>
References: <47a4f3080707130119xbee4477y3e96d27763f793aa@mail.gmail.com>
	<20070713223111.P2242@besplex.bde.org>
MIME-Version: 1.0
Content-Type: TEXT/PLAIN; charset=US-ASCII; format=flowed
Cc: freebsd-fs@freebsd.org, Brian Chu <chub@freebsd.org>
Subject: Re: msdosfs header unification patch
X-BeenThere: freebsd-fs@freebsd.org
X-Mailman-Version: 2.1.5
Precedence: list
List-Id: Filesystems <freebsd-fs.freebsd.org>
List-Unsubscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-fs>,
	<mailto:freebsd-fs-request@freebsd.org?subject=unsubscribe>
List-Archive: <http://lists.freebsd.org/pipermail/freebsd-fs>
List-Post: <mailto:freebsd-fs@freebsd.org>
List-Help: <mailto:freebsd-fs-request@freebsd.org?subject=help>
List-Subscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-fs>,
	<mailto:freebsd-fs-request@freebsd.org?subject=subscribe>
X-List-Received-Date: Sat, 14 Jul 2007 11:18:50 -0000

On Fri, 13 Jul 2007, Bruce Evans wrote:

> On Fri, 13 Jul 2007, Brian Chu wrote:
> % ...
> % -		/* Check backup FSInfo?					XXX 
> */
> % +
> % +    /* Unnecessary to check the backup FSInfo because there isn't
> % +     * an physical backup copy of the FSInfo block.  There's only a
> % +     * backup of the FSInfo block number, which was checked (bsBPB) above.
> % +     */
> %  	}
>
> But there is a physical backup FSinfo.
>
> The normal layout is:
>
> 	main boot sector (sector 0)
> 	fsinfo (sector 1, but can be anywhere)
> 	second boot sector (next sector? -- no pointer to this)
> 	   fsisig4 checks/clobbers here
> 	backup main boot sector (sector 3, but can be anywhere)

Actually, it is sector 6.  Sectors 3-5 are unused.  Microsoft docs say not
to use any other layout (for portability, since some utilities hard-code 6)
...

> 	backup fsinfo (next sector? -- no pointer to this, since backup
> 	   main boot sector can't have a different pointer than main bs)
> 	backup second boot sector (next sector?)
> 	   backup fsisig4 would check/clobber here
>
> newfs_msdos omits the second boot sector and creates the following layout:
>
> 	main boot sector (sector 0)
> 	fsinfo (sector 1)
> 	backup main boot sector (sector 2)
> 	   fsisig4 checks/clobbers here
> 	backup fsinfo (sector 3)
> 	   backup fsisig4 would check/clobber outside of all boot sectors

... so this is a very unusual layout and shouldn't be produced by default.

Bruce