From owner-freebsd-stable@FreeBSD.ORG  Fri Jul 15 22:03:39 2005
Return-Path: <owner-freebsd-stable@FreeBSD.ORG>
X-Original-To: freebsd-stable@freebsd.org
Delivered-To: freebsd-stable@freebsd.org
Received: from mx1.FreeBSD.org (mx1.freebsd.org [216.136.204.125])
	by hub.freebsd.org (Postfix) with ESMTP id 4388B16A41C
	for <freebsd-stable@freebsd.org>; Fri, 15 Jul 2005 22:03:39 +0000 (GMT)
	(envelope-from mkb@mkbuelow.net)
Received: from luzifer.incubus.de (incubus.de [80.237.207.83])
	by mx1.FreeBSD.org (Postfix) with ESMTP id D1CDE43D46
	for <freebsd-stable@freebsd.org>; Fri, 15 Jul 2005 22:03:38 +0000 (GMT)
	(envelope-from mkb@mkbuelow.net)
Received: from drjekyll.mkbuelow.net (p54AA8680.dip0.t-ipconnect.de
	[84.170.134.128])
	(using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits))
	(No client certificate requested)
	by luzifer.incubus.de (Postfix) with ESMTP id 607B4323E4;
	Sat, 16 Jul 2005 00:06:30 +0200 (CEST)
Received: from drjekyll.mkbuelow.net (mkb@localhost.mkbuelow.net [127.0.0.1])
	by drjekyll.mkbuelow.net (8.13.3/8.13.3) with ESMTP id
	j6FM3nsL015084; Sat, 16 Jul 2005 00:03:49 +0200 (CEST)
	(envelope-from mkb@drjekyll.mkbuelow.net)
Message-Id: <200507152203.j6FM3nsL015084@drjekyll.mkbuelow.net>
From: Matthias Buelow <mkb@incubus.de>
To: Lowell Gilbert <freebsd-stable-local@be-well.ilk.org>
In-Reply-To: Message from Lowell Gilbert
	<freebsd-stable-local@be-well.ilk.org> 
	of "15 Jul 2005 16:15:27 EDT." <44br539674.fsf@be-well.ilk.org> 
X-Mailer: MH-E 7.84; nmh 1.0.4; XEmacs 21.4 (patch 17)
Date: Sat, 16 Jul 2005 00:03:49 +0200
Sender: mkb@mkbuelow.net
Cc: John-Mark Gurney <gurney_j@resnet.uoregon.edu>, freebsd-stable@freebsd.org
Subject: Re: dangerous situation with shutdown process 
X-BeenThere: freebsd-stable@freebsd.org
X-Mailman-Version: 2.1.5
Precedence: list
List-Id: Production branch of FreeBSD source code <freebsd-stable.freebsd.org>
List-Unsubscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-stable>, 
	<mailto:freebsd-stable-request@freebsd.org?subject=unsubscribe>
List-Archive: <http://lists.freebsd.org/pipermail/freebsd-stable>
List-Post: <mailto:freebsd-stable@freebsd.org>
List-Help: <mailto:freebsd-stable-request@freebsd.org?subject=help>
List-Subscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-stable>,
	<mailto:freebsd-stable-request@freebsd.org?subject=subscribe>
X-List-Received-Date: Fri, 15 Jul 2005 22:03:39 -0000

Lowell Gilbert <freebsd-stable-local@be-well.ilk.org> writes:

>We keep trying to point out that barriers *can't* be enforced on the
>hardware with many (most, and apparently an increasing percentage of)
>ATA drives.  There is no semantic on these drives that allows you to
>guarantee the journal block will be written before the corresponding
>data block.  If you are sure that your drives do this properly, then
>you are safe, but in that case there's no reliability problem with
>softupdates, either.

See my other mail(s) about other systems using cache disabling/enabling
to make up for a drive that ignores (or does not implement) a flush
command.

Then the advice of "disable the wb-cache on disks to ensure data safety"
doesn't make sense:

Either

 * the drive supports disabling the write-back-cache,
   then this method can be used to flush data to the platters,

or else

 * the drive does not support disabling the write-back-cache, or lies
   about it, then the advice to disable the write-back-cache for
   softupdates is meaningless.

I know my drive allows disabling of the write cache, as, apparently, the
majority of IDE/SATA drives do.

mkb.