From owner-freebsd-geom@FreeBSD.ORG  Wed Jul  5 21:58:44 2006
Return-Path: <owner-freebsd-geom@FreeBSD.ORG>
X-Original-To: freebsd-geom@freebsd.org
Delivered-To: freebsd-geom@freebsd.org
Received: from mx1.FreeBSD.org (mx1.freebsd.org [216.136.204.125])
	by hub.freebsd.org (Postfix) with ESMTP id 1E94516A4E0
	for <freebsd-geom@freebsd.org>; Wed,  5 Jul 2006 21:58:44 +0000 (UTC)
	(envelope-from mikej@rogers.com)
Received: from H43.C18.B96.tor.eicat.ca (H43.C18.B96.tor.eicat.ca
	[66.96.18.43]) by mx1.FreeBSD.org (Postfix) with ESMTP id 4E77643D49
	for <freebsd-geom@freebsd.org>; Wed,  5 Jul 2006 21:58:41 +0000 (GMT)
	(envelope-from mikej@rogers.com)
Received: from [127.0.0.1] (desktop.home.local [172.16.0.200])
	by H43.C18.B96.tor.eicat.ca (Postfix) with ESMTP id C468211446;
	Wed,  5 Jul 2006 17:58:08 -0400 (EDT)
Message-ID: <44AC3618.80600@rogers.com>
Date: Wed, 05 Jul 2006 17:58:48 -0400
From: Mike Jakubik <mikej@rogers.com>
User-Agent: Thunderbird 1.5.0.4 (Windows/20060516)
MIME-Version: 1.0
To: "R. B. Riddick" <arne_woerner@yahoo.com>
References: <20060705211453.47043.qmail@web30308.mail.mud.yahoo.com>
In-Reply-To: <20060705211453.47043.qmail@web30308.mail.mud.yahoo.com>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit
X-SpamToaster-Information: This messages has been scanned by SpamToaster
	http://www.digitalprogression.ca
X-SpamToaster: Found to be clean
X-SpamToaster-SpamCheck: not spam, SpamAssassin (not cached, score=-2.49,
	required 3.5, ALL_TRUSTED -1.80, BAYES_00 -2.60,
	DK_POLICY_SIGNSOME 0.00, DNS_FROM_RFC_ABUSE 0.20,
	DNS_FROM_RFC_POST 1.71)
X-SpamToaster-From: mikej@rogers.com
X-Spam-Status: No
Cc: freebsd-geom@freebsd.org
Subject: Re: new class / geom_cache / request for comments
X-BeenThere: freebsd-geom@freebsd.org
X-Mailman-Version: 2.1.5
Precedence: list
List-Id: GEOM-specific discussions and implementations
	<freebsd-geom.freebsd.org>
List-Unsubscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-geom>,
	<mailto:freebsd-geom-request@freebsd.org?subject=unsubscribe>
List-Archive: <http://lists.freebsd.org/pipermail/freebsd-geom>
List-Post: <mailto:freebsd-geom@freebsd.org>
List-Help: <mailto:freebsd-geom-request@freebsd.org?subject=help>
List-Subscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-geom>,
	<mailto:freebsd-geom-request@freebsd.org?subject=subscribe>
X-List-Received-Date: Wed, 05 Jul 2006 21:58:44 -0000

R. B. Riddick wrote:
> Yes, I think so...
> But geom_cache is just useful, when file system's buffer cache cannot help.
>
> E. g.:
> A degraded RAID5 on 4 consumers (3 good plus 1 failed).
> When we want to get a data block, that resides on the failed consumer, we have
> to read all corresponding blocks (2+1) in order to rebuild the missing block.
> When we do a sequential read, we would have to read the consumers, that hold
> the data blocks twice (2 x 2).
> So the geom_cache could help here (2+1 real reads plus 2 from the cache), if
> the provider is not too busy.
>   

Wouldn't it make more sense to modify geom_raid5 to include this 
behavior, instead of writing a new and separate geom class that has only 
one useful function?