From owner-freebsd-arch@FreeBSD.ORG  Mon Mar 31 21:34:30 2008
Return-Path: <owner-freebsd-arch@FreeBSD.ORG>
Delivered-To: arch@freebsd.org
Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:4f8:fff6::34])
	by hub.freebsd.org (Postfix) with ESMTP id 5C849106567A;
	Mon, 31 Mar 2008 21:34:30 +0000 (UTC)
	(envelope-from mfouts@danger.com)
Received: from mx.danger.com (wall.danger.com [216.220.212.140])
	by mx1.freebsd.org (Postfix) with ESMTP id 45B8F8FC21;
	Mon, 31 Mar 2008 21:34:30 +0000 (UTC)
	(envelope-from mfouts@danger.com)
Received: from danger.com (exchange3.danger.com [10.0.1.7])
	by mx.danger.com (Postfix) with ESMTP id 62F3440A445;
	Mon, 31 Mar 2008 14:34:21 -0700 (PDT)
X-MimeOLE: Produced By Microsoft Exchange V6.5
Content-class: urn:content-classes:message
MIME-Version: 1.0
Content-Type: text/plain;
	charset="us-ascii"
Content-Transfer-Encoding: quoted-printable
Date: Mon, 31 Mar 2008 14:34:29 -0700
Message-ID: <B95CEC1093787C4DB3655EF330984818051D0A@EXCHANGE.danger.com>
In-Reply-To: <200803312006.m2VK6Aom028133@apollo.backplane.com>
X-MS-Has-Attach: 
X-MS-TNEF-Correlator: 
Thread-Topic: Flash disks and FFS layout heuristics
Thread-Index: AciTaslf/zlwdF8FTa+bHVN44JtuagAC86dQ
References: <20080330231544.A96475@localhost>
	<200803310135.m2V1ZpiN018354@apollo.backplane.com>
	<B95CEC1093787C4DB3655EF330984818051D03@EXCHANGE.danger.com>
	<200803312125.29325.qpadla@gmail.com>
	<200803311915.m2VJFSoR027593@apollo.backplane.com>
	<B95CEC1093787C4DB3655EF330984818051D09@EXCHANGE.danger.com>
	<200803312006.m2VK6Aom028133@apollo.backplane.com>
From: "Martin Fouts" <mfouts@danger.com>
To: "Matthew Dillon" <dillon@apollo.backplane.com>
Cc: Christopher Arnold <chris@arnold.se>, arch@freebsd.org, qpadla@gmail.com,
	freebsd-arch@freebsd.org
Subject: RE: Flash disks and FFS layout heuristics
X-BeenThere: freebsd-arch@freebsd.org
X-Mailman-Version: 2.1.5
Precedence: list
List-Id: Discussion related to FreeBSD architecture <freebsd-arch.freebsd.org>
List-Unsubscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-arch>,
	<mailto:freebsd-arch-request@freebsd.org?subject=unsubscribe>
List-Archive: <http://lists.freebsd.org/pipermail/freebsd-arch>
List-Post: <mailto:freebsd-arch@freebsd.org>
List-Help: <mailto:freebsd-arch-request@freebsd.org?subject=help>
List-Subscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-arch>,
	<mailto:freebsd-arch-request@freebsd.org?subject=subscribe>
X-List-Received-Date: Mon, 31 Mar 2008 21:34:30 -0000

=20

> -----Original Message-----
> From: Matthew Dillon [mailto:dillon@apollo.backplane.com]=20
> Sent: Monday, March 31, 2008 1:06 PM
> To: Martin Fouts
> Cc: qpadla@gmail.com; freebsd-arch@freebsd.org; Christopher=20
> Arnold; arch@freebsd.org
> Subject: RE: Flash disks and FFS layout heuristics
>=20
>=20
>     But how do you index that information?  You can't simply=20
> append the information to the NAND unless you also have a way to=20
> access it.  So does the filesystem have to scan the NAND (or
significant=20
> portions of it) in order to build an index of the filesystem topology
in=20
> system memory?
>=20
>     No matter what you do you have to index the information=20
> *SOMEWHERE*.

And NAND devices have a *SOMEWHERE* that makes them different than other
persistent storage devices in ways that make them interesting to do file
systems for.

It's not _that_ you have to scan the NAND, by the way, it's _when_ you
scan the NAND that has the major impact on performance.