From owner-freebsd-arch@FreeBSD.ORG Sun Mar 30 20:31:26 2008 Return-Path: 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 9CAFE1065672 for ; Sun, 30 Mar 2008 20:31:26 +0000 (UTC) (envelope-from phk@critter.freebsd.dk) Received: from phk.freebsd.dk (phk.freebsd.dk [130.225.244.222]) by mx1.freebsd.org (Postfix) with ESMTP id 52B878FC20 for ; Sun, 30 Mar 2008 20:31:26 +0000 (UTC) (envelope-from phk@critter.freebsd.dk) Received: from critter.freebsd.dk (unknown [192.168.61.3]) by phk.freebsd.dk (Postfix) with ESMTP id DF50B17104; Sun, 30 Mar 2008 20:31:24 +0000 (UTC) Received: from critter.freebsd.dk (localhost [127.0.0.1]) by critter.freebsd.dk (8.14.2/8.14.2) with ESMTP id m2UKVNjh012859; Sun, 30 Mar 2008 20:31:24 GMT (envelope-from phk@critter.freebsd.dk) To: Matthew Dillon From: "Poul-Henning Kamp" In-Reply-To: Your message of "Sun, 30 Mar 2008 13:16:56 MST." <200803302016.m2UKGuZA015127@apollo.backplane.com> Date: Sun, 30 Mar 2008 20:31:23 +0000 Message-ID: <12858.1206909083@critter.freebsd.dk> Sender: phk@critter.freebsd.dk Cc: Kirk McKusick , 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 List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Sun, 30 Mar 2008 20:31:26 -0000 In message <200803302016.m2UKGuZA015127@apollo.backplane.com>, Matthew Dillon w rites: > The right way to deal with flash is *NOT* to require that the filesystem > be smart about flash storage, but instead to implement an intermediate > storage layer which linearizes the writes to flash and removes all > random erases from the critical path. Your description of a simplified version of what is commonly called a "Flash Adaptation Layer", is a very good example of why there is a clear difference between "camera grade" flash devices, like most CF cards, and the new generation of "SSD" devices, like the M-Tron disk now in my laptop. The Camera grade Flash devices get lousy random write performance because they implement in essense what you describe, only in a more complete fashion where they have error correction, both the data and on the bitmaps. The newer generation of SSD devices do things much smarter than that, which is why their random write performance is much better than camera-grade devices. See my earlier emails for references to how to do the really smart thing. -- Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 phk@FreeBSD.ORG | TCP/IP since RFC 956 FreeBSD committer | BSD since 4.3-tahoe Never attribute to malice what can adequately be explained by incompetence.