From owner-freebsd-fs@FreeBSD.ORG Sun Oct 5 20:15:41 2014 Return-Path: Delivered-To: freebsd-fs@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:1900:2254:206a::19:1]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by hub.freebsd.org (Postfix) with ESMTPS id 39111AA6 for ; Sun, 5 Oct 2014 20:15:41 +0000 (UTC) Received: from msa04a.plala.or.jp (msa04.plala.or.jp [IPv6:2400:7800:0:5010::4]) by mx1.freebsd.org (Postfix) with ESMTP id AE005392 for ; Sun, 5 Oct 2014 20:15:40 +0000 (UTC) Received: from i58-95-58-27.s02.a026.ap.plala.or.jp ([58.95.58.27]) by msa04b.plala.or.jp with ESMTP id <20141005200935.HIIA12371.msa04b.plala.or.jp@i58-95-58-27.s02.a026.ap.plala.or.jp>; Mon, 6 Oct 2014 05:09:35 +0900 Date: Mon, 06 Oct 2014 05:09:34 +0900 Message-ID: <864mvigtxd.wl%poyopoyo@puripuri.plala.or.jp> From: poyopoyo@puripuri.plala.or.jp To: Warren Block Subject: Re: Mirrored SSDs for ZIL/SLOG - safety, flushing, capacitors In-Reply-To: References: Mail-Followup-To: Warren Block , javocado , FreeBSD Filesystems User-Agent: Wanderlust/2.15.9 (Almost Unreal) SEMI/1.14.6 (Maruoka) FLIM/1.14.9 (=?ISO-2022-JP-2?B?R29qGyQoRCtXGyhC?=) APEL/10.8 Emacs/24.3 (amd64-portbld-freebsd10.0) MULE/6.0 (HANACHIRUSATO) MIME-Version: 1.0 (generated by SEMI 1.14.6 - "Maruoka") Content-Type: text/plain; charset=US-ASCII X-VirusScan: Outbound; msa04b; Mon, 6 Oct 2014 05:09:35 +0900 Cc: FreeBSD Filesystems X-BeenThere: freebsd-fs@freebsd.org X-Mailman-Version: 2.1.18-1 Precedence: list List-Id: Filesystems List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Sun, 05 Oct 2014 20:15:41 -0000 At Fri, 3 Oct 2014 05:59:48 -0600 (MDT), Warren Block wrote: > I think the Crucial M550 SSDs have additional backup capacitors also. > However, I'm still skeptical about the value of a small SSD power backup > versus a true UPS for the entire system. I have read an interesting article on this topic: http://www.anandtech.com/show/8528 Micron M600 (128GB, 256GB & 1TB) SSD Review (The Truth About Micron's Power-Loss Protection) The latter part of this article reports Micron's position of this specific function. In my understanding, Micron's consumer-class SSDs such as M550 have very small backup capacitors which would not be worth flushing dirty DRAM cache, but completing ongoing NAND programming. This means while programming at power loss would not corrupt NAND, "successfully written" host write would be lost due to cache loss. It does not provide enough peace of mind required for logging device. It looks like metadata journaling or softupdates for filesystem, not a full journaling implementation. -- kuro