From owner-freebsd-stable@FreeBSD.ORG Thu Nov 2 09:50:20 2006 Return-Path: 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 5B9C916A412 for ; Thu, 2 Nov 2006 09:50:20 +0000 (UTC) (envelope-from fredrik.widlund@qbrick.com) Received: from mail.qbrick.com (mail.qbrick.com [62.13.40.40]) by mx1.FreeBSD.org (Postfix) with ESMTP id C649F43D58 for ; Thu, 2 Nov 2006 09:50:19 +0000 (GMT) (envelope-from fredrik.widlund@qbrick.com) Received: from localhost (localhost [127.0.0.1]) by mail.qbrick.com (Postfix) with ESMTP id C677D519AB; Thu, 2 Nov 2006 10:49:37 +0100 (CET) Received: from mail.qbrick.com ([127.0.0.1]) by localhost (mail0.p0.w0.local [127.0.0.1]) (amavisd-new, port 10024) with LMTP id 65805-01-18; Thu, 2 Nov 2006 10:49:36 +0100 (CET) Received: from [10.43.0.2] (fkwd0.p0.u3.local [10.43.0.2]) by mail.qbrick.com (Postfix) with ESMTP id 6D7F7519A8; Thu, 2 Nov 2006 10:49:36 +0100 (CET) Message-ID: <4549BF59.3000405@qbrick.com> Date: Thu, 02 Nov 2006 10:50:17 +0100 From: Fredrik Widlund User-Agent: Thunderbird 1.5.0.7 (Windows/20060909) MIME-Version: 1.0 To: Bob Willcox References: <45470D95.5020801@qbrick.com> <454718DD.8060108@qbrick.com> <45471ACC.2030604@fer.hr> <4547421D.2010206@qbrick.com> <45477FCC.9050901@qbrick.com> <4547DD7E.4030509@qbrick.com> <20061101210243.GG98766@rancor.immure.com> In-Reply-To: <20061101210243.GG98766@rancor.immure.com> X-Enigmail-Version: 0.93.2.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit X-Virus-Scanned: by amavisd-new at qbrick.com Cc: freebsd-stable@freebsd.org Subject: Re: SAS Raid - mfi driver X-BeenThere: freebsd-stable@freebsd.org X-Mailman-Version: 2.1.5 Precedence: list List-Id: Production branch of FreeBSD source code List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Thu, 02 Nov 2006 09:50:20 -0000 Because the card itself will deal with the buffered writes independently of the kernel activity the risk should be less than using softupdates. Words like "screwed" seems to me to be exaggerated in the generic case. I our case specific you would need to understand the nature of what we are doing to be able to make a comment like that. For example data is redundant (exists in many copies), consists of very large sequencial files, we have plenty of backup power, and the greatest risk is fbsd locking up/crashing. Anyway our specific case is not of interest here, I just wanted to share our experiences with the LSI MegaSAS with other fbsd users so they understand why they get a severe performance degradation if they try to use such a card w/o a bbu, and what their options are. The generic case of how great the risk really is of corrupting filesystems completely using caches of any kind on the way to secondary storage still is interesting to me, so if you could elaborate here that would be great! Kind regards, Fredrik Widlund Bob Willcox wrote: > On Wed, Nov 01, 2006 at 12:34:22AM +0100, Fredrik Widlund wrote: > >> Yes, it forces writeback even when the controller has no BBU. Choosing >> WBack itself will default back to WThru. It's dangerous, but I guess it >> should be much less dangerous than using for example softupdates. >> > > I don't see how it could be *less* dangerous than using softupdates. Any > loss of power while writing and it seems to me that you are going to be > screwed w/o a BBU. > > [snip] > >