From owner-freebsd-questions Sun Jun 24 21:59:14 2001 Delivered-To: freebsd-questions@freebsd.org Received: from obsidian.spvi.com (hs244.spvi.com [64.132.76.244]) by hub.freebsd.org (Postfix) with ESMTP id 65D5037B409 for ; Sun, 24 Jun 2001 21:59:11 -0700 (PDT) (envelope-from steve@spvi.com) Received: from spvi.com (hs243.spvi.com [64.132.76.243]) by obsidian.spvi.com (8.11.3/8.11.3) with ESMTP id f5P53No67273; Mon, 25 Jun 2001 00:03:23 -0500 (EST) (envelope-from steve@spvi.com) Message-ID: <3B36C515.BDA08193@spvi.com> Date: Sun, 24 Jun 2001 23:59:09 -0500 From: Steve Spicklemire Reply-To: steve@spvi.com Organization: SPVI X-Mailer: Mozilla 4.75C-CCK-MCD {C-UDP; EBM-APPLE} (Macintosh; U; PPC) X-Accept-Language: en MIME-Version: 1.0 To: Mike Meyer Cc: "Christopher W. Aiken" , questions@FreeBSD.ORG Subject: Re: UPDATE: FreeBSD 4.3 --> pits References: <15158.16531.104080.954141@guru.mired.org> Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Sender: owner-freebsd-questions@FreeBSD.ORG Precedence: bulk List-ID: List-Archive: (Web Archive) List-Help: (List Instructions) List-Subscribe: List-Unsubscribe: X-Loop: FreeBSD.ORG What effect does this flag have on RAIDish drives (e.g., 3ware two drive RAID10 and/or vinum setups)? Can folks achieve good performance and reasonable reliability at the same time? thanks! -steve Mike Meyer wrote: > > Christopher W. Aiken types: > > Thanks Kent. I added hw.ata.wc="1" to the /etc/defaults/loader.conf > > file and I seem to be back in business. Why on earth did FBSD 4.3 > > change this default? I would hazard a guess that a lot of people > > were hit with this one. The average "off the shelf" PX has EIDE/IDE > > drives. > > It's a reliability vs. speed issue. If your drives have WC enabled, > they can *lie* about whether or not writes are actually on disk > instead of in cache. The delay can be indefinite, depending on other > activity on the drive. The net result is that there the only limit on > how much data you can lose on a power failure is how much work you've > done. > > The speed hit was sufficient that this has been changed. Drives are > now left in whatever mode they are found in - which usually means WC > enabled, as that's how manufacturers ship them. Enabling soft updates > on file systems mounted on drives with WC enabled is not > recommended. If you really want a little extra performance, you can > mount the file systems async. Yes, that's considered dangerous - but > turning on WC means you've already decided you want speed instead of > reliability. > > Final note - SCSI drives support a feature called "tagged queueing" > that takes the sting out of disable WC on the drive. IDE drives are > starting to appear with this feature, and the ata driver supports it, > but the drive implementations seem flaky. See the tuning man page of > your system for more information. > > -- > Mike Meyer http://www.mired.org/home/mwm/ > Independent WWW/Perforce/FreeBSD/Unix consultant, email for more information. > > To Unsubscribe: send mail to majordomo@FreeBSD.org > with "unsubscribe freebsd-questions" in the body of the message To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-questions" in the body of the message