Date: Sun, 24 Jun 2001 23:59:09 -0500 From: Steve Spicklemire <steve@spvi.com> To: Mike Meyer <mwm@mired.org> Cc: "Christopher W. Aiken" <cwaiken@icubed.com>, questions@FreeBSD.ORG Subject: Re: UPDATE: FreeBSD 4.3 --> pits Message-ID: <3B36C515.BDA08193@spvi.com> References: <15158.16531.104080.954141@guru.mired.org>
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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 <cwaiken@icubed.com> 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 > -- > Mike Meyer <mwm@mired.org> 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
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