Date: Sun, 24 Jun 2001 14:33:39 -0500 From: Mike Meyer <mwm@mired.org> To: "Christopher W. Aiken" <cwaiken@icubed.com> Cc: questions@freebsd.org Subject: Re: UPDATE: FreeBSD 4.3 --> pits Message-ID: <15158.16531.104080.954141@guru.mired.org> In-Reply-To: <32131968@toto.iv>
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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
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