Date: Tue, 3 Mar 1998 22:44:52 +0100 (MET) From: Wilko Bulte <wilko@yedi.iaf.nl> To: julian@whistle.com (Julian Elischer) Cc: shimon@simon-shapiro.org, hackers@FreeBSD.ORG Subject: Re: SCSI Bus redundancy... Message-ID: <199803032144.WAA03955@yedi.iaf.nl> In-Reply-To: <34FC66A3.2781E494@whistle.com> from Julian Elischer at "Mar 3, 98 12:22:59 pm"
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As Julian Elischer wrote... > Simon Shapiro wrote: > > > > > I have seen an interesting solution some time ago; Instead of battery, the > > spindle motor (on the disk) was used to generate the power needed to flush > > the caches. then the motor leads will be clamped, and the spidle shut down > > quickly (normal procedure nowdays). This was done on a 14" spindle that > > had a bit more inertia than todays' disks. But the circuitry consumed more > > power too. > > > > spindle inertia is no longer used for power. > > I quote from the Qantum fireball programmer's manual.. > > "You may apply the power in any order, or open either the power or > return line with no loss of data or damage to the disk drive. However, > data may be lost in the sector being written at the time of power > loss." > > Other drives have similar stories to tell. > > We queried the manufacturers closely on this > and they confirmed that they cease all operation IMMEDIATLY on > recognition of power loss. Once Upon A Time, When Power Supplies Were Still Powersupplies there were 2 signals available: AC_OK and DC_OK. Whenever your logic (disk) saw AC_OK negate, it was time to cleanup. After some time, dependent on how big your powersupply capacitors were, how loaded the PS was etc you saw DC_OK negate. Current drives can only sense powergood by looking at their DC power inputs. This is of course lousy, because as soon as you see power drop, you better park the heads and lock out any write current to the heads. A spiral written by a retracting head on top of your precious data leaves lots to be desired from a data integrity standpoint. This not even takes into account writing out unflushed cache data, possibly requiring a seek. Drive write caches are Evil. Every write cache without good battery backup is Evil. Talk to a DBMS guy about enabling disk write caches. Put sneakers on and be prepared to run fast... But then again, with VM systems that have megabytes worth of unflushed data the best way to loose your data is to pull the plug from your server ;-) Wilko _ ______________________________________________________________________ | / o / / _ Bulte email: wilko @ yedi.iaf.nl http://www.tcja.nl/~wilko |/|/ / / /( (_) Arnhem, The Netherlands - Do, or do not. There is no 'try' --------------- Support your local daemons: run [Free,Net,Open]BSD Unix -- To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message
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