Date: Thu, 21 Sep 2000 12:35:32 -0700 From: Julian Elischer <julian@elischer.org> To: Archie Cobbs <archie@whistle.com> Cc: Soren Schmidt <sos@freebsd.dk>, mbendiks@eunet.no, terry@lambert.org, fs@freebsd.org Subject: Re: disable write caching with softupdates? Message-ID: <39CA6304.2781E494@elischer.org> References: <200009211900.MAA18117@bubba.whistle.com>
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Archie Cobbs wrote:
>
> What we do on the InterJet right now is detect a power failure and,
> if detected, freeze the system immediately. This guarantees that
> the disk won't write a corrupted sector (because our power supply
> has 80ms or so of residual power -- enough for the disk to finish
> writing). Disabling write caching guarantees that the sectors
> believed to be written by soft-updates actually are when the system
> freezes. BOTH guarantees are necessary in order to guarantee a
> valid disk.
for those who claim that (actually 60) mSecs is too long for
a single write, I calculated that I needed time toallow for a seek,
a write of the first few sectors of a write, another seek to an
alternate sector if ther is a badblock in the set, a firther write,
followed by a return seek to the original sequence of blocks.
Including rotational latencies I decided that 60mSec would cover
us for "enough" cases..
If you have 2 separate bad blocks in a single logical write
you are probably ok but it is getting tight, especially in Japan
where the (sometimes 90V) mains voltage means that the power supply
is REALLY going to give you 60mS.. in the USA you get 80 and in AUS
(250V) you get about 200mSecs :-)
Incidentally, if the mains return before the system dies
(within about 80mS) we continue on and sync the disks....
Archies comment about MOST drives not supporting safe writing of
accepted work is an understatement. When I was testing and selecting
drives for the interjet, I found NO drives that would guarantee that
data accepted for writing would be written in the case of power
failure.
>
--
__--_|\ Julian Elischer
/ \ julian@elischer.org
( OZ ) World tour 2000
---> X_.---._/ presently in: Perth
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