From owner-freebsd-hackers  Tue Mar  3 16:53:03 1998
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Date: Tue, 03 Mar 1998 16:57:21 -0800 (PST)
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From: Simon Shapiro <shimon@simon-shapiro.org>
To: Karl Denninger <karl@mcs.net>
Subject: Re: SCSI Bus redundancy...
Cc: hackers@FreeBSD.ORG, Julian Elischer <julian@whistle.com>,
        Wilko Bulte <wilko@yedi.iaf.nl>
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On 04-Mar-98 Karl Denninger wrote:
 
...

> Well, if you have a big enough cap on the output side of the regulator
> you 
> can write the rest of the current sector.  But these days, trying to get
> that on the circuit board is basically impossible.

There is a problem with that too.  Increased capacitance does funny things
to switching power supplies.  I think the whole idea went the way of good
sound when Hi-Fi turned into solidstate stereo.

 ...

> This is one of the reasons I like the CMD RAID controllers.  They have a
> nice big cache on them (with appropriate SIMMs), but they ALSO have an
> input for a 6V gelcel battery, and an internal *charging circuit* to 
> manage it.

Question is;  Do they actually know how to do something useful with that
cache?  Or is the reset that comes from the BIOS wipe them clean?  Before
you answer, try it.  I have seen batteries on computers before.  They do
not always do what you think they should.

> In addition, they have inputs on them to sense UPS health (if you have
> one).
> 
> You therefore get three levels of protection:
> 
> 1)    If the UPS goes onto battery, the unit starts "watching" things.
> 
> 2)    If it gets a low power warnings (ie: the "2 minute warning")
>       it flushes the cache and goes into write-through mode.  Now
>       you're "safe" if you get screwed.  
> 
> 3)    If you get dumped without warning, the battery is there and it will
>       pick up the pieces when power returns.
> 
> Note that if you have no battery connected or its discharged (the
> controller 
> is smart enough to know), getting a 2-minute warning flushes the cache
> and 
> quiesces the controller IMMEDIATELY.
> 
> These controllers will not operate without either a battery or UPS, and 
> both are (of course) preferred.

That's the way I like it.  Except, if the O/S is not in the loop, it may do
stupid things.


----------


Sincerely Yours, 

Simon Shapiro
Shimon@Simon-Shapiro.ORG                      Voice:   503.799.2313

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