Path: utzoo!attcan!uunet!mcsun!unido!tub!fauern!sun1.ruf.uni-freiburg.de!seifert
From: seifert@sun1.ruf.uni-freiburg.de (Ulrich Seifert)
Newsgroups: comp.os.os2.misc
Subject: Re: HPFS and cache controllers
Keywords: HPFS cache
Message-ID: <1990Sep15.154653.14659@sun1.ruf.uni-freiburg.de>
Date: 15 Sep 90 15:46:53 GMT
References: <1990Sep6.141036.11041@sun1.ruf.uni-freiburg.de> <57439@microsoft.UUCP>
Distribution: comp
Organization: Rechenzentrum der Universitaet Freiburg, Deutschland
Lines: 43

gordonl@microsoft.UUCP (Gordon LETWIN) writes:

>This HPFS can be configured to do write behinds; I think that may be the
>default.  This delays writes until the disk goes idle for a couple of
>seconds or until the data gets too old (risky to leave unwritten for
>too long a period of time) or until the cache gets too dirty.  The
>performance effect of this is threefold: ...

Gordon,
thank you very much for your explanations. Although I haven't seen benchmark
results for the performance of the cache system yet it the technical part
sounds very good.

Maybe this is the right place to post a related question.
Let us assume we have a power failure. Well, probably everybody thinks the 
damage from programs that unknowingly died in the machine is much bigger than
damage arising from programs dieing unknowingly in the machine will bemuch
bigger than everything what happens in the cache. Maybe but if we are doing
transaction processing we should be sure that the data safely arrive on the 
disk after a commit.
What I want to say is *NOT* that all caching with write behinds is nonsense.
But maybe it is not really enough. I think there are many possible situations
where an application should be notified that there is a problem with the power
supply. Of course, I assume that the computer is equipped with an UPS. Most
of the UPSs give a kind of warning if anything is wrong.
I think it would be a great idea for OS/2 to set up a watchdog kind of 
interface to the UPS and broadcast a warning to very running application (for
example within the PM to begin with). The file subsystem could save the cache
and switch to a no-write-behind mode until the UPS signals that everything is
o.k. again. The applications could write a kind of restart ile for the case
that the power doesn't stabelizes any more or ignore the message and the user 
could go and check the status of his coffee machine instead of hurrying from
one machine to the next and shutting everything down in the correct order.

Wouldn't that be fantastic (and maybe one argument more to give OS/2 a try?).

What do you think out there, maybe we can convince IBM/Microsoft that such a
thing is a must-have.

Ulf

Ulf