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From: mjr@hussar.dco.dec.com (Marcus J. Ranum)
Newsgroups: comp.unix.ultrix
Subject: Re: Maxtor disks & DEC5000
Keywords: Help
Message-ID: <1990Oct14.205157.27090@decuac.dec.com>
Date: 14 Oct 90 20:51:57 GMT
References: <768@inesc.UUCP> <1990Oct11.211023.2207@decuac.dec.com> <2933@canisius.UUCP>
Sender: news@decuac.dec.com (Network News)
Reply-To: mjr@hussar.dco.dec.com (Marcus J. Ranum)
Organization: Digital Equipment Corp., Ultrix Resource Center
Lines: 38

In article <2933@canisius.UUCP> pavlov@canisius.UUCP (Greg Pavlov) writes:
>In article <1990Oct11.211023.2207@decuac.dec.com>, I write:
>> 	Depending on the way you anticipate using your system, you might
>> want to increase the size of your buffer cache, and change the write
>> scheduling policy to delay scheduling dirty buffers for write.....
>> 
>   What is the downside of this ?  Am I taking a bigger chance on ending up
>   with a corrupted database, for instance ?   What are potential performance
>   downsides ?

	The possible downside of the delayed write is a slightly better
chance of losing something. Off the top of my head I can't think of a case
where delaying scheduling dirty blocks for write would be a performance
hit. Unless your machine is really unstable I wouldn't worry about losing
a block or 2 every so often. :)

	Bumping the buffer cache can be a performance hit if it causes you
to start running short of memory for jobs you may have running. If you
increase the buffer cache to, say, 20% and notice that you're starting
to swap where you weren't before, you're hurting your performance. If you
have a machine, for example, that is set up as a file server - nobody
logs in and starts running X-windows on it - then boosting the cache 
may make a lot of sense. If *all* you're doing is file service, you can
steal a riff from Plan 9 and just use everything you can for cache. :)

>    I have played with the tunefs parameters many times  - particularly rotat-
>    ional delay.  I have noticed that I do not see any difference (well, no 
>    more than 2-5%) regardless of what I set them to and on what type/brand of
>    disk (RA70's, RA90's, RDnn's, Fuji M2333k, various CDC and HP).

	Yep, I've never managed to squeeze the legendary Massive Benefits
of tweaking rotational delay out of the disks, either. I did manage to make
the disks on my 3100 measurably slower one afternoon, so I suppose I could
say I reaped massive benefits by undoing the damage.

mjr.
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