Path: utzoo!utgpu!news-server.csri.toronto.edu!mailrus!cs.utexas.edu!swrinde!zaphod.mps.ohio-state.edu!mips!prls!pyramid!infmx!aland
From: aland@infmx.UUCP (Dr. Scump)
Newsgroups: comp.unix.i386
Subject: Re: ESDI caching disk controllers: Reprise
Summary: other issues besides simple caching
Keywords: ESDI cache
Message-ID: <4180@infmx.UUCP>
Date: 5 May 90 04:29:32 GMT
References: <274@caslon.cs.arizona.edu> <873@sixhub.UUCP> <1424@ssbn.WLK.COM> <1736@serene.UUCP>
Reply-To: aland@infmx.UUCP (alan denney)
Distribution: na
Organization: INFORMIX Professional Services ("Peace thru Normalization")
Lines: 32

In article <1736@serene.UUCP> rfarris@serene.UU.NET (Rick Farris) writes:
>In article <1424@ssbn.WLK.COM> bill@ssbn.WLK.COM (Bill Kennedy) writes:
>
>> With a 768K cache memory on one card I routinely get 32% cache hits
>> and with 4Mb on the one in ssbn I get 48-52% cache hits.
>
>That brings up an interesting point.  I think I remember learning
>that without a cache-hit rate of 90% or better, you're generally
>better off without a cache, due to the overhead involved in cache
>misses.  
>
>Yet Bill tells us that subjectively the machine seems faster.  Is
>this a special case?  Is the 90% number only effective with the much
>smaller differential in access speed between cache and core?  Does
>the larger spread between core and disk mean that a lower hit rate is
>effective?  ...

That surprises me, but I'm no expert.  Keep in mind, however, that
some controllers do other things besides simple caching: readahead,
sorted writeback, DMA, adjustable bus speeds, etc.  The caching 
controllers I'm using (Consensys PowerStor) make the machines 
*scream*, even when simple cache hit percentages aren't that high.

>Rick Farris   RF Engineering  POB M  Del Mar, CA  92014   voice (619) 259-6793

--
Alan Denney  @  Informix Software, Inc.          "We're homeward bound
aland@informix.com  {pyramid|uunet}!infmx!aland   ('tis a damn fine sound!)
-----------------------------------------------   with a good ship, taut & free
 Disclaimer:  These opinions are mine alone.      We don't give a damn, 
 If I am caught or killed, the secretary          when we drink our rum
 will disavow any knowledge of my actions.        with the girls of old Maui."