ESDI caching disk controllers: Reprise

[Dr. Scump]

In article <1736 at serene.UUCP> rfarris at serene.UU.NET (Rick Farris) writes:
>In article <1424 at ssbn.WLK.COM> bill at 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

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