ESDI caching disk controllers: Reprise

[Bob Gustwick]

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.

this is really an implementation question.  a cache miss should be
a fast search thru some sort of table/tree/etc.  a 50/50 hit/miss
ratio should still provide a great performance increase.  why?
a search thru memory is orders of magnitudes cheaper in wall
clock time than a disk seek, unless the cache is implemented very
poorly.