Cache Hit Ratio definition?

HI all, I have been using Hit Ratio for a while and never paid extra attention to its formular until recently. I got confused by definitions from different sources for database Cache Hit Ratio (CHR).

consistent gets         399366
db block gets           3134
physical reads          290511

From Oracle course I took. It was defined by

CHR=100*(db block gets + consistent reads) / (db block gets + consistent reads

        + physical reads)

logical reads = (db block gets + consistent reads)

then the CHR for my DSS = 58%

on the other side, from a book titled "Advanced Oracle Tuning and Administration" by Aronoff, Loney and Sonawalla from Oracle Press. CHR was defined by

CHR=100*(db block gets + consistent reads - physical reads) / (db block gets +

        consistent reads)

then CHR = 27.8%

Mathematically, those two formulea are NOT identical. What is really going on here or I am missing something? To me the first formular makes more sense: it measures proportion of logical reads over the total. The second one measures proportions of actual reads from the database buffers over the total logical reads. When following tuning guideline of 80%, which one should be used?

Thanks

Jizhong
jizwang_at_cc.umanitoba.ca Received on Thu Jan 08 1998 - 00:00:00 CST