Buffer Cache Hit Ratio Discrepency

I am attempting to tune one of our Oracle 7.3.2.3 database instances and I am having some trouble with the Buffer Cache Hit Ratio. Using the calculation specified in Oracle Education material that I received in the "Administer the Oracle7 Database II" class that I attended, I simply retrieve the 3 values from the v$sysstat table (db block gets, consistent gets, and physical reads) and perform the calculation: hit ratio = 1 - (physical reads / (db block gets + consistent gets)). We calculate out at about 15 percent hit. All of our other systems are at 90+. We kept adding to db_block_buffers, but there was no effect.

We also have a product called DBVision for Oracle (from Platinum) and their monitor states that we are well above 90 percent hit ratio. I contacted their technical support to determine how they did the calculation. To get the physical reads, they take the sum of phyrds from the v$filestat table. One would think that this should be the same or near the same as the value of physical reads row in the v$sysstat table. In most of our systems they are, but in this particular instance the values are off by 90 percent. Can you please shed some light on this?

The above mentioned instance seems to be performing at an acceptable level. I have to believe that the DBVision calculation is the more accurate of the two buffer cache hit ratios based on the performance that we are seeing, but I am very uncomfortable with the discrepancy.

Thanks for you help. Received on Thu Nov 20 1997 - 00:00:00 CST