Buffer Cache Hit Ratio goes down to Negative values

Today I faced an interesting issue in my production database. Daily we check the Buffer Cache Hit Ratio of the database. Today I got the negative value for the hit ratio.

I am using the below formula for calculating the buffer cache hit ratio.

PHYSICAL_READS                1  -   ------------------------------------------------------                                 DB_BLOCK_GETS + CONSISTENT_GETS

               

I tried with different formulae that I got from the google. But it is of no use. The I searched it in the metalink and I got the answer in it. It was a bug 2645071 stated in the 'Oracle Database Performance Tuning Guide 10g Release 1 (10.1)'  and found the  correct formula to calculate the Buffer Cache Hit Ratio

'physical reads cache'              1 - -----------------------------------------------------------------------------                        'consistent gets from cache' + 'db block gets from cache'

.

Where,

‘physical reads cache’=’physical reads’-‘physical reads direct’-‘physical reads direct(lob)’

‘consistent gets from cache’= ‘consistent gets’

‘db block gets from cache’= ‘db block gets’

physical reads direct:

  This Oracle metric is the number of reads directly from the disk bypassing the buffer cache. For example if there is any request for large data, data-intensive operations such as parallel query, reads of the disk blocks bypass the buffer cache to maximize the transfer rates.

High disk reads indicates the small db buffer cache size.