Not only does its free buffer search algorithm favor replacing cloned buffers, but Oracle tries to limit the number of clones per buffer. Oracle wants the number of clones per buffer to not exceed the hidden instance parameter _db_bock_max_cr_dba, which has a default value of 6. However, if the cloning situation becomes intense, a buffer can easily have more than six clones.

But what if file number 9, block number 3755 in Figure 6-12 had 120 clones, rather than 6? If the system were experiencing CBC latch contention along with the 120 clones, we would be motivated to find out what's so special about this block and why so many processes are interested in accessing it.

For a buffer to be cloned, a process must be interested in an earlier version of the buffer. This implies two actions. First, the buffer must have been changed. Second, it is being queried. And for the contention to become significant, the buffer must be very popular; otherwise, only a few clones would be created. Popular buffers are popular for a reason. Usually, with the information shown in Figure 6-13, a DBA (or someone who knows the applications well) will know why the buffer is so popular. Because Oracle's hashing algorithm is based on the file number and block number, the solutions are frighteningly limited.