By default, the hot and cold regions are split down the middle 50/50. with the midpoint truly at the middle. However, this is configurable through the _db_percent_hot_default parameter. Table 6-3. later in this chapter, describes the more significant touch-count instance parameters.

As other buffer headers are inserted into the midpoint or promoted, buffer headers naturally migrate from the hot region toward the cold region's LRU chain endpoint. After a buffer header is inserted, the only way to remain in the cache for a long time is to be repeatedly promoted. I'll detail this in the upcoming "Buffer Promotion" section.

Because the window scheme used in the modified LRU algorithm is no longer used, the hidden instance parameter _small_table_threshold became deprecated. However, in Oracle Database 11g, it is being used again, but for a different purpose. Starting with this version, the _small_table_threshold parameter is the threshold for a server process to start issuing direct reads. Direct reads can increase performance because blocks read from disk are not placed into the buffer cache, but instead are processed in the server process's PGA memory. However, it is more selfish read and can actually slow performance, because no other server process will benefit from the IO activity. My tests have shown that Oracle Database 11g does not always respect changes to this parameter.