Figure 6-20. Flowchart of a server process searching for a free buffer as the result of a physical IO block read and LRU chain latch acquisition. If a database writer is scanning an LRU, it is the same, except that an unpopular free buffer is not replaced, but skipped, and scanning continues.

A buffer header's life in an LRU chain begins at the midpoint. Because other buffer headers are being replaced and inserted into the midpoint, as buffers get promoted, a buffer header naturally migrates toward the LRU end of the LRU chain. As presented in the previous section, the only way for a buffer header to get promoted is to be identified as popular. Another significant event occurs when a buffer crosses the midpoint, moving from the hot region into the cold region.

When a buffer crosses into the cold region, its touch count is reset to a default of 1 (_db_aging_cool_count). This has the affect of cooling a hot buffer, something no buffer who desires to remain in the cache wants to occur. Increasing this parameter will artificially increase buffer value, thereby increasing the likelihood of buffer movement.5 So, by default, when a buffer header crosses into the cold region, it must be touched at least once to match the promotion criteria (_db_aging_hot_criteria).