Figure 6-18. Oracle's touch-count algorithm determines buffer header popularity based on the number of times it is touched. Notice the FTS window concept is no longer necessary and has been removed.

There are three key touch-count algorithm aspects: midpoint insertion, touch count incrementation, and buffer promotion. The following sections look at each of these aspects.

The single most radical departure from the modified LRU algorithm is known as midpoint insertion. Each LRU chain is divided into a hot and cold region. When a buffer is read from disk and a free buffer has been found, the buffer and buffer header replace the previous buffer and buffer header contents, and then the buffer header is moved to the LRU chain midpoint. Single-block read, multiblock read, fast-full scan, or full-table scan-it makes no difference. The buffer header is not inserted at the MRU end of the LRU chain, but rather at the midpoint. This ensures the LRU chain is not obliterated by a large influx of a single object's blocks being read into the buffer cache.