LRU chains are much, much longer than CBCs. A 1GB 8KB buffer cache with 126,126 buffers and 16 LRU chains (the default) means that each LRU chain is, on average, 7,888 buffer headers long. A smaller system with a 250MB 8KB buffer cache containing 31,563 buffers and 8 LRU chains (by default) means that each LRU is around 3,945 buffer headers long. There will be some dirty buffers on the write list, but Oracle tries to keep that list very short. So although the numbers in these examples are not exact, they do present a good picture of how long LRU chains can be. This is important to keep in mind, because when diagramming LRU chains, as in Figure 6-15, they are usually drawn with only a few buffer headers.

It is no problem that dirty buffers reside on an LRU chain. In fact, it would be a massive performance problem if they couldn't reside on an LRU chain. One of the objectives of the LRU chains is to help keep popular buffers in the cache, and many dirty buffers are also very popular. During a database checkpoint though, every dirty buffer does get written to disk and becomes free once again.

Figure 6-15. This is an example of Oracle's LRU chains. There are 2 LRU chain latches and 6 LRU chains, protecting 20 buffer headers. Each buffer header is associated with a single free, pinned, or dirty buffer.