Option A in Figure 3-5 shows the entire hashing structure is protected-its access is controlled-by a single latch. While this will ensure serialization, with even limited concurrency, it will also ensure significant latch contention. As option B shows, a hashing structure can naturally be divided into group of chains, with each group being protected by its own latch. Each latch in option B in Figure 3-5 will be responsible for serialized access to a specified range of chains. In this example, the number of chains will be either three or four.

The hashing structures described in Table 3-1 will give you an idea of just how large Oracle's CBC memory structures can be. For example, the Oracle9i production database contains 1.4 million chains, not 10 chains as shown in Figure 3-5. These are massive memory structures! To reduce the likelihood that a process will be contending for a latch, based on the number of buffers the DBA set, Oracle created 8,192 CBC latches! While this may seem like a lot, each latch must still ensure controlled access to an average of 170 chains.

Table 3-1. Selected Oracle cache buffer chain attributes. All numbers are Oracle defaults, except the chosen block size and the chosen number of block buffers.