A buffer is free when it matches the block on disk. I commonly refer to a free buffer as a mirrored buffer, because it mirrors what is on disk. Figure 6-2 shows just how simple it is to determine the number of free buffers in the buffer cache. A free buffer may indeed be empty (for example, after an instance restart), but it will most likely contain real block information, such as rows. A free buffer can be replaced without any corruption because there is a copy on disk. Of course, if a transaction commits, then at a minimum, the buffer change must be recorded in an online redo log.

Figure 6-2. The number of free buffers can be determined by issuing a simple v$bh-based query looking at the status column.

A free buffer can be very unpopular. Perhaps a query needed to look at a single row, and therefore required the block to be brought into the cache, yet the buffer was never accessed again. On the other hand, a free buffer can also be very popular. For example, if a particular block is repeatedly queried, it becomes relatively popular, yet still free, because the buffer has not been changed. If you keep the free buffer definition simple and pure, many of Oracle's algorithms also become clearer, which makes understanding, detecting, and resolving contention easier.