Figure 6-3 shows dirty buffers having a status of either xcur or write. I will detail current and consistent mode buffers in the upcoming section on cache buffer chains. The xcur status means a process has changed a current mode buffer's status to this status, and processes can now update rows in the buffer, although the rows are still subject to other conditions, such as row-level locking. My tests demonstrate an exclusive mode does not prevent multiple users from changing multiple rows in the same buffer; it simply signifies the current mode buffer can be changed. This becomes critical in a Real Application Clusters (RAC) environment, where there can be multiple shared current mode buffers (mode of scur), but only one exclusive current mode buffer in the entire RAC database.

Figure 6-3. The number of dirty buffers can be determined by issuing a simple v$bh-based query looking at the dirty column.

When a buffer is pinned, it cannot be replaced. Another way to look at pinning is as kind of an unofficial lock on a buffer. Since a buffer is not a relational structure, standard locking mechanisms do not apply. Pinning relates to a specific buffer, whereas latches or mutexes can control access to an entire group of buffers. Pinning can be used in conjunction with latching and locking to ensure the proper serialization, protection, and concurrency control are achieved.