When a process requires access to a database block that is not in the buffer pool, the database engine must replace another buffer to make room for it. The server searches for a buffer to replace.
The ideal replacement candidate is a buffer that is unlocked and unmodified. Replacing an unmodified buffer requires only one step: writing the new contents into the buffer. If a buffer contains modified data, it must first be evicted before it can be replaced. Evicting the buffer requires two steps: writing the buffer's contents to disk, then writing new contents into the buffer. It is therefore slower and requires more overhead as shown in the following figure.
Figure 31. Evicting buffers
When searching for a replacement candidate, the server searches a maximum of ten buffers. If the server fails to find an unlocked, unmodified buffer, the server evicts the first unlocked, modified buffer that it finds.
