An Oracle buffer is an Oracle segment's cached block. As you might expect, an Oracle buffer starts off containing the same information as in the Oracle block. A buffer's contents depend on the type of segment and whether it's a segment header block.

Figure 6-1 is just one way to model an Oracle data block. Oracle buffers are indeed cached in Oracle's buffer cache. While I will delve deeper into data blocks later in this chapter, my point here is that a buffer simply represents the block on disk. But if a block is accessed, a difference between the buffered block and the on-disk block can occur.

While there are many buffer states, as indicated by the state column in v$bh, they can be practically summarized into three modes: free, dirty, and pinned. The algorithms I'll present in this chapter are closely associated with a buffer's state.