Let's walk through an abstracted scenario. I'm leaving out a lot of detail, but this story will work fine for our purposes. Suppose you are a server process who must query a row, which resides in a specific block. Based on the SQL statement and the data dictionary, you know the file and block number. You're all about speed, so you're hoping the block already resides in the buffer cache. To check this, you need to get the buffer's buffer cache memory address, which resides in its buffer header.

To find the buffer header, you must access the CBC structure. You hash on the file and block number, which points you to a hash bucket. Based on this hash bucket, you look up the associated CBC latch and contend for it. After a few spins, you are able to acquire the latch, so you begin your sequential CBC search. The first buffer header is not the buffer you're interested in, and unfortunately, there is no second buffer header in this cache chain, so you know the buffer does not currently reside in the buffer cache.

You release the CBC latch and make a call to the operating system, asking for your data block. While you're waiting, you're posting a db file sequential read wait event. Finally, you receive the block from the operating system and hold it in your PGA memory. Because you did not make a direct read, before you, or any other server process, can access the buffer, it must be properly inserted into the buffer cache and update all the appropriate structures.