What are buffer busy waits?

If two processes try (almost) simultaneously the same block and the block is not resident in the buffer cache, one process will allocate a buffer in the buffer cache, lock it and read the block into the buffer. The other process is locked until the block is read. This wait is refered to as buffer busy wait.

The type of buffer that causes the wait can be queried with v$waitstat, which lists the waits per buffer type for buffer busy waits only.

What types of buffers is waited for?

BlockDescription
segment headerThe problem is probably a freelist contention.
data blockIncreasing the size of the database buffer cache can help to reduce these waits; but this can also point to freelist contention.
undo headerIf you don't use Undo TableSpaces, you probably have too few rollback segments.
undo block