Oracle Internals: An Introduction

Buffer Locks

A form of enqueue locking is used to protect cached database blocks. For each buffer in the database buffer cache, there is a buffer header. The buffer headers constitute a fixed array in the permanent memory part of the shared pool. These buffer headers act as the resource structures for buffer locks. Sessions manipulate buffer headers, and thus buffers, via dynamically allocated structures known as buffer handles. The buffer handles act as the lock structures for buffer locks.

Buffer locks are taken only in shared and exclusive modes.^[3] The buffer headers implement a two-way linked list of the buffer handles for sessions that are using the buffer, and another for the buffer handles of sessions waiting for the buffer. Sessions waiting for a buffer lock report either buffer busy waits, or buffer busy due to global cache waits, or write complete waits. The parameters for buffer busy waits are shown in Table 4.6.

Table 4-6. Wait Parameters (buffer busy waits)

Parameter	Description
p1	The file number of the database block.
p2	The block number of the database block in its file.
p3	The reason for the wait. A or 1014 indicates that the buffer is locked exclusively by a session that is busy reading a block from disk into the buffer, and that the read has not yet completed. A reason of is used for consistent gets, whereas 1014 is used for current mode block gets. Any other number indicates that the buffer is locked exclusively for modification by another session.