Oracle Database Tips by Donald Burleson

Oracle10g Grid Computing with RAC
Chapter 7 - Cache Fusion and Inter Instance Coordination

Lock Modes

When a block is brought into the local cache of an instance, it is acquired with the local role. But, if a dirty buffer for the same data block is present in a remote instance, a past image is created in the remote instance before the data block is sent to the requesting instance's cache. Therefore, the data block resource acquires a global role.

For recovery purposes, instances that have past images will keep those past images in their buffer cache, until the master instance prompts the lock to release them. When the buffers are discarded, the instance holding the past image will write a BWR (or 'block written redo') to the redo stream. The BWR indicates that the block has already been written to disk and is not needed for recovery by the instance. Buffers are discarded when the disk write is initiated on the master instance. The master instance is where the current status and position of the data block is maintained.

So far, we have seen how a GCS resource maintains its access mode and its role. There is another feature called the "buffer state", which is discussed in the next section.

Block Access Modes and Buffer States

The buffer state indicates the status of a buffer in the local cache of an instance. Information about the buffer state can be seen in the dynamic performance view v$bh. The buffer state of a block relates to the access mode of the block. For example, if a buffer state is in exclusive current (XCUR) state, it indicates that the instance owns the resource in exclusive mode.