Oracle Database Tips by Donald Burleson

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

Block Transfers Using Cache Fusion - Example

Figure 7.6 (b) Data Block Shipping using Cache Fusion (Lock conversions)

In Stage (6), user B issues the same select against the 'salesman' table on instance 2. Instance 2 will request a consistent read copy of the buffer from another instance, which happens to be the current master.

Therefore, instance 1 will ship the block to instance 2, where it will be acquired with SG1. Then, at instance 1, the lock will be converted to SG1.

In Stage (7), user C on instance C updates the same row. Therefore, instance 3 acquires an exclusive lock and instances 1 and 2 will be downgraded to NG1 (Null global with past image). Instance 3 will have exclusive mode with a global role.

In Stage (8), the checkpoint is initiated and a 'write to disk' takes place at instance 3. Instance 1 and instance 2 will discard their past images. At instance 3, the lock mode will become exclusive with a local role.

The stages above illustrate that consistency is maintained even though the same block is requested at different levels. These operations are transparent to the application. All the mode and role conversions are handled by Oracle without any human configuration.

If there are considerable cross-instance updates and queries for the same set of blocks, blocks are moved across without resorting to disk read or disk writes. However, there will be considerable lock conversions, which may be expensive, though they are less expensive than disk read/writes.