Oracle Tips by Burleson

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

Cache Fusion Scenarios

Scenario 3

As shown in Figure 7.5(c), instance 2 first sends a write request to the GCS. This might be due to a user-executing checkpoint on instance 2. Note that there is a past image for the block on instance 2. The GCS forwards the request to instance 1 (the current block holder). The GCS remembers that a write at the system change number (SCN) is pending and it also remembers that it has to notify the nodes that have PIs of the same block.

Instance 1 then receives the write request and writes the block to disk. Instance 1 completes the write and notifies the GCS. Instance 1 also informs the GCS that the resource role can become local because the instance has completed the write of the current block. After completion of the protocol, all PIs of the block should be discarded.

Figure 7.5 (c) Write blocks to disk – GCS Processing

Upon receipt of the notification, the GCS orders all PI holders to discard (or flush) their PIs. Discarding, in this case, means that upon receipt of the message the PI holder records that the current block has been written and the buffer is released. The PI is no longer needed for recovery. The buffer is essentially free and the resource previously held in null mode is closed.