Oracle Database Tips by Donald Burleson

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

Cache Fusion Scenarios

Scenario 2

As shown in Figure 7.5(b), instance 1 intends to modify or update the data block and submits a request to GCS. The GCS transmits the request on to the holder (Instance 2).

Upon receiving the message, instance 2 sends the block to instance 1. Before sending the block, the resource is downgraded to null mode and the changed (dirty) buffer is kept as a PI. Thus, the role changes to global (G) because the block is dirty.

Along with the block, instance 2 informs the requestor that it retained a PI copy and a null resource. The same message also specifies that the requestor can take the block held in exclusive mode and with a global role

(X, G).

Figure 7.5 (b) Write/Write Cache Fusion - GCS processing

Upon receipt of the block and the resource dispositions, instance 1 informs the GCS of the mode and role (X, G). Note that the data block is not written to disk before the resource is granted to the other instance. That is, DBWR is not involved in the cache coherency scheme at this stage.