This illustrates the scenario of an instance requesting a changed block for modification.
The three main elements shown are The Global Cache Service, Instance 1, Instance 2, and a shared disk.
In Step 1 instance 2 sends a write request to the GCS. The GCS forwards the write request to instance 1, the current block holder. When instance 1 receives the write request instance 1 writes the block to disk. Then instance 1 records the completion of the write operation and informs the GCS that the resource role can become local. The GCS then orders all instances that hold PI holders to discard, or flush, their PIs because the PIs are no longer needed for recovery.
Further details about this operation are available in the surrounding text.