Global Cache Coordination Example

The scenario described in the slide assumes that the data block has been changed, or dirtied, by the first instance. Furthermore, only one copy of the block exists clusterwide, and the content of the block is represented by its SCN.

1. The second instance attempting to modify the block submits a request to the GCS.
2. The GCS transmits the request to the holder. In this case, the first instance is the holder.
3. The first instance receives the message and sends the block to the second instance. The first instance retains the dirty buffer for recovery purposes. This dirty image of the block is also called a past image of the block. A past image block cannot be modified further.
4. On receipt of the block, the second instance informs the GCS that it holds the block.

Note: The data block is not written to disk before the resource is granted to the second instance.