Write to Disk Coordination Example

The scenario described in the slide illustrates how an instance can perform a checkpoint at any time or replace buffers in the cache as a response to free buffer requests. Because multiple versions of the same data block with different changes can exist in the caches of instances in the cluster, a write protocol managed by the GCS ensures that only the most current version of the data is written to disk. It must also ensure that all previous versions are purged from the other caches. A write request for a data block can originate in any instance that has the current or past image of the block. In this scenario, assume that the first instance holding a past image buffer requests that the Oracle server write the buffer to disk:

1. The first instance sends a write request to the GCS.
2. The GCS forwards the request to the second instance, which is the holder of the current version of the block.
3. The second instance receives the write request and writes the block to disk.
4. The second instance records the completion of the write operation with the GCS.
5. After receipt of the notification, the GCS orders all past image holders to discard their past images. These past images are no longer needed for recovery.

Note: In this case, only one I/O is performed to write the most current version of the block to disk.