Global Cache Services
GCS also uses locks to coordinate shared data access by multiple instances. These are different from the enqueue locks used by GES. These GCS locks protect only data blocks in the global cache (previously known as parallel cache, and the locks were known as PCM locks).
A GCS lock can be acquired in shared mode or in exclusive mode. Each lock element can have the lock role set to either local or global. If the lock role is local, the block can be handled as it is usually handled in a single instance. Local mode reads the block from the disk if it is not seen in the buffer cache and can write the block to disk when it holds the lock mode X.
In a global role, three lock modes are possible—shared, exclusive, and null. The instance ...
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