- When instance A needs a block of data to modify, it reads the bock from disk, before reading it must inform the GCS (DLM). GCS keeps track of the lock status of the data block by keeping an exclusive lock on it on behalf of instance A
- Now instance B wants to modify that same data block, it to must inform GCS, GCS will then request instance A to release the lock, thus GCS ensures that instance B gets the latest version of the data block (including instance A modifications) and then exclusively locks it on instance B behalf.
- At any one point in time, only one instance has the current copy of the block, thus keeping the integrity of the block.
GCS maintains data coherency and coordination by keeping track of all lock status of each block that can be read/written to by any nodes in the RAC. GCS is an in memory database that contains information about current locks on blocks and instances waiting to acquire locks. This is known asParallel Cache Management (PCM). The Global Resource Manager (GRM) helps to coordinate and communicate the lock requests from Oracle processes between instances in the RAC. Each instance has a buffer cache in its SGA, to ensure that each RAC instance obtains the block that it needs to satisfy a query or transaction. RAC uses two processes the GCS and GES which maintain records of lock status of each data file and each cached block using a GRD.