RAC Dynamic Resource Manager (DRM) 

RAC Dynamic Resource Manager (DRM)

The gc cr grant 2-way and gc current grant 2-way wait events occur when locks need to be obtained in one instance but the resource master is in another instance. The GRD keeps track of the number of times a lock was requested from an instance other than where the resource master is located.

If the requests frequently come from the same instance, Dynamic Resource Mastering (DRM) will relocate the resource master to the instance dominating the requests.

By re-mastering the resource to another instance, the number of gc cr grant 2-way and gc current grant 2-way wait events are reduced.

The end users will not have to wait for cross-instance messaging to complete to be able to obtain the lock. GRD calls this technique resource affinity where a resource and its master have a strong attraction to one instance in the cluster. Resource affinity is often seen in Oracle RAC deployments where application partitioning is performed, a topic discussed in the next chapter. The gv$gcspfmaster_info view shows which objects have been the subjects of DRM: 

SQL> select

  2     data_object_id,

  3     gc_mastering_policy,

  4     current_master+1 as current_mast_inst,   

  5     remaster_cnt

  6  from

  7     gv$gcspfmaster_info;

DATA_OBJECT_ID GC_MASTERIN CURRENT_MAST_INST REMASTER_CNT

-------------- ----------- ----------------- ------------

            57 Affinity                    1            1

            58 Affinity                    2            1

            59 Affinity                    3            1

The data_object_id column can be joined to dba_objects.object_id to know which database object is involved.

It should be noted that the current_master and previous_master columns are one number off the instance identifier as they start counting from zero. The example query above adds one to obtain the instance id. Also, these current and previous master columns may show a value of 32767. If current_master is 32767, then the object was subject to DRM but no longer is. If previous_master is 32767, then the object was not subject to DRM, but now is.