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
shows which objects have been the subjects of DRM:
current_master+1 as current_mast_inst,
DATA_OBJECT_ID GC_MASTERIN CURRENT_MAST_INST
-------------- ----------- -----------------
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
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.
Learn RAC Tuning
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Oracle RAC Performance tuning,
a book that provides real world advice for resolving
the most difficult RAC performance and tuning issues.
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