When using Real application Clusters (RAC), Each instance has its own SGA and buffer cache. RAC will ensure that these block changes are co -ordinated to maximize performance and to ensure data intergrity. Each copy of the buffer also called as a cache resource has a master which is one of the nodes of the cluster.
In database releases before 10g (10.1.0.2) once a cache resource is mastered on an instance, a re-mastering or a change in the master would take place only during a reconfiguration that would happen automatically during both normal operations like instance startup or instance shutdown or abnormal events like Node eviction by Cluster Manager. So if Node B is the master of a cache resource, this resource will remain mastered on Node B until reconfiguration.
10g introduces a concept of resource remastering via DRM. With DRM a resource can be re-mastered on another node say from Node B to Node A if it is found that the cache resource is accessed more frequently from Node A. A reconfiguration is no longer the only reason for a resource to be re-mastered.
In 10gR1 DRM is driven by affinity of files and in 10gR2 it is based on objects.
Sample LMD trace file during a DRM operation
attributes are intentionally undocumented since they may change
depending on the version. These attributes should not be changed
without discussing with Support.
@DRM is driven by the following
@ 1.) _gc_affinity_time = Time in minutes at which statistics will be evaluated (default = 10 mins)
@ 2.) _gc_affinity_limit = # of times a node accesses a file/object (default = 50)
@ 3.) _gc_affinity_minimum = minimum # of times per minute a file/object is accessed before affinity kicks in
@ (default = 600 per minute per cpu )
It is important to note that
Generally DRM should not be disabled unless Oracle Support/Development has suggested turning it off due to some known issues.
@To disable DRM, set
@To disable DRM, set
@_gc_affinity_time=0 # Only if DB version is 10.1 or 10.2
@_gc_undo_affinity=FALSE # Only if Db version is 10.2
@_gc_policy_time=FALSE # Only if DB version is 11.1 or higher
@_gc_affinity_time has been renamed to _gc_policy_time in 11g
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