DRM - Dynamic Resource Mastering 有多可靠?

Oracle的DRM - Dynamic Resource Mastering，动态资源Re-Mastering，设计的目的是带来性能提升，但是在不同版本中带来的Bug非常之多，导致了大量的性能问题。

引用Metalink的介绍：

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.

在Oracle的建议中，在10g R1中，建议关闭DRM，通过如下步骤设置初始化参数，重启数据库生效：
alter system set "_gc_undo_affinity"=false scope=spfile sid='*';
alter system set "_gc_affinity_time"=0 scope=spfile sid='*';

可以通过如下三个主要视图来查询DRM的相关统计信息：
X$KJDRMREQ：Dynamic Remastering Requests
X$KJDRMAFNSTATS：File Remastering Statistics
X$KJDRMHVSTATS：Hash Value Statistics

此外，LMD进程负责完成DRM的操作，检查LMD进程的跟踪文件可以找到相关的信息作为诊断参考。

_gc_affinity_time 设置为 0 就禁用了DRM的检测，虽然Oracle建议在Oracle Database 10gR2中启用该参数，但是在10.2.0.4中仍然存在BUG，会导致严重的性能问题，一个Workaround仍然是禁用DRM

Parameter Name:	_gc_affinity_time
Description:	if non zero, enable dynamic object affinity
Type:	NUMBER	Obsoleted:	FALSE
Can ALTER SESSION:	FALSE	Can ALTER SYSTEM:	FALSE