RAC cache fusion机制实现原理分析
本文将详细介绍RAC cache fusion机制实现原理，需要了解更多的朋友可以参考下
在单实例中，进程要想修改数据块，必须在数据块的当前版本（Currentcopy）上进行修改RAC环境也一样这便涉及到一系列问题：
如何获得数据块的版本在集群节点间的分布图？
如何知道哪个节点拥有的是当前版本？
如何完成传递过程？
这一系列问题的解决依靠内存融合技术（cachefusion）
cachefusion通过高速的privateinterconnect，在实例间进行数据块传递
这是RAC最核心的工作机制，他把所有实例的SGA虚拟成一个大的SGA区
每当不同的实例请求相同的数据块，这个数据块就需要在实例间进行传递
在Oracle7的OPS中，这种传递是通过磁盘完成的，也叫“Disk-BasedPing”
也就是第1个实例必须先把这个数据块写回磁盘，然后第2个实例再从磁盘上读取这个数据块
这种依靠磁盘来完成数据传递极大影响系统性能
在Oracle8i引入“Net-BasedPing”通过PrivateInterconnect来传递数据块
但是8i只能传递没有修改过的数据块，对于“脏块”还是要通过磁盘来传递，这一点和OPS一样
在Oracle9i的cachefusion，所有的数据块，无论修改的或者没有修改的，都可以通过PrivateInterconnect传递
系统系能得到极大的改善
在cachefusion中，每个数据块都会被映射成一个cachefusion资源，或者说是一个PCM资源
PCM资源实际上是一个数据结构，资源的名称就是DBA(数据块地址)
每个进程对数据的请求都是分步完成：
①把DBA转换成PCM资源名称
②把这个PCM资源请求提交给DLM（分布式锁管理器）
③DLM进行GlobalLock的申请、释放活动，只有进程获得了PCMLock，才能继续下一步
也就是实例首先要获得数据块的使用权
整个cachefusion有两个服务组成：GCS和GES
⑴GCS服务负责数据块在实例间的传递
由后台进程LMSn完成
⑵GES服务负责锁管理
在多个实例之间协调对数据块的访问顺序，保证数据的一致性访问
由后台进程LMD完成
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