Oracle RAC OCR 与健忘症
OCR就好比Windows的一个注册表，存储了所有与集群，RAC数据库相关的配置信息。而且是公用的配置，也就是说多个节点共享相同的配置
OCR就好比Windows的一个注册表，存储了所有与集群，，RAC数据库相关的配置信息。而且是公用的配置，也就是说多个节点共享相同的配置信息。因此该配置应当存储于共享磁盘。本文主要基于Oracle 10g RAC描述了集群的OCR以及OCR产生的健忘问题。
一、OCR的特点
类似于Windows注册表，用于存储所有与集群，RAC数据库相关的配置信息
被多个节点所共享，因此，只能存储于共享磁盘。支持单disk以及镜像方式来存放。大小通常100MB-1GB。
在Oracle 10g中，只能存储于裸设备或者ocfs文件系统，以及nfs,gfs文件系统。Oracle 11g中可以直接存放在asm中。
整个集群及RAC数据库配置需要在OCR中来进行维护。换句话说，就像windows注册表的导入导出，修改、更新键值等。
通常情况下，OCR中的配置信息会随着使用工具对其进行自动更新。如SRVCTL,DBCA,OEM,NETCA等。
而OCR的配置与维护则通常包括OCR的校验，备份，查看OCR的内容，添加移出OCR文件，重定位，修复OCR文件。
OCR磁盘最多只能有两个，一个Primary OCR 和一个Mirror OCR，两个OCR 磁盘互为镜像，以防止OCR 磁盘的单点故障。
注：集群，我们通常指的是clusterware，而RAC数据库，即是基于集群之上的数据库。
二、OCR包含的内容
OCR中通常包含下列内容
节点成员信息
数据库实例，节点，以及其他的映射关系
ASM
资源配置信息(vip,services等等)
服务特性(Service characteristics)
Oracle集群中相关进程的信息
CRS控制的第三方应用程序信息
三、OCR的工作过程
由于OCR存放于共享存储，因此在Cluster中的每个节点都通过本地OCR进程访问OCR缓存在其内存中维护着的一个副本。同时由于对OCR
的所有操作必须确保OCR内容完整性，所以在ORACLE Clusterware运行过程中，并不是所有结点都能操作OCR Disk。 只有一个OCR进程对共
享存储中的OCR进行读写操作。这个节点叫作OCR Master结点。此进程负责刷新（refresh）其自己拥有的本地缓存以及Cluster中其他节点
的OCR cache。也就是说，OCR客户端查询都是通过本地OCR进程来查询本地的一个OCR副本，而当客户端需要更新OCR时，它们将通过本地OCR
进程与那个扮演读写OCR文件的进程OCR Master进行交互。
#下面是一个两节点的RAC，可以看出ocr master节点起初在第一个节点，后来转移到第二个节点，再后来又到第一个节点。
#这是由于节点一关闭或节点一上的集群处于不可用状态导致ocr master节点发生了转移。
oracle@bo2dbp:/u01/oracle/crs/log/bo2dbp/cssd> cat ocssd.log | grep "master node"
[ CSSD]CLSS-3001: local node number 1, master node number 1
[ CSSD]CLSS-3001: local node number 1, master node number 1
[ CSSD]CLSS-3001: local node number 1, master node number 1
[ CSSD]CLSS-3001: local node number 1, master node number 1
[ CSSD]CLSS-3001: local node number 1, master node number 2
[ CSSD]CLSS-3001: local node number 1, master node number 1
oracle@bo2dbp:~> grep -i "master node" $ORA_CRS_HOME/log/bo2dbp/cssd/ocssd.log | tail -1
[ CSSD]CLSS-3001: local node number 1, master node number 1
OCR客户端应用有：Oracle通用安装器（OUI）、SRVCTL、企业管理器（EM）、DBCA、DBUA、NetCA和虚拟网络协议助理（VIPCA）。
此外，OCR维护管理着CRS内部中定义的各种应用程序的资源的依赖和状态信息，特别是Database、Instance、Services和节点的应用程序。
OCR配置文件的名字是ocr.loc，Linux下位于/etc/oracle/ocr.loc。
后附OCR结构图
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