官档地址：
http://docs.oracle.com/cd/E11882_01/rac.112/e41960/admcon.htm#RACAD1111
A cluster comprises multiple interconnected computers or servers that appear as if they are one server to end users and applications. Oracle RAC enables you to cluster an Oracle database. Oracle RAC uses Oracle Clusterware for the infrastructure to bind multiple servers so they operate as a single system.An Oracle RAC database can have up to 100 instances.
一个集群包括多个互联主机或服务，它们作为一个服务整体去处理用户和应用的请求。oracle rac 能够让你将一个oracle数据库聚集起来。oracle rac使用oracle clusterware作为一个基础去绑定多个服务，使之可以作为一个系统来进行操控。一个oracle rac集群数据库最多能够拥有100个实例。
Oracle RAC databases differ architecturally from noncluster Oracle databases in that each Oracle RAC database instance also has:
At least one additional thread of redo for each instance
An instance-specific undo tablespace
In Oracle RAC, each Oracle instance must run on a separate server.
oracle rac集群数据库在结构上与非集群数据库是不同的。每个oracle rac集群数据库实例都有：
1、每个实例至少一个额外的redo线程
2、每个实例都有自己的undo表空间
在oracle rac集群数据库中，每个oracle实例都必须运行在各自的服务器上。
Traditionally, an Oracle RAC environment is located in one data center. However, you can configure Oracle RAC on an extended distance cluster, which is an architecture that provides extremely fast recovery from a site failure and allows for all nodes, at all sites, to actively process transactions as part of a single database cluster. In an extended cluster, the nodes in the cluster are located in two buildings that are separated by greater distances (anywhere from across the street, to across a campus, or across a city). For availability reasons, the data must be located at both sites, thus requiring the implementation of disk mirroring technology for storage.
通常，一个oracle rac环境被放置在一个数据中心。然而，你可以在一个扩展的远程集群上配置oracle rac环境，这个集群的所有节点、所有地方都可以从一个故障点非常快地恢复过来，作为一个集群数据库的一部分去迅速处理事务。在一个扩展集群中，节点是在距离很远的两个地方的。出于有效性考虑，数据必须存储在两个地方，因此就需要启用磁盘镜像技术来进行存储。
You can install Oracle Clusterware independently or as a prerequisite前提 to the Oracle RAC installation process. Oracle Database features, such as services, use the underlying潜在的 Oracle Clusterware mechanisms机制 to provide advanced capabilities容量. Oracle Database also continues to support select third-party第三方的 clusterware products on specified platforms.
你可以独立地安装oracle集群软件，这是一个oracle安装的前提条件。oracle数据库在这方面有一些特点，例如，服务是通过oracle集群软件机制来提供初始的大小。oracle数据库在特定的操作系统支持第三方的集群软件。
▪ Understanding Cluster-Aware Storage Solutions
An Oracle RAC database is a shared everything database. All data files, control files, SPFILEs,Foot 2 and redo log files in Oracle RAC environments must reside on cluster-aware shared disks, so that all of the cluster database instances can access these storage components. Because Oracle RAC databases use a shared everything architecture, Oracle RAC requires cluster-aware storage for all database files.
In Oracle RAC, the Oracle Database software manages disk access and is certified for use on a variety of storage architectures. It is your choice how to configure your storage, but you must use a supported cluster-aware storage solution. Oracle Database provides the following file storage options for Oracle RAC:
Oracle Automatic Storage Management (Oracle ASM)
Oracle recommends this solution to manage your storage.
A certified cluster file system, including OCFS2 and Oracle Cluster File System (OCFS for Windows)
OCFS2 is available for Linux, and OCFS for Windows is available for Windows platforms. However you may optionally随意的 use a third-party cluster file system or cluster-aware volume群集卷 manager that is certified for Oracle RAC.
Certified network file system (NFS) file servers
理解集群存储方案
oraclerac数据库是一个共享型数据库。在oraclerac环境中的所有的数据文件、控制文件、动态参数文件和重做日志文件，都必须共享地保存在磁盘中，所有的数据库实例都能够访问这些存储组件，因为oracle染成数据库使用一个共享型结构。oracle rac需要能够存储所有数据库文件的存储器。
在oracle rac中，oracle数据库软件可以管理磁盘的接口，并且在一个多样化的存储结构上经使用者验证。你可以选择如何配置你的存储器，但是你必须使用一个oracle支持的集群存储结构方案。oracle数据库为rac提供了下列文件存储方案：
1、oracle自动存储管理系统（ASM）oracle建议使用这个方案来管理你的存储器
2、一个合适集群文件系统，包括OCFS2 和oracle数据库集群文件系统（OCFS是给windows用的）
OCFS2可以在linux上使用，，orcf 可以在windows平台上使用。然而你可以随意的使用一个oraclerac认证的第三方的集群文件系统或者集群卷来管理
3、经认证的文件系统（NFS）和文件服务。
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由于精力有限，官档我只挑部分段落。我的英文水平不是很好，如果各位觉得我翻译的有问题，请即时联系我。
