rac备份及恢复的重要概念之一——Redo Threads和Streams

rac数据库的备份和恢复，与单实例Oracle数据库的备份和恢复没有根本的不同，但区别还是有的，如果大家理解了Redo Threads和Streams概念，也就没什么了，下面这段文字清晰了解释了两者的概念和rac及单实例间的不同：

Redo information generated by an instance is called a thread of redo. All log files for that instance belong to this thread. An online redo log file belongs to a group, and the group belongs to a thread. If the database is running in ARCHIVELOG mode, the thread is nothing but all the archived redo log files. If the server is running in NOARCHIVELOG mode, then the thread is effectively the size of the online redo log files. A record in the controlfile describes each and every online redo log file. Details about the log file group and thread association details are stored in the controlfile.
A redo stream consists of all the threads of redo information ever recorded. The stream forms a timeline of changes performed to the database. In a single-instance database, the terms thread and stream refer to the same thing because a single instance has only one thread. Oracle RAC databases have multiple threads of redo—that is, each active instance has one active thread.In Oracle RAC, the threads are parallel timelines and together form the stream.