第二章 体系结构

第二章 体系结构[align=center][/align]
2.2.3Temp Files
1.Oracle will use temporary files to store the intermediate results of a large sort operation, or result set,when there is insufficient memory to hold it all in RAM.
当内存不足以在RAM中保存一个大规模排序操作的中间结果或结果集，Oracle使用临时文件来存储它们。
2.Permanent data objects, such as a table or an index, will never be stored in a temporaty file, but the contents of a temporary table or index would be.
3.Each and every change you make to an object will be recorded in the redo logs-these transaction logs can be replayed at a later date in order to 'redo a transaction'. Temporary files are exclueded from this process. Temporary files never have redo generated for them, although they do have UNDO generated.
2.2.4Control Files
1.The control file is a fairly small file that contains a directory of the other files Oracle needs. The parameter file tells the instance where the control files are, the control files tell the instance where the database and online redo log files are.
2.Control files should be multiplexed either by hardware or by Oracle when RAID or mirroring is not available-more than one copy of them should exist and they should be stored on separate disks, to avoid losing them in the event you hav a disk failure.
2.2.5Redo Log Files
2.2.5.1.Online redo log
1.Every Oracle database has at least two online redo log files. These online redo log files are fixed in size and are used in a circular fashion. Oracle will write to log file1, and when it gets to the end of this file, it will switch to log file2, and rewrite the contents of that file from start to end.
2.Since the redo logs are used to recover transactions in the event of a failure, we must assure ourselves that we won't need the contents of a redo log file in the event of a failure before we reuse it.
因为重做日志是用来在失败时进行恢复处理用的，必须保证，在重用之前，不需要在失败时重做日志的内容。
3.数据库缓冲区高速缓存是数据库块临时存储数据的地方。这是Oracle的SGA的结构。当块被读取时，它们就存储于这个缓存区高速缓存中——希望将来会允许不必物理地重读它们。缓冲区高速缓存是第一个，也是最重要的性能优化设备，它单独存在，以便使非常慢的物理I/O进程显得更快一些。当通过更新块中的一行来修改一个块时，这些修改在内存中，对缓冲区高速缓存中的块进行操作。重做这个修改的足够信息存储在重做日志缓冲器，即另一个SGA数据结构中。当提交修改，将修改永久化时，Oracle不会再任何SGA中修改块，并将它们写到磁盘上。相反，它只是将重做日志缓冲器中的内容写到联机重做日志中。
4.只要修改的块在缓冲区高速缓存中而不在磁盘上，就需要联机重做日志的内容，以防止数据库失败。
5.这是DBWn开始活动的地方。它是Oracle后台进程，负责在缓冲区高速缓存溢出的时候创造空间，最重要的是，执行检查点。检查点是缓冲区高速缓存到磁盘上脏（修改过的）块的刷新。
2.2.5.2Archived Redo log
1.The Oracle database can run in one of two modes-NoArchiveLog mode and ArchiverLog mode. A system is not a production system unless it is in ArchiveLog mode.
2.A database that is not in ArchiveLog mode will, some day, lose data.
3.The difference between these two modes is simply what happens to a redo log file when Oracle goest to reuse it. 'Will we keep a copy of that redo or should Oracle just overwrite it, losing it forever?'