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CHAPTER 9
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This chapter describes two of the most important pieces of data in an Oracle database: redo and undo. Redo is the information Oracle records in online (and archived) redo log files in order to replay your transaction in the event of a failure. Undo is the information Oracle records in the undo segments in order to reverse, or roll back, your transaction. In this chapter, we will discuss topics such as how redo and undo (rollback) are generated, and how they fit into transactions, recovery, and so on. We ll start off with a high-level overview of what undo and redo are and how they work together. We ll then drill down into each topic, covering each in more depth and discussing what you, the developer, need to know about them. The chapter is slanted toward the developer perspective in that we will not cover issues that a DBA should be exclusively in charge of figuring out and tuning. For example, we won t cover how to find the optimum setting for RECOVERY_PARALLELISM or the FAST_START_MTTR_TARGET parameters. Nevertheless, redo and undo are topics that bridge the DBA and developer roles. Both need a good fundamental understanding of the purpose of redo and undo, how they work, and how to avoid potential issues with regard to their use. Knowledge of redo and undo also helps both DBAs and developers better understand how the database operates, in general. In this chapter, I will present the pseudo-code for these mechanisms in Oracle and a conceptual explanation of what actually takes place. I will not cover every internal detail of what files get updated with what bytes of data. What actually takes place is a little more involved, but having a good understanding of the flow of how it works is valuable and will help you to understand the ramifications of your actions.
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Note Time and time again, I get questions regarding the exact bits and bytes of redo and undo. People seem to
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want to have a very detailed specification of exactly, precisely, what is in there. I never answer those questions. Instead, I focus on the intent of redo and undo, the concepts behind redo and undo. I focus on the use of redo and undo not on the bits and bytes. I myself do not dump redo log files or undo segments. I do use the supplied tools, such as Log Miner to read redo and flashback transaction history to read undo, but that presents the information to me in a human-readable format. So, we won t be doing internals in this chapter but rather building a strong foundation.
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CHAPTER 9 REDO AND UNDO
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Redo log files are crucial to the Oracle database. These are the transaction logs for the database. Oracle maintains two types of redo log files: online and archived. They are used for recovery purposes; their purpose in life is to be used in the event of an instance or media failure. If the power goes off on your database machine, causing an instance failure, Oracle will use the online redo logs to restore the system to exactly the committed point it was at immediately prior to the power outage. If your disk drive fails (a media failure), Oracle will use both archived redo logs and online redo logs to recover a backup of the data that was on that drive to the correct point in time. Moreover, if you accidentally truncate a table or remove some critical information and commit the operation, you can restore a backup of the affected data and recover it to the point in time immediately prior to the accident using online and archived redo log files. Archived redo log files are simply copies of old, full online redo log files. As the system fills up log files, the ARCH process makes a copy of the online redo log file in another location, and optionally puts several other copies into local and remote locations as well. These archived redo log files are used to perform media recovery when a failure is caused by a disk drive going bad or some other physical fault. Oracle can take these archived redo log files and apply them to backups of the data files to catch them up to the rest of the database. They are the transaction history of the database.
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Note With the advent of Oracle 10g, we now have flashback technology. This allows us to perform flashback
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queries (query the data as of some point in time in the past), un-drop a database table, put a table back the way it was some time ago, and so on. As a result, the number of occasions in which we need to perform a conventional recovery using backups and archived redo logs has decreased. However, the ability to perform a recovery is the DBA s most important job. Database recovery is the one thing a DBA is not allowed to get wrong.
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Every Oracle database has at least two online redo log groups with at least a single member (redo log file) in each group. These online redo log groups are used in a circular fashion. Oracle will write to the log files in group 1, and when it gets to the end of the files in group 1, it will switch to log file group 2 and begin writing to that one. When it has filled log file group 2, it will switch back to log file group 1 (assuming you have only two redo log file groups; if you have three, Oracle would, of course, proceed to the third group). Redo logs, or transaction logs, are one of the major features that make a database a database. They are perhaps its most important recovery structure, although without the other pieces such as undo segments, distributed transaction recovery, and so on, nothing works. They are a major component of what sets a database apart from a conventional file system. The online redo logs allow us to effectively recover from a power outage one that might happen while Oracle is in the middle of a write. The archived redo logs let us recover from media failures when, for instance, the hard disk goes bad or human error causes data loss. Without redo logs, the database would not offer any more protection than a file system.
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