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s an Oracle DBA, one of your fundamental tasks is to regularly back up databases. Backups involve making copies of your database to re-create the database if necessary. They provide the basis of all database recoveries no backup, no recovery. One of the best things you can do to help yourself as a DBA is to focus on a tried-and-tested strategy for backing up the database, because the more time you spend planning backups, the less time you ll spend recovering the database from a mishap. You can perform database backups in two different ways: use Oracle s Recovery Manager (RMAN) interface or use operating system utilities. I give RMAN-based backups much more attention in this chapter because of the many benefits they offer compared with operating system-based, user-created backups. Database administrators frequently use tape devices for Oracle backups, because of their convenience and also because tape backups are easy to archive for safekeeping. If you want to use RMAN with tape devices, you need to use a media management layer (MML). Oracle Corporation offers its own media management tool, called Oracle Secure Backup, free with the Oracle server. In this chapter, you ll learn how to install, configure, and integrate Oracle Secure Backup with RMAN to perform sophisticated backups. You need to consistently check and verify backups to make sure they re correct and usable during a recovery. The latter part of this chapter is devoted to a review of database corruption and the many ways to test for it. I ll also briefly review Oracle Data Guard and the concept of standby databases. Let s begin with an overview of Oracle database backups.

PCBs contain the shared memory, which is mapped into the local address space of all the processors The various processors compete for control of the system bus, which is the only resource through which they can access the shared memory If all the processor modules are similar, and they all have equal rights to control of the system bus, the system is called a symmetric multiprocessor (SMP) SMPs use a memory access architecture called Uniform Memory Access (UMA), so-called because the speed of access to the shared memory is the same for all processors In order for multiple processors to cooperate peacefully in the same box, there must be a mechanism for preventing bus contention Contention occurs when two or more processors try to control the bus at the same time, and can cause a system to fail in unpredictable ways, at both the hardware and software levels.

Database backups are used to avoid the loss of data, so it s essential to have a backup system in place. Backups involve keeping copies of the key Oracle database files: datafiles, the control file, and the archived redo log files. Physical backups involve the copying of database files. You can perform physical backups in two main ways: Use operating system utilities like cp and dd to back up files to perform user-managed backups. You use a combination of operating system backup commands and SQL*Plus commands to back up the database files. Use the Oracle-provided utility Recovery Manager to perform the backups. RMAN can be used in the command-line mode, as well as through the OEM Database Control interface. RMAN can do everything that user-managed backups can, and it provides several additional capabilities. You also don t need to keep track of the backed-up datafiles and archived redo log files with RMAN, since RMAN itself manages all that information.

In this chapter, I introduce you to using RMAN through the command line so you understand the concepts behind it. Once you gain proficiency in using the tool, feel free to use the Database Control interface to manage RMAN-based backups. Although I ll focus on using RMAN in this chapter, I briefly discuss user-managed backups toward the end of the chapter. Although disk storage prices keep dropping, tape storage is still the cheaper way to store large amounts of data offsite. If you re using RMAN, you need a third-party media manager to make a backup to a tape device. Before you start dealing with the mechanics of backups, you need to understand certain terms associated with backups.

All multiprocessing buses define hardware rules to prevent bus contention A properly designed card for a particular bus contains hardware that abides by the appropriate bus rules The prevention of bus contention is not sufficient to ensure proper operation of a shared memory system A mechanism must exist that allows each processor to lock a data structure from concurrent alteration by other processors The mechanism is often based on software flags Each lockable data structure is associated with a flag When a processor wants exclusive access to a data structure, it must set the associated software flag Before doing so, it must wait for the flag to be cleared, indicating no one else is using it As soon as the flag is tested and found cleared, the processor can set it.

A clear understanding of the types of backups and backup concepts is extremely important for a successful recovery. Here, I ll review some terminology related to Oracle database backups.