Use RMAN to restore datafiles from backup sets or from image copies on disk, either to their current location or to a new location. The more data that accumulates in memory without being written to disk, the longer the instance recovery time, since a crash or media failure will force Oracle to apply redo data from the current online log to recover those changes. Backup and recovery in general refers to the various strategies and operations involved in protecting your database against data loss and reconstructing the data should that loss occur. To learn how to perform complete media recovery using O/S methods, see "Performing Complete Media Recovery". See Also: To learn how to back up the control file, see "Backing Up the Control File After Structural Changes". At one p.m. your disk drive fails.
A backup is a safeguard against data loss or corruption. You should consider logical backups as an additional tool within your overall backup and recovery strategy. You should consider logical backups as an additional tool within your overall backup and recovery strategy.
You should probably back up this tablespace frequently; in this way, you will not have to apply as much redo data during recovery. If the database is mounted during recovery, rollback occurs only when the database is opened. Oracle systematically goes through the redo log to determine which changes it needs to apply to which blocks, and then changes the blocks.
See Also: To learn how to make online backups using O/S methods, see "Backing Up Online Tablespaces and Datafiles". To protect against redo log failure, Oracle allows the redo log to be multiplexed. Logical backups store information about the schema objects created for a database. For example, a user issues an invalid SQL statement. with the primary site, as updates are performed at the primary. Running the database in ARCHIVELOG mode has the following consequences: Running the database in NOARCHIVELOG mode has the following consequences: Note:
The redo log keeps a record of this change while the rollback segment stores the old value.
Unlike crash and instance recovery, media recovery is executed on your command. The low SCN records the lowest SCN recorded in the log file, while the high SCN records the highest SCN in the log file.
If the physical makeup of the database changes, e.g., a new datafile or redo log file is created, then Oracle modifies the database's control file to reflect the change. Since you are not completely recovering the database to the most current time, you must tell Oracle when to terminate recovery. A database's datafiles, which belong to logical structures called tablespaces, contain the database data. Every time you mount an Oracle database, its control file is used to identify the datafiles and online redo log files that must be opened for database operation.
It also contains consistency information that is used during recovery, such as: When multiplexing the control file, you configure Oracle to write multiple copies of it in order to protect it against loss. You can perform complete recovery on a database, tablespace, or datafile. You do not need to issue this command when using RMAN. Depending on the size and value of the lost information, the results can be devastating. Unfortunately, one of your archived redo logs is corrupted. For more information on the syntax of SQL commands, see the Oracle8i SQL Reference.
If you make periodic backups of a database, then should you lose some of the data on your original database, you can perform media recovery using the backups. If the media manager is capable of making a proxy copy, then it can perform the data transfer in backup and restore operations. The background process DBWn, known as the database writer or db writer, writes modified buffers to disk. See Also: To learn how to use the Export and Import utilities, see Oracle8i Utilities. To learn how to manage these structures for backup and recovery, see Chapter 2, "Managing Data Structures". See Also: To learn how to perform incomplete recovery with RMAN, see "Performing Incomplete Recovery". To learn how to perform O/S TSPITR, see Appendix B, "Performing Operating System Tablespace Point-in-Time Recovery". To learn the basics of administering these data structures, see the Oracle8i Administrator's Guide. After resetting the online redo logs, make a whole database backup. If the backup was a consistent backup, then you can also restore the backup without performing recovery.
As the diagram illustrates, you generate new changes in the new incarnation of the database, eventually reaching SCN 4000. When Oracle multiplexes the online redo log, it maintains two or more copies of the redo log on different disks. Remote backup systems provide a wide range of availability, allowing the transaction processing to continue even if the primary site is destroyed by a fire, flood or earthquake. This copy includes important parts of your database such as the control file and datafiles.
For an overview of Recovery Manager features, see Chapter 4, "Recovery Manager Concepts". When statement failure occurs, Oracle automatically undoes any effects of the statement and returns control to the user. The most common types of failures causing data loss are: A logical failure in the handling of a statement in an Oracle program.
When developing your backup strategy, you must decide what information you want to copy. When performing media recovery, you can recover the whole database, a tablespace, or a datafile. Media recovery uses redo records or (if you use RMAN) incremental backups to recover restored datafiles either to the present or to a specified non-current time. In other words, you do not apply all of the redo data generated since the most recent backup.
RMAN is a powerful and versatile program that allows you to make a backup or image copy of your data. In any case, you always use a restored backup to perform the recovery. DBMS - Mapping Cardinalities Using E-R Diagram, DBMS - exceptions in a list comprehension, DBMS - Dictionary Comprehension in Python, DBMS - Closure of a set of Functional Dependencies, DBMS - Comparison of Ordered Indexing and Hashing, DBMS - Transaction Isolation and Atomicity, DBMS- Multiple Granularity Locking Protocol, DBMS- Operating System Role in Buffer Management, DBMS- Failure with loss of Nonvolatile Storage, How Much does it Hiring Remote React Developers Key Aspects & Cost, How to Handle Every FULL STACK DEVELOPMENT Challenge with Ease Using These Tips. You restore the SCN 1000 backup and prepare for complete recovery. After these steps are completed, issue either the RMAN recover command or the SQL*Plus RECOVER statement. You can make a backup of the whole database at once or supplement a whole database backup with backups of individual tablespaces, datafiles, control files, and archived logs. The remote site must be synchronized with the primary site, as updates are performed at the primary. An inconsistent backup is a backup of one or more database files that you make while the database is open or after the database has been shut down abnormally. See Also: To learn more about managing important data structures such as the online redo logs, see Chapter 2, "Managing Data Structures". Oracle will not allow you to apply logs from an old incarnation to the new incarnation. You can: The basic RMAN recovery commands are restore and recover. A backup is a safeguard against unexpected data loss and application errors; should you lose your original data, you can use the backup to make it available again. In contrast, logical backups contain data that you extract using the Oracle Export utility and store in a binary file. Use the Enterprise Backup Manager (EBU) for Oracle7 databases. For example, a user changes a row value in a table from 5 to 7. The data of logical database structures such as tables and indexes is physically located in the blocks of the datafiles allocated for a database. Because Oracle will not apply an archived redo log to a datafile unless the SCN and timestamps match, the RESETLOGS operations prevents you from corrupting your datafiles with old archived logs. Oracle does not archive the filled online redo log files before reusing them in the cycle. Starting at noon, one change to the database is made every second. Oracle has integrity checks that prevent you from opening the database until all datafiles are consistent with one another. For example, if a user adds a row to a table, but the server crashes before it can save the change to disk, Oracle can use the redo record for this transaction to update the data block to reflect the new row.
At SCN 4000, the database crashes. A media manager is a vendor-supplied software package that allows you to back up to archival media such as tape. If the user process fails while modifying the database, Oracle background processes undo the effects of uncommitted transactions. The datafile is divided into smaller units called data blocks. To learn how to make backups and copies with RMAN, see Chapter 8, "Making Backups and Copies with Recovery Manager". You can combine types of backups in a variety of ways. Opening the database with the RESETLOGS option informs Oracle that you want to discard some redo and prevents Oracle from ever applying the discarded redo in any recovery you might do in the future. See Also: For more information on parameters that you can use to influence the MTTR, see Oracle8i Tuning.
To learn how to use RMAN to make backups, see Chapter 8, "Making Backups and Copies with Recovery Manager". Oracle writes to these logs in a circular fashion: first it writes to one log file, then switches to the next available file when the current log is full. Whenever a transaction modifies a data block, a rollback segment records the state of the information before it changed. RMAN automatically selects the appropriate incremental backups or redo logs and applies them. To learn how to manage these data structures, see the Oracle8i Administrator's Guide. Whenever you open a database, Oracle checks the datafile header information against the information stored in the control file to determine whether recovery is necessary. By making frequent backups, you ensure that you can restore at least some of your lost data. Whenever you perform incomplete recovery, you must reset the online redo logs when you open the database. Oracle assigns every redo log file a unique log sequence number to identify it. Every Oracle database has one or more physical datafiles. When you back up the entire database after shutting it down cleanly, it is called a consistent whole database backup. When an instance fails, Oracle does not write the data in the buffers of the SGA to the datafiles. The remote site is also called secondary site. You can use the set until command to perform incomplete media recovery. RMAN provides several advantages. : When the primary site fails, the backup site takes over the processing and becomes the new primary site. Use the Enterprise Backup Manager (EBU) for Oracle7 databases. In general, the rollback segments of a database store the old values of data changed by uncommitted transactions. In a single server environment, Oracle automatically performs crash recovery when the database opens at the next startup. Whenever you open the database with the RESETLOGS option, all datafiles get a new SCN and timestamp. In traditional backup methods, you must perform a full backup in which you back up all the data blocks ever used in a datafile. If your database must be open and available all the time, then inconsistent backups are your only option.