Detailed introduction to Oracle backup and recovery

This article brings you relevant knowledge about Oracle, which mainly organizes issues related to backup and recovery, including some content about data protection, Oracle backup technology comparison, etc., as follows Let's take a look, I hope it will be helpful to everyone.

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1. About data protection

Backup Is a data copy of the database that can be used to reconstruct the data. Backups can be physical or logical.

A physical backup is a copy of the physical files used to store and restore the database. These files include data files, control files, and archived redo logs. Most importantly, each physical backup is a copy of a file that stores database information to another location, whether it is on disk or an offline storage medium such as tape.

Logical backup contains logical data such as tables and stored procedures. Oracle Data Pump can be used to import logical data into binary files that can later be imported into a database. The Data Pump command line clients expdp and impdp use the DBMS_DATAPUMP and DBMS_METADATA PL/SQL packages.

Physical backup is the foundation of any reliable backup and recovery strategy. Logical backup is a useful supplement to physical backup in many cases, but without physical backup, it does not provide sufficient protection against data loss.

Unless otherwise stated, the term backup in the backup and recovery documentation refers to a physical backup. Backing up a database is the act of making a physical backup. The emphasis in backup and recovery documentation is almost exclusively on physical backups.

Most of the document focuses on backup and recovery based on RMAN (Recovery Manager). The most noteworthy are the following aspects:

1) Incremental backup
Incremental backup only stores data blocks that have changed since the previous backup. Therefore, they provide more compact backups and faster restores, thus reducing the need to apply redo during data file media recovery. If you enable block change tracking, you can improve backup performance by avoiding a full scan of each input data file. Use BACKUP INCREMENTAL to perform incremental backups.

2) Block Media Recovery

Can repair a data file with only a small number of corrupted data blocks without taking it offline or restoring it from backup. Use RECOVER BLOCK to perform block media recovery.

2. Comparison of Oracle backup technologies

A variety of technologies can be used to create backups of Oracle databases. The following compares the features of RMAN (Recovery Manager), user-managed backup and data pump (Data Pump) technology.

3. Oracle Flashback Technology

Oracle Flashback Technology provides a series of features to complement physical backup and recovery strategies.

Oracle Flashback technology provides an additional layer of data protection. Specifically, you can use the various features of Oracle Flashback to view past data states and rewind the database without the need to restore a backup or perform a point-in-time recovery. In summary, flashback features are more efficient and less disruptive than media recovery in most situations where they can be applied.

Oracle flashback technology allows you to use the following features:

1) Logical flashback feature
2) Flashback database

3.1. Logical flashback feature

Oracle database logical layer flashback feature does not depend on RMAN. Flashback is available regardless of whether RMAN is part of the backup strategy.

Most of Oracle's flashback features operate at the logical layer, allowing you to view and manipulate database objects. In addition to Oracle Flashback Drop, this logical flashback feature relies on undo data, which are values ​​that are overwritten by record and update operations for each database update.

Oracle database includes the following logical flashback features:

1) Oracle Flashback Query
You can specify a target time and run a query on the database to view the results that appear at the target time. To recover from an unwanted change such as an update to a table, you can choose a target time before the error and run a query to retrieve the contents of the missing row. The Oracle Database Development Guide explains how to use this feature.

2) Oracle Flashback Version Query

You can view all versions of all rows that exist in one or more tables within a specified time interval. You can also retrieve metadata about different versions of a row, including start and end times, operations, and the ID of the transaction that created the version. You can use this feature to recover lost data values ​​and audit changes to queried tables. The Oracle Database Development Guide explains how to use this feature.

3) Oracle Flashback Transaction Query

You can view changes made by a single transaction, or changes made by all transactions in a specified time period. The Oracle Database Development Guide explains how to use this feature.

4) Oracle Flashback Transaction
You can undo a transaction. Oracle Database determines the dependencies between transactions and actually creates an offsetting transaction to undo the unwanted changes. The database is reverted to a state as if the transaction and any transactions that depended on it had never occurred. The Oracle Database Development Guide explains how to use this feature.

5) Oracle Flashback Table
You can restore a table or a group of tables to a specified earlier point in time without taking any part of the database offline. In many cases, Flashback Tables eliminate the need to perform more complex point-in-time recovery operations. Flashback Tables restores a table while automatically maintaining associated properties such as current indexes, triggers, and constraints, in a manner that allows you to avoid finding and restoring database-specific properties. The chapter "Using Flashback Tables to Rewind Tables" explains how to use this feature.

6) Oracle Flashback Drop
can undo the effect of the DROP TABLE statement. The chapter "Using Flash Drop to Rewind a DROP Table" explains how to use this feature.

Flashback Data Archiving allows you to use some logical flashback features to access data long in the past. A flashback data archive contains one or more table spaces or portions of table spaces. When you create a flashback data archive, you can specify the name, retention period, and tablespace. You can also specify a default flashback data archive. The database automatically clears old historical data that exceeds the retention period.

Flashback archiving can be turned on and off for individual tables. By default, flashback archiving is turned off for each table.

3.2. Flashback Database

Flashback Database allows you to restore the database to a previous point in time.

At the physical level, Oracle Flashback Database provides a more efficient data protection option compared to database point-in-time recovery (DBPITR, database point-in-time recovery). If there are unwanted changes in the current data file, you can use the RMAN command FLASHBACK DATABASE to restore the contents of the data file to the past time. The end result is very similar to that of DBPITR, but is generally faster because it does not require restoring data files from backup and requires less redo than media recovery.

Flashback Database uses flashback logs to access past data block versions and certain information from archived redo logs. Flashback Database requires that a fast recovery area be configured for the database because flashback logs can only be stored there. Flashback logging is not enabled by default. The space used by the flashback log is automatically managed by the database and balanced with the space requirements of other files in the fast recovery area.

Oracle Database, along with Flashback Database and Backup and Recovery, also supports restore points. A restore point is an alias equivalent to a system change number (SCN). A restore point can be created at any time if it is anticipated that the need will be returned to some or all of the database to that time. A guaranteed restore point ensures that you can use Flashback Database to return the database to the restore point.

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