15 Backup and Recovery

Overview of Consistent Backups

A consistent backup of a database or part of a database is a backup in which all read/write datafiles and control files are checkpointed with the same SCN.

The only way to make a consistent whole database backup is to shut down the database with the NORMAL, IMMEDIATE, or TRANSACTIONAL options and make the backup while the database is closed. If a database is not shut down cleanly, for example, an instance fails or you issue a SHUTDOWN ABORT statement, then the database's datafiles are always inconsistent—unless the database is a read-only database.

Oracle makes the control files and datafiles consistent to the same SCN during a database checkpoint. The only tablespaces in a consistent backup that are allowed to have older SCNs are read-only and offline normal tablespaces, which are still consistent with the other datafiles in the backup because no changes have been made to them.

The important point is that you can open the database after restoring a consistent whole database backup without needing recovery because the data is already consistent: no action is required to make the data in the restored datafiles correct. Hence, you can restore a year-old consistent backup of your database without performing media recovery and without Oracle performing instance recovery. Of course, when you restore a consistent whole database backup without applying redo, you lose all transactions that were made since the backup was taken.

A consistent whole database backup is the only valid backup option for databases operating in NOARCHIVELOG mode, because otherwise recovery is necessary for consistency. In NOARCHIVELOG mode, Oracle does not archive the redo logs, and so the required redo logs might not exist on disk. A consistent whole backup is also a valid backup option for databases operating in ARCHIVELOG mode. When this type of backup is restored and archived logs are available, you have the option of either opening the database immediately and losing transactions that were made since the backup was taken, or applying the archived logs to recover those transactions.

Overview of Inconsistent Backups

An inconsistent backup is a backup in which the files being backed up do not contain all the changes made at all the SCNs. In other words, some changes are missing. This means that the files in the backup contain data taken from different points in time. This can occur because the datafiles are being modified as backups are being taken. Oracle recovery makes inconsistent backups consistent by reading all archived and online redo logs, starting with the earliest SCN in any of the datafile headers, and applying the changes from the logs back into the datafiles.

If the database must be up and running 24 hours a day, 7 days a week, then you have no choice but to perform inconsistent backups of the whole database. A backup of online datafiles is called an online backup. This requires that you run your database in ARCHIVELOG mode.

If you run the database in ARCHIVELOG mode, then you do not have to back up the whole database at one time. For example, if your database contains seven tablespaces, and if you back up the control file as well as a different tablespace each night, then in a week you will back up all tablespaces in the database as well as the control file. You can consider this staggered backup as a whole database backup. However, if such a staggered backup must be restored, then you need to recover using all archived redo logs that were created since the earliest backup was taken.

Whole Database Backups

A whole database backup is a backup of every datafile in the database, plus the control file. Whole database backups are the most common type of backup.

Whole database backups can be taken in either ARCHIVELOG or NOARCHIVELOG mode. Before performing whole database backups, however, be aware of the implications of backing up in ARCHIVELOG and NOARCHIVELOG modes.

Figure 15-1 illustrates the valid configuration options given the type of backup that is performed.

Figure 15-1 Whole Database Backup Options

Description of the illustration cncpt195.gif

A whole database backup is either a consistent backup or an inconsistent backup. Whether a backup is consistent determines whether you need to apply redo logs after restoring the backup.

Tablespace Backups

A tablespace backup is a backup of the datafiles that constitute the tablespace. For example, if tablespace users contains datafiles 2, 3, and 4, then a backup of tablespace users backs up these three datafiles.

Tablespace backups, whether online or offline, are valid only if the database is operating in ARCHIVELOG mode. The reason is that redo is required to make the restored tablespace consistent with the other tablespaces in the database.

Datafile Backups

A datafile backup is a backup of a single datafile. Datafile backups, which are not as common as tablespace backups, are valid in ARCHIVELOG databases. The only time a datafile backup is valid for a database in NOARCHIVELOG mode is if:

• Every datafile in a tablespace is backed up. You cannot restore the database unless all datafiles are backed up.

• The datafiles are read only or offline-normal.

  
  

  See Also: Oracle Database Recovery Manager Reference

  
  

RMAN with Online Backups

Because the database continues writing to the file during an online backup, there is the possibility of backing up inconsistent data within a block. For example, assume that either RMAN or an operating system utility reads the block while database writer is in the middle of updating the block. In this case, RMAN or the copy utility could read the old data in the top half of the block and the new data in the bottom top half of the block. The block is a fractured block, meaning that the data in this block is not consistent.

During an RMAN backup, the Oracle database server reads the datafiles, not an operating system utility. The server reads each block and determines whether the block is fractured. If the block is fractured, then Oracle re-reads the block until it gets a consistent picture of the data.

When you back up an online datafile with an operating system utility (rather than with RMAN), you must use a different method to handle fractured blocks. You must first place the files in backup mode with the ALTER TABLESPACE BEGIN BACKUP statement (to back up an individual tablespace), or the ALTER DATABASE BEGIN BACKUP statement (to back up the entire database). After an online backup is completed, you must run the ALTER TABLESPACE ... END BACKUP or ALTER DATABASE END BACKUP statement to take the tablespace out of backup mode.

When updates are made to files in backup mode, additional redo data is logged. This additional data is needed to repair fractured blocks that might be backed up by the operating system utility.

Control File Backups

Backing up the control file is a crucial aspect of backup and recovery. Without a control file, you cannot mount or open the database.

You can instruct RMAN to automatically backup the control file whenever you run backup jobs. The command is CONFIGURE CONTROLFILE AUTOBACKUP. Because the autobackup uses a default filename, RMAN can restore this backup even if the RMAN repository is unavailable. Hence, this feature is extremely useful in a disaster recovery scenario.

You can make manual backups of the control file by using the following methods:

• The RMAN BACKUP CURRENT CONTROLFILE command makes a binary backup of the control file, as either a backup set or an image copy.

• The SQL statement ALTER DATABASE BACKUP CONTROLFILE makes a binary backup of the control file.

• The SQL statement ALTER DATABASE BACKUP CONTROLFILE TO TRACE exports the control file contents to a SQL script file. You can use the script to create a new control file. Trace file backups have one major disadvantage: they contain no records of archived redo logs, and RMAN backups and copies. For this reason, binary backups are preferable.

  
  

  See Also: Oracle Database Backup and Recovery Advanced User's Guide Oracle Database Recovery Manager Reference

  
  

Archived Redo Log Backups

Archived redo logs are essential for recovering an inconsistent backup. The only way to recover an inconsistent backup without archived logs is to use RMAN incremental backups. To be able to recover a backup through the most recent log, every log generated between these two points must be available. In other words, you cannot recover from log 100 to log 200 if log 173 is missing. If log 173 is missing, then you must halt recovery at log 172 and open the database with the RESETLOGS option.

Because archived redo logs are essential to recovery, you should back them up regularly. If possible, then back them up regularly to tape.

You can make backups of archived logs by using the following methods:

• The RMAN BACKUP ARCHIVELOG command

• The RMAN BACKUP ... PLUS ARCHIVELOG command

• An operating system utility

  
  

  See Also: Oracle Database Backup and Recovery Advanced User's Guide Oracle Database Recovery Manager Reference

  
  

Complete Recovery

Complete recovery involves using redo data or incremental backups combined with a backup of a database, tablespace, or datafile to update it to the most current point in time. It is called complete because Oracle applies all of the redo changes contained in the archived and online logs to the backup. Typically, you perform complete media recovery after a media failure damages datafiles or the control file.

You can perform complete recovery on a database, tablespace, or datafile. If you are performing complete recovery on the whole database, then you must:

• Mount the database

• Ensure that all datafiles you want to recover are online

• Restore a backup of the whole database or the files you want to recover

• Apply online or archived redo logs, or a combination of the two

If you are performing complete recovery on a tablespace or datafile, then you must:

• Take the tablespace or datafile to be recovered offline if the database is open

• Restore a backup of the datafiles you want to recover

• Apply online or archived redo logs, or a combination of the two

Incomplete Recovery

Incomplete recovery, or point-in-time recovery, uses a backup to produce a noncurrent version of the database. In other words, you do not apply all of the redo records generated after the most recent backup. You usually perform incomplete recovery of the whole database in the following situations:

• Media failure destroys some or all of the online redo logs.

• A user error causes data loss, for example, a user inadvertently drops a table.

• You cannot perform complete recovery because an archived redo log is missing.

• You lose your current control file and must use a backup control file to open the database.

To perform incomplete media recovery, you must restore all datafiles from backups created prior to the time to which you want to recover and then open the database with the RESETLOGS option when recovery completes. The RESETLOGS operation creates a new incarnation of the database—in other words, a database with a new stream of log sequence numbers starting with log sequence 1.

Before using the OPEN RESETLOGS command to open the database in read/write mode after an incomplete recovery, it is a good idea to first open the database in read-only mode, and inspect the data to make sure that the database was recovered to the correct point. If the recovery was done to the wrong point, then it is easier to re-run the recovery if no OPEN RESETLOGS has been done. If you open the database read-only and discover that not enough recovery was done, then just run the recovery again to the desired time. If you discover that too much recovery was done, then you must restore the database again and re-run the recovery.

Datafile Media Recovery

Datafile media recovery is used to recover from a lost or damaged current datafile or control file. It is also used to recover changes that were lost when a tablespace went offline without the OFFLINE NORMAL option. Both datafile media recovery and instance recovery must repair database integrity. However, these types of recovery differ with respect to their additional features. Media recovery has the following characteristics:

• Applies changes to restored backups of damaged datafiles.

• Can use archived logs as well as online logs.

• Requires explicit invocation by a user.

• Does not detect media failure (that is, the need to restore a backup) automatically. After a backup has been restored, however, detection of the need to recover it through media recovery is automatic.

• Has a recovery time governed solely by user policy (for example, frequency of backups, parallel recovery parameters, number of database transactions since the last backup) rather than by Oracle internal mechanisms.

The database cannot be opened if any of the online datafiles needs media recovery, nor can a datafile that needs media recovery be brought online until media recovery is complete. The following scenarios necessitate media recovery:

• You restore a backup of a datafile.

• You restore a backup control file (even if all datafiles are current).

• A datafile is taken offline (either by you or automatically by Oracle) without the OFFLINE NORMAL option.

Unless the database is not open by any instance, datafile media recovery can only operate on offline datafiles. You can initiate datafile media recovery before opening a database even when crash recovery would have sufficed. If so, crash recovery still runs automatically at database open.

Note that when a file requires media recovery, you must perform media recovery even if all necessary changes are contained in the online logs. In other words, you must still run recovery even though the archived logs are not needed. Media recovery could find nothing to do — and signal the "no recovery required" error — if invoked for files that do not need recovery.

Block Media Recovery

Block media recovery is a technique for restoring and recovering individual data blocks while all database files remain online and available. If corruption is limited to only a few blocks among a subset of database files, then block media recovery might be preferable to datafile recovery.

The interface to block media recovery is provided by RMAN. If you do not already use RMAN as your principal backup and recovery solution, then you can still perform block media recovery by cataloging into the RMAN repository the necessary user-managed datafile and archived redo log backups.

RMAN Restore and Recovery

The basic RMAN recovery commands are RESTORE and RECOVER. Use RESTORE to restore datafiles from backup sets or from image copies on disk, either to their current location or to a new location. You can also restore backup sets containing archived redo logs, but this is usually unnecessary, because RMAN automatically restores the archived logs that are needed for recovery and deletes them after the recovery is finished. Use the RMAN RECOVER command to perform media recovery and apply archived logs or incremental backups.

RMAN automates the procedure for recovering and restoring your backups and copies.

User-Managed Restore and Recovery

If you do not use RMAN, then you can restore backups with operating system utilities and then run the SQL*Plus RECOVER command to recover the database. You should follow these basic steps:

1. After identifying which files are damaged, place the database in the appropriate state for restore and recovery. For example, if some but not all datafiles are damaged, then take the affected tablespaces offline while the database is open.

2. Restore the files with an operating system utility. If you do not have a backup, it is sometimes possible to perform recovery if you have the necessary redo logs dating from the time when the datafiles were first created and the control file contains the name of the damaged file.

   If you cannot restore a datafile to its original location, then relocate the restored datafile and change the location in the control file.

3. Restore any necessary archived redo log files.

4. Use the SQL*Plus RECOVER command to recover the datafile backups.

   
   

   See Also: Oracle Database Backup and Recovery Advanced User's Guide for details about how to restore and recover with operating system utilities and SQL*Plus

   
   

Overview of Oracle Flashback Database

Oracle Flashback Database lets you quickly recover an Oracle database to a previous time to correct problems caused by logical data corruptions or user errors.

If an Oracle managed disk area, called a flash recovery area is configured, and if you have enabled the Flashback functionality, then you can use the RMAN and SQL FLASHBACK DATABASE commands to return the database to a prior time. Flashback Database is not true media recovery, because it does not involve restoring physical files. However, Flashback is preferable to using the RESTORE and RECOVER commands in some cases, because it is faster and easier, and does not require restoring the whole database.

To Flashback a database, Oracle uses past block images to back out changes to the database. During normal database operation, Oracle occasionally logs these block images in Flashback logs. Flashback logs are written sequentially, and they are not archived. Oracle automatically creates, deletes, and resizes Flashback logs in the flash recovery area. You only need to be aware of Flashback logs for monitoring performance and deciding how much disk space to allocate to the flash recovery area for Flashback logs.

The amount of time it takes to Flashback a database is proportional to how far back you need to revert the database, rather than the time it would take to restore and recover the whole database, which could be much longer. The before images in the Flashback logs are only used to restore the database to a point in the past, and forward recovery is used to bring the database to a consistent state at some time in the past. Oracle returns datafiles to the previous point-in-time, but not auxiliary files, such as initialization parameter files.

Overview of Oracle Flashback Table

Oracle Flashback Table lets you recover tables to a specified point in time with a single statement. You can restore table data along with associated indexes, triggers, and constraints, while the database is online, undoing changes to only the specified tables. Flashback Table does not address physical corruption; for example, bad disks or data segment and index inconsistencies.

Flashback Table works like a self-service repair tool. Suppose a user accidentally deletes some important rows from a table and wants to recover the deleted rows. You can restore the table to the time before the deletion and see the missing rows in the table with the FLASHBACK TABLE statement.

You can revert the table and its contents to a certain wall clock time or user-specified system change number (SCN). Use Flashback Table with Oracle Flashback Version query and Flashback Transaction Query to find a time to which the table should be restored back to.

For Flashback Table to succeed, the system must retain enough undo information to satisfy the specified SCN or timestamp, and the integrity constraints specified on the tables cannot be violated. Also, row movement must be enabled.

The point of time in the past that you use Flashback Table to go to is controlled by the undo retention of the system. Initialization parameters specify the duration for which undo information is to be saved before being used for other transactions. The longer the undo information is available, the further back in time can the database version be available to Flashback. Typically, DBAs supply sufficient space to the undo tablespace to be able to Flashback for a couple of days.

Overview of Redo Application

Database buffers in the buffer cache in the SGA are written to disk only when necessary, using a least-recently-used (LRU) algorithm. Because of the way that the database writer process uses this algorithm to write database buffers to datafiles, datafiles could contain some data blocks modified by uncommitted transactions and some data blocks missing changes from committed transactions.

Two potential problems can result if an instance failure occurs:

• Data blocks modified by a transaction might not be written to the datafiles at commit time and might only appear in the redo log. Therefore, the redo log contains changes that must be reapplied to the database during recovery.

• After the roll forward phase, the datafiles could contain changes that had not been committed at the time of the failure. These uncommitted changes must be rolled back to ensure transactional consistency. These changes were either saved to the datafiles before the failure, or introduced during the roll forward phase.

To solve this dilemma, two separate steps are generally used by Oracle for a successful recovery of a system failure: rolling forward with the redo log (cache recovery) and rolling back with the rollback or undo segments (transaction recovery).

Overview of Transaction Recovery

You can run Oracle in either manual undo management mode or automatic undo management mode. In manual mode, you must create and manage rollback segments to record the before-image of changes to the database. In automatic undo management mode, you create one or more undo tablespaces. These undo tablespaces contain undo segments similar to traditional rollback segments. The main difference is that Oracle manages the undo for you.

Undo blocks (whether in rollback segments or automatic undo tablespaces) record database actions that should be undone during certain database operations. In database recovery, the undo blocks roll back the effects of uncommitted transactions previously applied by the rolling forward phase.

After the roll forward, any changes that were not committed must be undone. Oracle applies undo blocks to roll back uncommitted changes in data blocks that were either written before the failure or introduced by redo application during cache recovery. This process is called rolling back or transaction recovery.

Figure 15-3 illustrates rolling forward and rolling back, the two steps necessary to recover from any type of system failure.

Figure 15-3 Basic Recovery Steps: Rolling Forward and Rolling Back

Description of the illustration cncpt196.gif

Oracle can roll back multiple transactions simultaneously as needed. All transactions systemwide that were active at the time of failure are marked as terminated. Instead of waiting for SMON to roll back terminated transactions, new transactions can recover blocking transactions themselves to get the row locks they need.

Overview of Instance and Crash Recovery

Crash recovery is used to recover from a failure either when a single-instance database fails or all instances of an Oracle Real Application Clusters database fail. Instance recovery refers to the case where a surviving instance recovers a failed instance in an Oracle Real Application Clusters database.

The goal of crash and instance recovery is to restore the data block changes located in the cache of the terminated instance and to close the redo thread that was left open. Instance and crash recovery use only online redo log files and current online datafiles. Oracle recovers the redo threads of the terminated instances together.

Crash and instance recovery involve two distinct operations: rolling forward the current, online datafiles by applying both committed and uncommitted transactions contained in online redo records, and then rolling back changes made in uncommitted transactions to their original state.

Crash and instance recovery have the following shared characteristics:

• Redo the changes using the current online datafiles (as left on disk after the failure or SHUTDOWN ABORT)

• Use only the online redo logs and never require the use of the archived logs

• Have a recovery time governed by the number of terminated instances, amount of redo generated in each terminated redo thread since the last checkpoint, and by user-configurable factors such as the number and size of redo log files, checkpoint frequency, and the parallel recovery setting

Oracle performs this recovery automatically on two occasions:

• At the first database open after the failure of a single-instance database or all instances of an Oracle Real Applications Cluster database (crash recovery).

• When some but not all instances of an Oracle Real Application Clusters configuration fail (instance recovery). The recovery is performed automatically by a surviving instance in the configuration.

The important point is that in both crash and instance recovery, Oracle applies the redo automatically: no user intervention is required to supply redo logs. However, you can set parameters in the database server that can tune the duration of instance and crash recovery performance. Also, you can tune the rolling forward and rolling back phases of instance recovery separately.