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Step 9 - Configuring Your Database for Basic Backup and Recovery

Learn Oracle - Configuring Your Database for Basic Backup and Recovery

To take maximum advantage of Oracle’s features for automatic managing backup and recovery, configure your database as follows:

Use a flash recovery area, to automate storage management for most backup-related files.

Run your database in ARCHIVELOG mode, so that Oracle DBA can perform online backups and have data recovery options such as complete and point-in-time media recovery.

Use the flash recovery area as an archive log destination for your database. Oracle DBA must also set a number of policies governing which files are backed up, what format is used to store backups on disk, and when files become eligible for deletion from the flash recovery area. Finally, Oracle DBA should make a record of two pieces of information about your database: the DBID and the DB_UNIQUE_NAME. This information is needed in some disaster recovery situations, such as restoring a lost database control file.

Planning Space Usage and Location for the Flash Recovery Area

To better protect your data, put the flash recovery area on a separate disk from the working set files of your database, to avoid the disk being a single point of failure for your database.

The amount of disk space to allocate for the flash recovery area depends upon the size and activity levels of your database, which determine the size of your datafiles and redo log files, and your recovery objectives, which dictate what kinds of backups Oracle DBA take, when Oracle DBA take them, and how long Oracle DBA must keep them.

Retention Policy and the Flash Recovery Area

Space management in the flash recovery area is governed by a backup retention policy. Retention policy determines when files are obsolete, meaning that they are no longer needed to meet your data recovery objectives.

Retention policies can be based on redundancy of backups, or a recovery window. Under a redundancy-based policy, the flash recovery area considers a backup of a file to be obsolete only when a specified number of more recent backups of that file are present. For example, if Oracle DBA specify a retention policy requiring two backups of each file be kept, and make backups nightly starting on a Monday night, then after the Wednesday night backup succeeds, the Monday night backup becomes redundant because the Tuesday and Wednesday backups are available.

Under a recovery window-based policy, Oracle DBA specify a time interval measured in days, and files become obsolete only when they are no longer needed for successful complete recovery or point-in-time recovery to any point within that number of days into the past. For example, if Oracle DBA specify a recovery window of three days, a backup of all datafiles from at least three days ago must be retained, along with a full set of archived redo logs since that backup.



A redundancy-based retention policy makes it easier to predict space usage in the flash recovery area, but do not allow Oracle DBA to predict how far into the past you can recover your database. A recovery window-based policy offers better protection for your data, but can make storage requirements for backups harder to predict. As already noted, even a short recovery window combined with a poorly designed backup strategy can cause unexpectedly high space requirements. (A reasonable backup strategy would include more frequent datafile backups.) Oracle recommends the use of a recovery-window-based backup retention policy as part of your backup strategy.

Even after they are obsolete, files are generally not deleted from the flash recovery area until space is needed to store new files. As long as space permits, files recently moved to tape will remain on disk as well, so that they will not have to be retrieved from tape in the event of a recovery.

The automatic deletion of obsolete files and files moved to tape from the flash recovery area makes the flash recovery area a very convenient redo log archiving destination. Other archiving destinations require manual cleanup of archived redo logs no longer needed on disk for recovery. Sizing the Flash Recovery Area

An approach for sizing the flash recovery area is described at length in Oracle Database Backup and Recovery Basics, but as a general rule, the larger the flash recovery area, the more useful it becomes. Ideally, the flash recovery area should be large enough to hold a copy of all of your datafiles and control files, the online redo logs, and the archived redo log files needed to recover your database using the datafile backups kept under your retention policy. If your backup strategy includes incremental backups, described in "Incremental Backups of Datafiles" on page 9-10, add enough space to the flash recovery area to accommodate these files as well. If Oracle DBA can move some backups to tape, Oracle DBA can reduce the size of the flash recovery area somewhat, although retrieving those files from tape will cause longer database restore and recovery times.

Credentials for Performing Oracle Enterprise Manager Backup and Recovery

You must have the proper credentials to perform some of the configuration tasks for backup and recovery, and to schedule backup jobs and perform recovery. Two sets of credentials are required: the Oracle user you use when Oracle DBA log in to Enterprise Manager, and the host operating system user whose credentials Oracle DBA provide when performing backup and recovery tasks. The host operating system credentials are used when RMAN is started behind the scenes, to actually perform the tasks you specified or scheduled through Enterprise Manager.

If Oracle DBA log in to Enterprise Manager with SYSDBA privileges, any valid host operating system user who has execute permission for the RMAN command line client will suffice for scheduling and running RMAN tasks.

For those tasks where host operating system credentials are required, a Host Credentials form where they can be entered appears at the bottom of the page used to perform the task. This form always includes a checkbox labelled Save as Preferred Credential. If you check this box before performing your action, the credentials you provide will be stored persistently for the currently logged-in Oracle user, and re-used whenever you log in as that user and perform operations requiring host credentials. (If the database is down, however, as is required for some database recovery operations, Oracle DBA will have to enter host credentials manually.)

Configuring the Flash Recovery Area and ARCHIVELOG Mode

It is possible to configure a flash recovery area and set the database archiving mode when first creating the database. If you did not perform these tasks at database creation time, however, you can add them to an existing database with the procedure described here.

Configuring the flash recovery area and setting ARCHIVELOG mode requires the following steps:

1. Under the host operating system, create a directory to hold the flash recovery area. Make sure that the permissions on this directory allow Oracle to create files here.

2. From the Database Home page, go to the Maintenance page. On this page, under the Backup/Recovery heading, select Configure Recovery Settings. The Configure Recovery Settings page appears.

3. Under the Flash Recovery Area heading, enter the path to the flash recovery area location (the path to the directory on disk you created in step 1), and your desired flash recovery area size. Make sure the Apply changes to SPFile Only box is not checked, then click Apply to make these changes take effect.

4. Under the Media Recovery heading, check ARCHIVELOG Mode if it was not already checked. Below the ARCHIVELOG Mode checkbox is a list of up to ten possible log archiving locations. Enter USE_DB_RECOVERY_FILE_DEST as one of the destinations, to specify the flash recovery area as a destination. For ease of database management, the best practice is to use the flash recovery area as your only redo log archiving destination. Oracle DBA can, however, specify other locations on disk, filling them in starting with the first one.

5. Click Apply to make these changes take effect.

Configuring Backup Settings

Assuming Oracle DBA have a flash recovery area configured and are running in ARCHIVELOG mode, Oracle DBA can configure a number of settings and policies that determine how backups are stored, which data is backed up, how backups perform, and how long backups are retained before being purged from the flash recovery area. There are also settings Oracle DBA can configure to improve backup performance. This section provides change them through Enterprise Manager.

Understanding Backup Device Settings for Disk

The settings available on the Device property page of Configure Backup Settings affect how backups will be written to disk and to tape. For disk-based backups, you can configure the default format for storing backups, the location on disk where backups are stored, and whether backup tasks are run in parallel for improved performance. Backup File Types: Image Copies, Backup Pieces, and Backup Sets Database backups created by RMAN can be stored in one of two forms: image copies or backup sets.

Image copies are exact byte-for-byte copies of the files they back up. You can create an image copy by copying a file at the host operating system level. However, unlike copying files at the operating system level, creating image copy backups through Recovery Manager or Enterprise Manager causes a record of those image copies to be made in the RMAN repository. This allows RMAN to use these copies during database restore and recovery. RMAN can only use files in restore operations if they are recorded in the RMAN repository.

Backup sets are logical entities containing the results of running various backup tasks. In fact, a backup set consists of several physical files called backup pieces. Backup pieces use an Oracle-proprietary format to store the contents of one or several files from a backup job. Backup pieces cannot be meaningfully manipulated individually; they can only be accessed as part of backup sets. When datafiles are backed up into backup sets, data blocks that have not yet been used to store data are not written into the backup pieces, saving space. This is referred to as unused block compression. Unused block compression is fundamental to how backup sets are created, and cannot be disabled.

An additional binary data compression algorithm can be applied during the writing of backup sets to save more space, at some cost in performance. Using binary compression for backup sets stored on disk is recommended if conserving disk space is more important than backup speed.

Oracle backups to disk can be saved as image copies or backup sets. Image copy backups can only be created on disk. Backups to tape and similar media management devices must be stored as backup sets.

Parallelism and RMAN Backups RMAN depends upon server sessions, processes that run on the database server, to actually perform backup and restore tasks. Each server session in turn corresponds to an RMAN channel, representing one stream of data to or from a backup device. RMAN supports parallelism, the use of multiple channels and server sessions to carry out the work of one backup or recovery task. If a task can be usefully executed in parallel on your hardware, Oracle DBA can set the level of parallelism to use on that task. Oracle DBA can also set a default level of parallelism to use when working with specific devices. Proper exploitation of parallelism can greatly increase performance on backup and recovery tasks.

The Configure Backup Settings page contains three property pages: Device, Backup Set and Policy. The settings you pick here are defaults which can apply to all backup jobs. When performing individual backup tasks, you can override these defaults.

The Device property page is shown first by default. Review the following fields under the Disk Settings section:

Parallelism: For now, set this value to 1. Later, when Oracle DBA have had time to review the information in Oracle Database Backup and Recovery Advanced User's Guide on parallelism and performance in RMAN, Oracle DBA may want to change this value.

Disk Backup Location: Should be blank, to direct backups to the flash recovery area.

Disk Backup Type: Make sure that Backup Set is selected. One advantage to backing up Oracle datafiles to backup sets is that RMAN uses unused block compression to save space in backing up datafiles. Only those blocks in your datafiles that have been used to store data are included in the backup set. Oracle DBA can also provide the host credentials for the backup. Enter a username and password for a host operating system account that is a member of the DBA group. After these settings are filled in, Oracle DBA can click Test Disk Backup to make sure the credentials and backup location are correct.

The settings on the Backup Set property page should not be altered at this time.

Configuring Backup Policy Settings

From the Backup Settings page, choose the Policy property page. On this page, Oracle DBA can set the backup policies governing control file and SPFILE backups, tablespaces to exclude from whole database backup, and the backup retention policy. Configuring Backup Policies For now, check the box to Automatically back up the control file and server parameter file (SPFILE) with every backup and database structural change. The SPFIle and control file are critical to the operation of your database and RMAN, and are also relatively small compared to typical datafiles. Backing them up frequently imposes relatively little disk space overhead. Leave the Autobackup Disk Location field blank, so that the autobackups are sent to the flash recovery area.

Check the option to Optimize the whole database backup by skipping unchanged files such as read-only and offline datafiles that have been backed up. This will save space in the flash recovery area.

Check the Enable block change tracking for faster incremental backups box to take advantage of the block change tracking feature of Oracle, which substantially improves performance of incremental backups at a small cost of overhead during normal operations.

Configuring Exclusions from Backup Oracle DBA can list tablespaces to exclude from backup. If, for example, you have a read-only tablespace, you do not need to back it up. For now, make sure this list is empty, so that all tablespaces are backed up. Configuring Backup Retention Policy You can choose between three forms of retention policy:

Retain All Backups (in effect, no retention policy at all, so that all backups are kept in the flash recovery area until Oracle DBA explicitly delete them)

Retain backups that are necessary for a recovery to any time within the specified number of days (point-in-time recovery) for a recovery window-based retention policy

Retain at least the specified number of full backups for each datafile for a redundancy-based retention policy

For now, choose the recovery window-based retention policy, with a recovery window of 31 days. Verify that the Host Credentials section at the bottom of the page contains proper credentials. Then click OK to save the new settings.

Recording Your DBID and DB_UNIQUE_NAME

If you lose your database control file or SPFILE, Enterprise Manager can restore them from backup, as long as you can provide the DB_UNIQUE_NAME and DBID for your database.

To find out your DB_UNIQUE_NAME, from the database home page, from the Administration page, under Instance, click All Initialization Parameters. On the Current property page, for Filter enter DB_UNIQUE_NAME and click Go. The resulting page displays a row with DB_UNIQUE_NAME listed in the Name column, and the DB_ UNIQUE_NAME value for your database in the Value column.

To find out your DBID, from the database home page, select the Administration page, then under Storage, click Controlfiles, then select the Advanced property page. The Database ID field contains the value of the DBID. Record these two pieces of information offline in case they are needed for disaster recovery.
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STEP 9 - Next Topics
This introduces Oracle DBA to Oracle backup and recovery as performed through Enterprise Manager. The goals are to introduce the basic concepts of Oracle backup and recovery, show how to configure your database for backup and recovery using the Oracle-suggested disk-based backup strategy, and then walk Oracle DBA through recovery of a full database backup. Click on Oracle tuorial links below:
Backup, Restore and Recovery ConceptsBackup and Restore
Configuring Database for Basic Backup and RecoveryBackup and Restore
Backing Up Your Database
Restore and Recovery Operations
ORACLE DBA - DISCUSSION FORUMS
Oracle Database - PODCASTS
More Tutorials on Oracle dba ...



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