What is a Flash Recovery Area and how to configure it ? (文
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APPLIES TO:Oracle Server - Enterprise Edition - Version 10.1.0.2 to 11.2.0.2 [Release 10.1 to 11.2]Information in this document applies to any platform. ***Checked for relevance on 28-Nov-2012*** PURPOSEThis document explains the basic design and configuration of a Flash Recovery Area. SCOPEDBA,Support. DETAILS1. What is a Flash Recovery Area ?The flash recovery area is an Oracle-managed directory,file system,or Automatic Storage Management disk group that provides a centralized disk location for backup and recovery files. All the files you need to completely recover a database from a media failure are part of the Flash Recovery Area. Oracle creates?archived logs?and?flashback logs?in the flash recovery area. RMAN can store its?backup sets?and?image copies?in the flash recovery area,and it uses it when restoring files during media recovery. The flash recovery area also acts as a disk cache for tape. Flash recovery extends the functionality of Oracle Managed Files to all recovery related files (backup sets,image copies,and archived logs). Oracle Database automatically manages this storage,deleting files that are no longer needed. The user specifies only the location of a Flash Recovery Area,and the amount of disk space that Oracle is allowed to use for recovery related files. This feature is integrated with the 10G MMON feature,so that out of space conditions can be handled through the standard Oracle monitoring framework. Periodically copying backups to tape frees space in the flash recovery area for other files. Oracle recommends that you enable a recovery area to simplify backup management. The following recovery-related files are stored in the flash recovery area:? 2. Why should we use a Flash Recovery Area?As disk storage media is now competitive to tape with respect to purchase costs,disk-based backup is an optimal and preferable storage mechanism. By using disks for storage purposes,you gain significant benefits in terms of mass storage,and you can randomly access your data in milliseconds rather than hours. The previously expensive downtime in traditional recovery is exchanged for rapid data access and recovery times using cheap disk space.? 3. What should be the location of Flash Recovery Area ?The flash recovery area should place on a separate disk from the working set of database files. Otherwise,the disk becomes a single point of failure for your database. 4. What should be the size of Flash Recovery Area ?The larger the flash recovery area is,the more useful it becomes. Ideally,the flash recovery area should be large enough to contain the required files. The recovery area should be able to contain a copy of all datafiles in the database and the incremental backups used by your chosen backup strategy.? Formulas for estimating a useful flash recovery area size depend on whether:? If you plan to enable flashback logging,then note that the volume of flashback log generation is approximately the same order of magnitude as redo log generation. For example,if you intend to set DB_FLASHBACK_RETENTION_TARGET to 24 hours,and if the database generates 20 GB of redo in a day,then a rule of thumb is to allow 20 GB to 30 GB disk space for the flashback logs. The same rule applies to guaranteed restore points when flashback logging is enabled. For example,if the database generates 20 GB redo every day,and if the guaranteed restore point will be kept for a day,then plan to allocate 20 to 30 GB. For an example suppose that you want to determine the size of a flash recovery when the backup retention policy is set to?REDUNDANCY 1?and you intend to follow the Oracle Suggested Strategy of?using an incrementally updated backup.?You use the following formula to estimate the disk quota
Disk Quota =?
Size of a copy of database +? Size of an incremental backup +? Size of (n+1) days of archived redo logs +? Size of (y+1) days of foreign archived redo logs (for logical standby) +? Size of control file +? Size of an online redo log member * number of log groups +? Size of flashback logs (based on DB_FLASHBACK_RETENTION_TARGET value)? Where n is the interval in days between incremental updates and y is the delay in applying the foreign archived redo logs on a logical standby database: For Oracle suggested backup kindly refer Note.303861.1 Incrementally Updated Backup In 10G 5. Can I have same Flashback Recovery Area for multiple database ?Multiple databases can have the same value for DB_RECOVERY_FILE_DEST,but one of the following must be true:? 6. ASM (Automatic Storage Management) support for Flash Recovery Area :The Flash Recovery Area can be an ASM disk group. RMAN can back up and restore files that are stored in ASM disk groups.In fact,RMAN is the only way to back up and restore ASM files.?Backup sets and image copies can be created in ASM,either by automatically creating them in the Flash Recovery Area,or by explicitly specifying an ASM? 7. OMF (Oracle Managed Files) support for Flash Recovery Area :Backup/Restore of OMF files is easier with RMAN as RMAN does not use the long OMF file names in any commands.? 8. Flash Recovery Area for RAC :For RAC database the location and disk quota must be the same on all instances. To accomplish this,Oracle recommends that you place the flash recovery area on the shared ASM disks. In addition,you must set the DB_RECOVERY_FILE_DEST and DB_RECOVERY_FILE_DEST_SIZE parameters to the same values on all instances.? 9. What type of files can?be stored in? Flash Recovery Area ?The files in Flash Recovery Area are classified as permanent or transient.? 9. Initialization Parameters?required for Flash Recovery AreaTo enable the Flash Recovery Area,you must set the two initialization parameters:?
Note1: The Flash Recovery Area cannot be stored on a raw file system.?
Note2 : DB_RECOVERY_FILE_DEST_SIZE must be set before DB_RECOVERY_FILE_DEST.
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- Oracle Database Products?>?Oracle Database?>?Oracle Database?>?Oracle Database - Enterprise Edition?>?Flashback Recovery?>?Flash Recovery Area
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