Saturday, February 28, 2009

Flash Recovery Area

1. 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: -- Current control file -- Online redo logs -- Archived redo logs -- Flashback logs -- Control file autobackups -- Datafile and control file copies -- Backup pieces -- Foreign archived redo log ( An archived redo log received by a logical standby database for a LogMiner session.) 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. The advantage that we have over tape is that tape is a sequential access device and disk is a random access device. Hence the amount of time needed for restoring from the tape is eliminated or reduced. 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. If providing this much space is impractical, then it is best to create an area large enough to keep a backup of the most important tablespaces and all the archived logs not yet on tape. At an absolute minimum, the flash recovery area must be large enough to contain the archived redo logs not yet on tape. If the recovery area has insufficient space to store flashback logs and meet other backup retention requirements, then the recovery area may delete flashback logs to make room. Formulas for estimating a useful flash recovery area size depend on whether: -- Your database has a small or large number of data blocks that change frequently . -- You store backups only on disk, or on disk and tape. -- You use a redundancy-based backup retention policy, or a recovery window-based retention policy . -- You plan to use Flashback Database or a guaranteed restore point as alternatives to point-in-time recovery. 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: -- No two databases for which the DB_UNIQUE_NAME initialization parameters are specified have the same value for DB_UNIQUE_NAME. -- For those databases where no DB_UNIQUE_NAME is provided, no two databases have the same value for DB_NAME. When databases share a single recovery area in this way, the location should be large enough to hold the files for all databases. Add the values for DB_RECOVERY_FILE_DEST_SIZE for the databases, then allow for overhead such as mirroring or compression. 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 disk group for their location. 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. Backup sets and image copies can be created as OMF files by creating them in the Flash Recovery Area. RMAN can be used to migrate existing files to OMF. 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. To use the Flash Recovery feature, you must first configure the flash recovery area for each instance in your Oracle RAC cluster. 9. What type of files can be stored in Flash Recovery Area ? The files in Flash Recovery Area are classified as permanent or transient. -- Permanent Files The permanent files (assuming these are configured to be stored in the recovery area) are multiplexed copies of the current control file and online redo logs. These cannot be deleted without causing the instance to fail. -- Transient Files Transient files include archived redo logs, datafile copies, control file copies, control file autobackups, backup pieces and flashback logs. Oracle manages these files automatically for deletion whenever space is required in the Flash Recovery Area. They are deleted once they become obsolete under the retention policy or have been backed up to tape. Any transient file in the flash recovery area once backed up to tape even if not deleted are internally placed on a file can be deleted list. Until there is a backup of the file on disk made to a teriary storage device it cannot be obsolete. 9. Initialization Parameters required for Flash Recovery Area To enable the Flash Recovery Area, you must set the two initialization parameters: -- DB_RECOVERY_FILE_DEST_SIZE : It is the disk limit, which is the amount of space the flash recovery area is permitted to use. The minimum size of the Flash Recovery Area should be at least large enough to contain archive logs that have not been copied to tape. Note: This value does not include certain kinds of disk overhead: -Block 0 or the OS block header of each Oracle file is not included in this size, so make sure to allow an extra 10% for this data when computing the actual disk usage required for the Flash Recovery Area. -DB_RECOVERY_FILE_DEST_SIZE does not indicate the real size occupied on disk when the underlying filesystem is mirrored, compressed, or in some other way affected by overhead not known to Oracle. -- DB_RECOVERY_FILE_DEST: This initialization parameter is a valid destination to create the Flash Recovery Area. The destination can be defined as a directory, file system, or ASM disk group. 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. 10. Restrictions on Initialization Parameters for Flash Recovery Area - You cannot use the LOG_ARCHIVE_DEST and LOG_ARCHIVE_DUPLEX_DEST parameters to specify redo log archive destinations. You must always use the LOG_ARCHIVE_DEST_n parameters in case you have configured flash recovery area. - LOG_ARCHIVE_DEST_10 is implicitly set to USE_DB_RECOVERY_FILE_DEST if you create a recovery area and do not set any other local archiving destinations. - Multiple database can have the same DB_RECOVERY_FILE_DEST only if the DB_NAME are different or if the DB_NAME is same (example the primary and standby database) then the DB_UNIQUE_NAME parameter must be different for the databases. - For RAC the location of Flash Recovery Area must be on a cluster file system, ASM or a shared directory configured through NFS. The location and disk quota must be the same on all instances.

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I am an Oracle Certified Professional, Oracle 9i/10G DBA, having 4+ years of core DBA experience.