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db_flashback_retention_target tips

Oracle Tips by Burleson Consulting
October 23,  2008

Question:  If I set db_flashback_retention_target=600, this means i can restore database to any time between sysdate and sysdate-10/24?
 
 Let's assume I set db_flashback_retention_target=600; and db_recovery_file_dest_size=20g. but my transaction is much more than 20gb in 10 hours. what happens when the destination file becomes full?
 
When I reach the time limit for db_recovery_file_dest_size; will oracle continue to add new flashback logs or reuse older ones in circular manner?  Also, Will the size of "flashback recovery area" ever exceed the size of db_recovery_file_dest_size?

 

Answers:  The db_flashback_retention_target parameter specifies the upper limit (in minutes) on how far back in time the database may be flashed back.

db_flashback_retention_target

Default value ? 1440 (minutes)

Modifiable ? ALTER SYSTEM

Range of values ? 0 to 2 32 - 1 (maximum value represented by 32 bits)

db_flashback_retention_target specifies the upper limit (in minutes) on how far back in time the database may be flashed back. How far back one can flash back a database depends on how much flashback data Oracle has kept in the recovery area.

The following statement changes the flashback time from the default of one day to two days:

SQL> alter system set db_flashback_retention_target = 2880;


The following statement changes the flashback time from the default of 1 day to two days.

SQL> alter system set db_flashback_retention_target = 2880;

How far back you can flash back also depends on how much flashback data Oracle has kept in the flashback recovery area.

If you exceed the size of your flashback recovery area, the offending task will fail, and roll back with this error:

ORA-38700: Limit of %s flashback database logs has been exceeded.

If you ever receive the out-of-space warning (85) and critical alerts (97) because of space pressure in you flash recovery area, you have the following options:

  • Consider changing your backup retention and archive log retention policies.
     

  • Increase the size of the DB_RECOVERY_FILE_DEST_SIZE parameter to allocate more space to your current flash recovery area.
     

  • Use the BACKUP RECOVERY AREA command in the RMAN to back up the contents of the flash recovery area to a tape device.
     

  • Use the RMAN to delete unnecessary backup files. The RMAN commands CROSSCHECK and DELETE EXPIRED come in handy during this deletion process.

You can monitor the flash recovery area with the v$recovery_file_dest view:

select
   name,
   to_char(space_limit, '999,999,999,999') as space_limit,
   to_char(space_limit - space_used + space_reclaimable,
   '999,999,999,999') as space_available,
   round((space_used - space_reclaimable)/space_limit * 100, 1) as pct_full
from
   v$recovery_file_dest;
 

See the Oracle scripts collection for the full set of flashback monitoring scripts

The flash recovery area may contain the following files:
 

  • Datafile copies: The new RMAN command BACKUP AS COPY can be used to create image copies of all datafiles and automatically store in the flash recovery area.
     

  • Control file autobackups: The database places any control file backups it generates in the flash recovery area.
     

  • Archived redo log files: If you store Archived redo log files in the flash recovery area, Oracle will automatically delete the files.
     

  • Online redo log files: Oracle recommends that you save a multiplexed copy of your online redo log files in the flash recovery area. The following statements can create online redo logs in the flash recovery area:

    CREATE DATABASE, ALTER DATABASE ADD LOGFILE, ALTER DATABASE ADD STANDBY LOGFILE, and ALTER DATABASE OPEN RESETLOGS.
     

  • Current control files: Oracle also recommends that you store a multiplexed current control file in the flash recovery area.
     

  • RMAN files
     

  • Flashback logs: If you enable the flashback database feature, Oracle copies images of each altered block in every datafile into flashback logs stored in the flash recovery area.

Note: Oracle calls the multiplexed redo log files and control files in the flash recovery area permanent files, since they should never be deleted and are part of the live database. Oracle terms all the other files in the flash recovery area (recovery related files) transient files, since Oracle will delete them eventually after they have become obsolete or have already been copied to tape.


Related flashback errors for RAC include:

ORA-38767: flashback retention target parameter mismatch Cause: The value of parameters db_flashback_retention_target must be same in all instances.


 

 

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