PrecisionOharts DATABASE a\ rt 3 A |. I a\ \t o - - r) a\ \t L ''_\ Maximrze Av ailabilitv with Flashback Flashback was introduced in Oracle9i through the DBMS-FLASHBACK package to enable DBAs to view older versions of modified data, and through Oraclel0g Flashback Techniques were developed to become easier and flexible. I real life human errors and mistakes could cost the whole enterprise a valuable time rn'asted through system outage cased by standard recovery processes initiated by DBAs in order to bring back the lost data. Now Flashback can eliminate this issue by providing you with the capabilities to recover/ query, and compare old modified data with minimal downtime. What is Flashback? Flashback is a revolutionary technology in recovery; this technique had been introduced and enhanced over time to minimize applications downtime caused by human errors. Flashback can be used to recover committed changes if were done by mistake, recover dropped tables, and compare before and after image of data with minimal efforts. Flashback Techniques Oracle's flashback technologies have implemented different techniques that suite almost every type of human error requiring recovery. The types of Flash back currently supported by Oracle Database 119l . Flashback Database . Flashback Droo o Flashback Table . Flashback Query . Flashback Versions Query . Flashback Transactions Query Flashback Database DBAs may have situations when multiple tables were modified by batch jobs, or when related transaction take place and the result need to be undone, previously DBA had to perform point-in-time incomplete recovery by restoring backup and apply redo changes until a desired time to undo the changes, this process is time consuming and cause the database to had a longer downtime due to the restoration of all datafiles and apply a large number of archived logs. Using flashback database will reduce time for incomplete recovery using special architecture. Architecture When Flashback Database option is configured, the server starts a new buffer - Flashback Buffer - in the SGA, this buffer will log flashback data which are old images of data blocks before changes toke place. The size of Flashback Buffer depends on the size of the Log Buffer. A Background process named Recovery Writer (RVWR) will start also, this process will write flashback data from the flashback buffer to the Flashback database logs, and logs circular, which means they will be reused. Flashback logs are stored in location specified by DB_RECOVERY__FI LE_DEST parameter and have a lifespan depending on retention period parameter (in minutes) DB_FLASHBACK, RETENTION-TARGET. When yOu usE FLASHBACKDATABASE command, the flashback logs are used to restore to the data status before the given time or marker. Redo data is used to roll forward to the desired flashback time and fill the gaps. Flash Recovery Area The Flash recover area is a location on your storage media that hold flashback logs generated by the flashback process, in addition, it may hold several oracle files like RMAN backups, control file backups, archived Logs and more. DB_RECOVERY-FI LE*DEST initialization parameter will control where to create the recovery area. You must allocate this area on storage subsystem that has adequate space and enough resources to manage the load. The total size of the Flashback recovery area considers not only the database file, but also the archived files. Flashback Logs The Flashback database technique implements new type of logs named flashback logs; the logs are generated on regular interval and dumped in the location specified by init.ora parameter DB.,,RECOVERY__FILE_DEST. They store entire block structure for the data before modification and used to rewind data changes when executing FLASHBACK DATABASE command. Flashback Retention Your recovery requirements control lifespan of flashback logs in recovery area, init.ora parameter for this purpose DB_FLASH BACK_RETENTION-TARGET can be set to a number of seconds for how long you want to keep the logs, default value is 24 hours. This parameter can be altered dynamically using the ALTER SYSTEM SET SQL command. Undo Tablespace The size of the undo tablespace must be configured to support the required retention timeframe. The parameter U N DO'TABLESPACE specifies the tablespace name. The instance can have multiple undo tablespace, but one active undo tablespace at startup only. f-'";A lY t'''*J-q" utt u Flash Recovery Area Data base Flashback Logs Fig. 1 : Flashback Architecture