8/6/2019 Flashback Chart

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Maximrze Av ailabilitv with FlashbackFlashback was introduced in Oracle9i through the DBMS-FLASHBACK package

to enable DBAs to view older versions of modified data, and through Oraclel0gFlashback 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 revolutionarytechnology in recovery; this techniquehad been introduced and enhanced

over time to minimize applicationsdowntime caused by human errors.Flashback can be used to recovercommitted changes if were done bymistake, recover dropped tables, andcompare before and after image ofdata with minimal efforts.

Flashback Techniques

Oracle's flashback technologies haveimplemented different techniques thatsuite almost every type of humanerror requiring recovery. The types ofFlash back currently supported byOracle Database 119l

. Flashback Database

. Flashback Drooo Flashback Table. Flashback Query. Flashback Versions Query. Flashback Transactions Query

Flashback DatabaseDBAs may have situations whenmultiple tables were modified bybatch jobs, or when relatedtransaction take place and the resultneed to be undone, previously DBA

had to perform point-in-time

incomplete recovery by restoringbackup and apply redo changes until a

desired time to undo the changes, thisprocess is time consuming and causethe database to had a longerdowntime due to the restoration of all

datafiles and apply a large number ofarchived logs. Using flashbackdatabase will reduce time forincomplete recovery using specialarchitecture.

ArchitectureWhen Flashback Database option is

configured, the server starts a new

buffer - Flashback Buffer - in theSGA, this buffer will log flashback datawhich are old images of data blocksbefore changes toke place. The size ofFlashback Buffer depends on the sizeof the Log Buffer. A Backgroundprocess named Recovery Writer

(RVWR) will start also, this process willwrite flashback data from the flashbackbuffer to the Flashback database logs,and those logs are circular, which means

they will be reused. Flashback logs arestored in location specified byDB_RECOVERY__FI LE_DEST parameterand have a lifespan depending on

retention period parameter (in minutes)DB_FLASHBACK, RETENTION-TARGET.

When yOu usE FLASHBACKDATABASEcommand, the flashback logs are used torestore to the data status before thegiven 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 onyour storage media that hold flashbacklogs generated by the flashback process,

in addition, it may hold several oracle fileslike RMAN backups, control file backups,archived Logs and more.DB_RECOVERY-FI LE*DEST initialization

parameter will control where to create therecovery area. You must allocate this area

on storage subsystem that has adequatespace and enough resources to managethe load. The total size of the Flashback

recovery area considers not only thedatabase file, but also the archived files.

Flashback Logs

The Flashback database techniqueimplements new type of logs namedflashback logs; the logs are generated onregular interval and dumped in thelocation specified by init.ora parameterDB.,,RECOVERY__FILE_DEST. They storeentire block structure for the data beforemodification and used to rewind datachanges when executing FLASHBACK

DATABASE command.

Flashback RetentionYour recovery requirements controllifespan of flashback logs in recoveryarea, init.ora parameter for this purpose

DB_FLASH BACK_RETENTION-TARGET

can be set to a number of seconds forhow long you want to keep the logs,

default value is 24 hours. This parameter

can be altered dynamically using theALTER SYSTEM SET SQL command.

Undo Tablespace

The size of the undo tablespace must beconfigured to support the requiredretention timeframe. The parameterU N DO'TABLESPACE specifies thetablespace name. The instance can havemultiple undo tablespace, but one activeundo tablespace at startup only.

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Flash Recovery AreaData base

Flashback Logs

Fig. 1 : Flashback Architecture