Enabling Flashback Database
As its name implies, Flashback Database offers the
capability to quickly "flash" a database back to its prior state as
of a specified point in time. Oracle does this by retaining a copy of any
modified database blocks in flashback logs in the Flash Recovery Area. A
new flashback log is written to the Flash Recovery Area on a regular basis
(usually hourly, even if nothing has changed in the database), and these logs
are typically smaller in size than an archived redo log. Flashback logs have a
file extension of .FLB.
When a Flashback Database request is received, Oracle then
reconstructs the state of the database just prior to the point in time
requested using the contents of the appropriate flashback logs. Then the
database's archived redo logs are used to fill in the remaining gaps between
the last backup of the datafile and the point in time desired for recovery.
The beauty of this approach is that no datafiles need to
be restored from backups; further, only the few changes required to fill in
the gaps are automatically applied from archived redo logs. This means that
recovery is much quicker than traditional incomplete recovery methods, with
much higher database availability.
It is worth noting the few prerequisites that must be met
before a database may utilize Flashback Database features:
-
The database must have flashback logging enabled, and therefore a
Flash Recovery Area must have been configured. (For a RAC environment, the
Flash Recovery Area must also be stored in either ASM or in a clustered file
system.)
-
Since archived redo logs are used to "fill in the gaps"
during Flashback Database recovery, the database must be running in ARCHIVELOG
mode.
Activating Flashback Database. Once the Flash
Recovery Area has been configured, the next step is to enable Flashback Database
by issuing the ALTER DATABASE FLASHBACK ON;
command while the database is in MOUNT EXCLUSIVE mode, similar to activating a
database in ARCHIVELOG mode.
Setting the Flashback Retention Target. Once
Flashback Database has been enabled, the DB_FLASHBACK_RETENTION_TARGET initialization parameter
determines exactly how far a database can be flashed back. The default value is
1440 minutes (one full day), but this can be modified to suit the needs of your
database. For purposes of illustration, I have set my demonstration database's
setting to 2880 minutes (two full days).
Deactivating Flashback Database. Likewise, issuing
the ALTER DATABASE FLASHBACK OFF; command
deactivates Flashback Backup and Recovery. Just as in the activation process, note
that this command must be issued while the database is in MOUNT EXCLUSIVE mode.
See Listing
2.3 for queries that display the status of the Flash Recovery Area,
status of the related initialization parameters, and whether the database has
been successfully configured for flashback.
Storing Backups In Flash Recovery Area
Now that I have enabled the Flash Recovery Area and enabled
flashback logging, I can next turn my attention to preparing the database to
use flashback logs during a Flashback Database recovery operation.
Listing
2.4 lists the RMAN commands I will need to issue to configure the
database for Flash Recovery Area and Flashback Database use. Notice that I have
not CONFIGUREd a FORMAT directive for the RMAN channels used to create database
backups; for these examples, I am going to let RMAN place all backup components
directly in the Flash Recovery Area.
Listing
2.5 implements Oracle's recommended daily RMAN backup scheme using
datafile image copies and incrementally-updated backups. (See the previous
article in this series for a full discussion of this technique.)
Finally, Listing
2.6 shows the abbreviated results of the first cycle's run of this
backup scheme. Note that Oracle uses OMF naming standards for each backup
component file - in this example, datafiles, the "extra copy" of the
archived redo logs, and control file autobackups - stored in the Flash Recovery
Area.
Flashback Database: An Example
Now that I have enabled flashback logging and have created
sufficient backup components that are being managed in the Flash Recovery Area,
it is time to demonstrate a Flashback Database operation.
Let's assume a worst-case scenario: One of my junior developers
has been enthusiastically experimenting with logical units of work on what he thought
was his personal development database, but instead mistakenly applied a
transaction against the production database. He has just accidentally deleted
several thousand entries in the SH.SALES and SH.COSTS tables - just in time to
endanger our end-of-quarter sales reporting schedule, of course! Here is the
DML statements issued, along with the number of records removed:
DELETE FROM sh.sales
WHERE prod_id BETWEEN 20 AND 80;
10455 rows deleted
Executed in 89.408 seconds
DELETE FROM sh.costs
WHERE prod_id BETWEEN 20 AND 80;
6728 rows deleted
Executed in 18.086 seconds
COMMIT;
Commit complete
Executed in 0.881 seconds
Flashback Database to the rescue! Since I know the
approximate date and time that this transaction was committed to the database, I
will issue an appropriate FLASHBACK DATABASE command from within an RMAN
session to return the database to that approximate point in time. Here is a
more complete listing of the FLASHBACK DATABASE command set:
FLASHBACK [DEVICE TYPE = <device type>] DATABASE
TO [BEFORE] SCN = <scn>
TO [BEFORE] SEQUENCE = <sequence> [THREAD = <thread id>]
TO [BEFORE] TIME = '<date_string>'
Note that I can return the database to any prior point in
time based on a specific System Change Number (SCN), a specific redo
log sequence number (SEQUENCE), or to a specific date and time (TIME).
If I specify the BEFORE directive, I am telling RMAN to flash the database back
to the point in time just prior to the specified SCN, redo log, or time,
whereas if the BEFORE directive is not specified, the database will be flashed
back to the specified SCN, redo log, or time as of that specified point in
time, i.e., inclusively.
First, I queried my database's Flashback Logs to determine
which ones are available, found the log just prior to the user error and
decided to flash back the database based on that log's starting SCN. Listing 2.7
contains the query I ran against V$FLASHBACK_DATABASE_LOGFILE to obtain this
information.
Just as I would do during a normal point-in-time incomplete
recovery, I then shut down the database by issuing the SHUTDOWN IMMEDIATE command, and then restarted
the database and brought it into MOUNT mode via the STARTUP MOUNT command. Instead of having to
perform a restoration of datafiles as in a normal incomplete recovery, I
instead simply issue the appropriate FLASHBACK DATABASE command to take the
database back to the SCN I desired.
Once the flashback is completed, I could have continued to
roll forward additional changes from the archived redo logs available; however,
I simply chose to open the database at this point in time via the ALTER
DATABASE OPEN RESETLOGS; command. Here are the actual results from the RMAN
session:
C:>rman nocatalog target sys/@zdcdb
Recovery Manager: Release 10.1.0.2.0 - Production
Copyright (c) 1995, 2004, Oracle. All rights reserved.
connected to target database: ZDCDB (DBID=1863541959)
using target database controlfile instead of recovery catalog
RMAN> FLASHBACK DEVICE TYPE = DISK DATABASE TO SCN = 2127725;
Starting flashback at 08-DEC-04
allocated channel: ORA_DISK_1
channel ORA_DISK_1: sid=158 devtype=DISK
starting media recovery
media recovery complete
Finished flashback at 08-DEC-04
RMAN> alter database open resetlogs;
database opened
To see what is really going on during the flashback and
recovery process, I have also included a portion
of the database's alert log. Note that Oracle automatically cleaned up after
itself: Since they are of no use any longer after the RESETLOGS operation,
Oracle even deleted the outmoded Flashback Logs from the Flashback Recovery
Area.
Conclusion
Oracle 10g's Flash Recovery Area simplifies the storage
and handling of backup components and flashback logs, and the new Flashback
Database features provide any Oracle DBA with a much improved, faster option
for incomplete database recovery. The next article in this series will delve
into the details of using Oracle 10g's expanded Logical Flashback features,
including some intriguing capabilities for recovering from logical errors at a
much more granular level than Flashback Database provides.
References and Additional Reading
While there is no substitute for direct experience, reading
the manual is not a bad idea, either. I have drawn upon the following Oracle
10g documentation for the deeper technical details of this article:
B10734-01 Oracle Database
Backup and Recovery Advanced User's Guide
B10735-01 Oracle Database
Backup and Recovery Basics
B10750-01 Oracle Database
New Features Guide
B10770-01 Oracle Database
Recovery Manager Refererence
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