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Editor to share with you how flashback database chooses the flashback log that needs to be applied. I hope you will get something after reading this article. Let's discuss it together.

The flashback database process consists of two stages: flashback restore and flashback recover. Flashback restore is apply flashback log,flashback recover and apply redolog.

Recently, when using flashback database, there is a question: which flashback logfile did flashback restore start apply from?

To make this problem clear, let's start with the following two implicit parameters

_ flashback_barrier_interval: the default value is 30 minutes, indicating how often a tag is written to the flashback log. The main content of this tag is the SCN number at that time. Without exception, flashback restore must use the SCN number as the end point of the restore. Flashback log is applied in the reverse order of the scn number from large to small. A higher value of _ flashback_barrier_interval indicates that the data block before image will write to flashback log at a lower frequency when the database changes. So the less IO Flashback log writes consumes on the database, the more redolog you need to apply when flashback recover. If the setting of _ flashback_barrier_interval is relatively small, before image will be written to flashback log,Flashback log writes more frequently and the IO consumption of the database will be higher, but the redolog that needs to be applied in flashback recover will be reduced, because the scn contained in the tag is closer to the target scn we want to flashback. For example, the following tags are already available in flashback log (suppose the tag is written every 30 minutes)

The generation time marker value of the tag

08:00 scn=100

08:30 scn=130

09:00 scn=155

09:30 scn=172

Suppose the time now is 9:38, and we want to flashback to 8:50, then the flashback restore will end at 08:30 in the order of 09apply 30-> 09 redolog-> 08:30. We must rely on redolog from 08:30 to 08:50, that is, we must apply at least 20 minutes of Redolog. If we shorten the write interval of the mark to 10 minutes by using _ flashback_barrier_interval Then flashback recover can only apply redolog for 10 minutes at most. For the effect of _ flashback_barrier_interval on the number of flashback log writes, please see http://blog.itpub.net/53956/viewspace-1602019/.

_ flashback_verbose_info: the default is FALSE, and we set it to TRUE to output more logs during the flashback process

In order to reduce the length, the whole test process was not included, but some illustrative excerpts were made.

-execute flashback database

Flashback database to scn 12723361569813

Log output at flashback database (including alert.log and .trc files)

* Information output of alert.log

Mon Apr 27 14:08:13 2015

Flashback database to scn 12723361569813

Flashback Restore Start

Mon Apr 27 14:08:27 2015

Flashback Restore Complete

Flashback Media Recovery Start

Started logmerger process

Mon Apr 27 14:08:27 2015

Flashback mount Marker scn during SCN 12723361569676 27.03 Mb/sec

Total waited on sequential reads to complete 0.01s

Number of random reads: 1 = > 16Kb per read

Random read speed: 16Kb in 0.04s = > 0.00 Mb/sec

Total waited on random reads to complete 0.01s

No async reads.

Number of position calls/cache misses: 1/1

Record buffer resizes = 0

-

Excluding datafile 7 from restore portion of flashback because it

Does not have any changes before the flashback target.

Excluding datafile 8 from restore portion of flashback because it

Does not have any changes before the flashback target.

-Flashback Parameters-

Target (to-before) scn: 0x0b92.63725c16 [2962.1668439062]

Restore (to-before) scn: 0x0b92.63725c16 [2962.1668439062]

Current inc 1, Restore target inc 1, Recovery target inc 1

Recovery start checkpoint:

Scn: 0x0b92.637251ed [2962.1668436461] 04/27/2015 13:54:24

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