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Posts Tagged ‘flashback_on’

Flashback database – dont forget the standby

Posted by John Hallas on September 21, 2016

Today an  application team had a problem with a production system and they wanted the database restored to a point a couple of days ago. It is a configuration management system and they figured they could lose any changes . Their call.

I did all the good things

  • Checked we had a change in place
  • Blacked out primary and standby in OEM
  • Checked flashback was on  –
select flashback_on from v$database;
  • Checked how far we could flashback to
set lines 120
col oldest_flashback_scn for 99999999999999999999
col oldest_flashback_time for a30
to_char(oldest_flashback_time, 'DD-MM-YYYY HH24:MI:SS')
from v$flashback_database_log;
  • Checked that standby had flashback on.

I stopped managed recovery on standby, shutdown that database, stopped primary and mounted and flashed back to my desired time. Everything was fine.

I went to standby, mounted it and ran

flashback standby database to timestamp
to_timestamp ('19/09/2016 12:00:00','dd/mm/yyyy hh24:mi:ss');
ERROR at line 1:
 ORA-38729: Not enough flashback database log data to do FLASHBACK.


That stumped me. Obviously I knew what the message meant and I knew I had not run the script to see how far we could flashback to on standby but why did we have a problem?

Flashback retention was set to 1440  – 24 hours on both primary and standby but there had been no space pressure on primary so it had retained sufficient  flashback logs to go back 6 days if required

It did not take a lot of digging around to find the explanation. Normally we have one database per server (physical or virtual) in production. We then map that  to a dedicated standby server. In this case we had 3 databases running. Due to performance considerations we run 2 primaries and a standby on one side and 2 standbys and a primary on the other. This database was not the key/busiest database and so the standby was sharing with another standby and the primary for the biggest database. That had generated a lot of logfiles which had caused space to be consumed and the flashback logs had been tidied to meet the 1440 minute parameter setting.

A rebuild of standby is under way currently.

I always try to learn lessons from incidents like this. In this case there were two unusual requirements – flashback production and go back 48 hours.

My lessons learned

Use db_recovery_file_dest_size for each database where we have more than one and set them appropriately to the size of each database.

Could we have some sort of automated routine that creates a guaranteed restore point every hour and then drops it 48 hours later. That needs thinking about as we could risk running out of space.




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