DGMGRL was never difficult to use; however, it was not providing enough information before switchover/failover to make sure it happened without issues. Even if “show configuration verbose” was indicating that everything had the SUCCESS status, switchover could lead to issues during role changes.
However, Data Guard Broker in #DB12c introduced a new command, VALIDATE DATABASE, which tremendously lightens the process of information gathering to make sure role change operations succeed. It is very helpful in the case of RAC database usage in Data Guard configuration.
Let’s see a test example of the command Data Guard configuration with a primary two instance RAC database and a standalone standby database.
DGMGRL> show configuration
Configuration - dg_d
Protection Mode: MaxPerformance
Databases:
d - Primary database
d1 - Physical standby database
Fast-Start Failover: DISABLED
Configuration Status:
SUCCESS
DGMGRL> show database d
Database - d
Role: PRIMARY
Intended State: TRANSPORT-ON
Instance(s):
d1
d2
Database Status:
SUCCESS
DGMGRL> show database d1
Database - d1
Role: PHYSICAL STANDBY
Intended State: APPLY-ON
Transport Lag: 0 seconds (computed 0 seconds ago)
Apply Lag: 0 seconds (computed 0 seconds ago)
Apply Rate: 205.00 KByte/s
Real Time Query: OFF
Instance(s):
d1
Database Status:
SUCCESS
The DG configuration has the SUCCESS status. Let’s validate the standby database to see if it is ready for switchover:
DGMGRL> validate database verbose d1
Database Role: Physical standby database
Primary Database: d
Ready for Switchover: Yes
Ready for Failover: Yes (Primary Running)
Capacity Information:
Database Instances Threads
d 2 2
d1 1 2
Warning: the target standby has fewer instances than the
primary database, this may impact application performance
Temporary Tablespace File Information:
d TEMP Files: 2
d1 TEMP Files: 1
Flashback Database Status:
d: Off
d1: Off
Data file Online Move in Progress:
d: No
d1: No
Standby Apply-Related Information:
Apply State: Running
Apply Lag: 0 seconds
Apply Delay: 0 minutes
Transport-Related Information:
Transport On: Yes
Gap Status: No Gap
Transport Lag: 0 seconds
Transport Status: Success
Log Files Cleared:
d Standby Redo Log Files: Cleared
d1 Online Redo Log Files: Cleared
Current Log File Groups Configuration:
Thread # Online Redo Log Groups Standby Redo Log Groups
(d) (d1)
1 2 1
2 2 1
Future Log File Groups Configuration:
Thread # Online Redo Log Groups Standby Redo Log Groups
(d1) (d)
1 2 1
2 2 1
Current Configuration Log File Sizes:
Thread # Smallest Online Redo Smallest Standby Redo
Log File Size Log File Size
(d) (d1)
1 50 MBytes 50 MBytes
2 50 MBytes 50 MBytes
Future Configuration Log File Sizes:
Thread # Smallest Online Redo Smallest Standby Redo
Log File Size Log File Size
(d1) (d)
1 50 MBytes 50 MBytes
2 50 MBytes 50 MBytes
Apply-Related Property Settings:
Property d Value d1 Value
DelayMins 0 0
ApplyParallel AUTO AUTO
Transport-Related Property Settings:
Property d Value d1 Value
LogXptMode ASYNC ASYNC
Dependency <empty> <empty>
DelayMins 0 0
Binding optional optional
MaxFailure 0 0
MaxConnections 1 1
ReopenSecs 300 300
NetTimeout 30 30
RedoCompression DISABLE DISABLE
LogShipping ON ON
Automatic Diagnostic Repository Errors:
Error d d1
No logging operation NO NO
Control file corruptions NO NO
SRL Group Unavailable NO NO
System data file missing NO NO
System data file corrupted NO NO
System data file offline NO NO
User data file missing NO NO
User data file corrupted NO NO
User data file offline NO NO
Block Corruptions found NO NO
As you can see, the database is ready for switchover. There is no lag, and the DG broker has gathered other important bits and pieces, like clearance of standby and redo logs, temp files (there seems to be a bug showing 2 tempfiles for the 2 instance RAC database – does it select data from gv$tempfile?), and the status of data file movements related to the new #db12c feature.
Let’s see what will happen if transfer to standby is turned off:
DGMGRL> edit database d set state='TRANSPORT-OFF';
Succeeded.
DGMGRL> validate database d1
Database Role: Physical standby database
Primary Database: d
Ready for Switchover: No
Ready for Failover: Yes (Primary Running)
...
The standby database is not ready. But what is the reason for the status? DG log has more information:
08/20/2013 13:45:28
Primary completed health check
EDIT DATABASE d SET STATE = TRANSPORT-OFF
08/20/2013 13:45:41
Switchover to d1 not possible, verification returned error ORA-16466
See database alert log for more details
The alert log doesn’t contain much information. There is only a command to disable standby destination, and the error description gives the following:
$ oerr ora 16466
16466, 0000, "invalid switchover target"
// *Cause: The switchover target was not a valid, enabled, or active
// standby database.
// *Action: Fix the problem in the switchover target and reissue the command
The issue is not clearly stated, but the Data Guard configuration should certainly be checked and fixed before a switchover operation.
I tested several different scenarios. One of them was to shut down the standby database the with abort clause (I could not even get results from “validate database d1” since it was not available) and start it back up with disabled transport from the primary database. Data Guard command “show database d1” indicated that lags were unknown (which is related to NULL values in v$dataguard_stats). However, “validate database d1” was showing a constant Apply Lag of “286 days 11 hours 35 minutes 12 seconds”. It was not growing and stated the same thing until I enabled transport from primary. Where did it come from? Who knows…
DGMGRL> show database d1
Database - d1
Role: PHYSICAL STANDBY
Intended State: APPLY-ON
Transport Lag: (unknown)
Apply Lag: (unknown)
Apply Rate: 0 Byte/s
Real Time Query: OFF
Instance(s):
d1
Database Status:
SUCCESS
DGMGRL> validate database d1
Database Role: Physical standby database
Primary Database: d
Ready for Switchover: No
Ready for Failover: Yes (Primary Running)
Capacity Information:
Database Instances Threads
d 2 2
d1 1 2
Warning: the target standby has fewer instances than the
primary database, this may impact application performance
Temporary Tablespace File Information:
d TEMP Files: 2
d1 TEMP Files: 1
Flashback Database Status:
d: Off
d1: Off
Standby Apply-Related Information:
Apply State: Running
Apply Lag: 286 days 11 hours 35 minutes 12 seconds
Apply Delay: 0 minutes
Current Log File Groups Configuration:
Thread # Online Redo Log Groups Standby Redo Log Groups
(d) (d1)
1 2 1
2 2 1
Future Log File Groups Configuration:
Thread # Online Redo Log Groups Standby Redo Log Groups
(d1) (d)
1 2 1
2 2 1
Lag came back to 0 seconds when transport was enabled again.
There are definitely areas that should be improved to provide a clear understanding of VALIDATE DATABASE output and messages in drc (and alert…) logs, but this new feature improved and simplified information gathering for Data Guard configuration. This change will allow us to save time during possible role change operations.
Happy Data Guard’ing!
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