With Data Guard, you have the choice between Physical and Logical Standby databases. Let’s see the differences! My demo starts with a Physical Standby, that is then converted into a Logical Standby (therefore the name of the database):
[oracle@uhesse1 ~]$ dgmgrl sys/oracle@prima
DGMGRL for Linux: Version 11.2.0.1.0 - 64bit Production
Copyright (c) 2000, 2009, Oracle. All rights reserved.
Welcome to DGMGRL, type "help" for information.
Connected.
DGMGRL> show configuration;
Configuration - myconf
Protection Mode: MaxPerformance
Databases:
prima - Primary database
logst - Physical standby database
Fast-Start Failover: DISABLED
Configuration Status:
SUCCESS
For now, logst is still a Physical Standby. It is called that way, because the datafiles of prima and logst are physically identical. I can even restore them from one side to the other:
DGMGRL> edit database logst set state=apply-off; Succeeded. DGMGRL> exit [oracle@uhesse1 ~]$ sqlplus sys/oracle@prima as sysdba SQL*Plus: Release 11.2.0.1.0 Production on Fri Feb 20 11:43:07 2015 Copyright (c) 1982, 2009, Oracle. All rights reserved. Connected to: Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - 64bit Production SYS@prima > select name from v$datafile where file#=4; NAME -------------------------------------------------- /u01/app/oracle/oradata/prima/users01.dbf SYS@prima > alter database datafile 4 offline; Database altered.
Now I copy the datafile from the standby server uhesse2 to the primary server uhesse1 – there are different ways to do that, but scp is one:
SYS@logst > select name from v$datafile where file#=4; NAME -------------------------------------------------- /u01/app/oracle/oradata/logst/users01.dbf SYS@logst > exit Disconnected from Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - 64bit Production [oracle@uhesse2 ~]$ scp /u01/app/oracle/oradata/logst/users01.dbf uhesse1:/u01/app/oracle/oradata/prima/users01.dbf The authenticity of host 'uhesse1 (192.168.56.10)' can't be established. RSA key fingerprint is e9:e7:5b:8b:b2:33:42:26:89:03:54:0c:16:0d:98:57. Are you sure you want to continue connecting (yes/no)? yes Warning: Permanently added 'uhesse1,192.168.56.10' (RSA) to the list of known hosts. oracle@uhesse1's password: users01.dbf 100% 5128KB 5.0MB/s 00:00 [oracle@uhesse2 ~]$
When I try to online the datafile again on prima, it is like if I would have restored it from backup:
SYS@prima > alter database datafile 4 online; alter database datafile 4 online * ERROR at line 1: ORA-01113: file 4 needs media recovery ORA-01110: data file 4: '/u01/app/oracle/oradata/prima/users01.dbf' SYS@prima > recover datafile 4; Media recovery complete. SYS@prima > alter database datafile 4 online; Database altered.
The datafiles and also the archived logfiles are physically identical on both sites here, only the controlfiles are different. v$database (like v$datafile, by the way) derives its content from the controlfile:
SYS@prima > select name,dbid,database_role from v$database; NAME DBID DATABASE_ROLE -------------------------------------------------- ---------- ---------------- PRIMA 2012613220 PRIMARY SYS@prima > connect sys/oracle@logst as sysdba Connected. SYS@logst > select name,dbid,database_role from v$database; NAME DBID DATABASE_ROLE -------------------------------------------------- ---------- ---------------- PRIMA 2012613220 PHYSICAL STANDBY
Now I will convert it into Logical Standby:
DGMGRL> edit database logst set state=apply-off;
Succeeded.
DGMGRL> exit
[oracle@uhesse1 ~]$ sqlplus sys/oracle@prima as sysdba
SQL*Plus: Release 11.2.0.1.0 Production on Fri Feb 20 17:29:16 2015
Copyright (c) 1982, 2009, Oracle. All rights reserved.
Connected to:
Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - 64bit Production
SYS@prima > exec dbms_logstdby.build
PL/SQL procedure successfully completed.
SYS@prima > connect sys/oracle@logst as sysdba
Connected.
SYS@logst > alter database recover to logical standby logst;
Database altered.
SYS@logst > shutdown immediate
ORA-01507: database not mounted
ORACLE instance shut down.
SYS@logst > startup mount
ORACLE instance started.
Total System Global Area 521936896 bytes
Fixed Size 2214936 bytes
Variable Size 314573800 bytes
Database Buffers 201326592 bytes
Redo Buffers 3821568 bytes
Database mounted.
SYS@logst > alter database open resetlogs;
Database altered.
SYS@logst > select name,dbid,database_role from v$database;
NAME DBID DATABASE_ROLE
-------------------------------------------------- ---------- ----------------
LOGST 3156487356 LOGICAL STANDBY
SYS@logst > exit
Disconnected from Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - 64bit Production
[oracle@uhesse1 ~]$ dgmgrl sys/oracle@prima
DGMGRL for Linux: Version 11.2.0.1.0 - 64bit Production
Copyright (c) 2000, 2009, Oracle. All rights reserved.
Welcome to DGMGRL, type "help" for information.
Connected.
DGMGRL> remove database logst;
Removed database "logst" from the configuration
DGMGRL> add database logst as connect identifier is logst;
Database "logst" added
DGMGRL> enable database logst;
Enabled.
DGMGRL> show configuration;
Configuration - myconf
Protection Mode: MaxPerformance
Databases:
prima - Primary database
logst - Logical standby database
Fast-Start Failover: DISABLED
Configuration Status:
SUCCESS
One significant change is that the DBID and the name is now different from the primary database as you see above. And the datafiles are no longer physically identical:
DGMGRL> edit database logst set state=apply-off; Succeeded. DGMGRL> exit [oracle@uhesse1 ~]$ sqlplus sys/oracle@prima as sysdba SQL*Plus: Release 11.2.0.1.0 Production on Fri Feb 20 17:38:56 2015 Copyright (c) 1982, 2009, Oracle. All rights reserved. Connected to: Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - 64bit Production SYS@prima > alter database datafile 4 offline; Database altered. SYS@prima > select name from v$datafile where file#=4; NAME -------------------------------------------------- /u01/app/oracle/oradata/prima/users01.dbf SYS@prima > host cp /u01/app/oracle/oradata/prima/users01.dbf /u01/app/oracle/oradata/prima/users01.old
I copy the original file because I know that the restore from logst will not work. It is just to show my point:
[oracle@uhesse2 ~]$ scp /u01/app/oracle/oradata/logst/users01.dbf uhesse1:/u01/app/oracle/oradata/prima/users01.dbf
oracle@uhesse1's password:
users01.dbf 100% 5128KB 5.0MB/s 00:00
SYS@prima > alter database datafile 4 online;
alter database datafile 4 online
*
ERROR at line 1:
ORA-01122: database file 4 failed verification check
ORA-01110: data file 4: '/u01/app/oracle/oradata/prima/users01.dbf'
ORA-01206: file is not part of this database - wrong database id
Exactly. logst is now an autonomous database that is just incidentally doing (nearly) the same DML as prima does. It is no longer Oracle-Block-wise the same as prima. The rowids from prima have no meaning on logst any more:
DGMGRL> edit database logst set state=apply-on;
Succeeded.
SYS@prima > insert into scott.dept values (50,'TEST','TEST');
insert into scott.dept values (50,'TEST','TEST')
*
ERROR at line 1:
ORA-00376: file 4 cannot be read at this time
ORA-01110: data file 4: '/u01/app/oracle/oradata/prima/users01.dbf'
SYS@prima > host cp /u01/app/oracle/oradata/prima/users01.old /u01/app/oracle/oradata/prima/users01.dbf
SYS@prima > alter database datafile 4 online;
alter database datafile 4 online
*
ERROR at line 1:
ORA-01113: file 4 needs media recovery
ORA-01110: data file 4: '/u01/app/oracle/oradata/prima/users01.dbf'
SYS@prima > recover datafile 4;
Media recovery complete.
SYS@prima > alter database datafile 4 online;
Database altered.
SYS@prima > insert into scott.dept values (50,'TEST','TEST');
1 row created.
SYS@prima > commit;
Commit complete.
SYS@prima > select rowid,dept.* from scott.dept where deptno=50;
ROWID DEPTNO DNAME LOC
------------------ ---------- -------------- -------------
AAADS8AAEAAAACNAAE 50 TEST TEST
This rowid is what we normally record in the redo entries and it would be sufficient to retrieve that row on the primary and also on a physical standby where we do “Redo Apply” (another term for “recover database”). But that rowid is different on logst:
SYS@logst > connect sys/oracle@logst as sysdba Connected. SYS@logst > select rowid,dept.* from scott.dept where deptno=50; ROWID DEPTNO DNAME LOC ------------------ ---------- -------------- ------------- AAADS8AAEAAAACOAAA 50 TEST TEST
That is why we need to put additional information – supplemental log data – into the redo entries on the primary. It will help the SQL Apply mechanism to retrieve the row there:
Logical Standby Architecture
The supplemental log data contains at least additionally the primary/unique key like on the picture. In the absence of primary/unique keys, every column of a modified row is written into the redo logs. That may impact the performance of the primary database. Another serious drawback of Logical Standby is that not every datatype and not every operation on the primary is supported for the SQL Apply mechanism. The number of unsupported datatypes decreases version by version, though.
The demo and the sketch above are from my presentation about Transient Logical Standby at the Oracle University Expert Summit 2015 in Dubai – really an amazing location! Hope you find it useful :-)
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