Posts Tagged Data Guard

What happens to the Standby when you move a datafile on the Primary?

In 12c, we have introduced online datafile movement as a new feature. Now does that impact an existing standby database? I got asked that yesterday during an online webinar. My answer was that I expect no impact at all on the standby database since redo apply doesn’t care about the physical placement of the datafile on the primary. But I added also that this is just an educated guess because I didn’t test that yet. Now I did:

You know, I like to practice what I preach: Don’t believe it, test it! :-)

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Data Guard Logical Standby – what does it mean?

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

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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Foreign Archived Log in #Oracle – what does it mean?

When you look into V$RECOVERY_AREA_USAGE, you see a strange row at the bottom:

SQL> select * from v$recovery_area_usage;

FILE_TYPE               PERCENT_SPACE_USED PERCENT_SPACE_RECLAIMABLE NUMBER_OF_FILES     CON_ID
----------------------- ------------------ ------------------------- --------------- ----------
CONTROL FILE                             0                         0               0          0
REDO LOG                                 0                         0               0          0
ARCHIVED LOG                         10.18                         0              73          0
BACKUP PIECE                             0                         0               0          0
IMAGE COPY                               0                         0               0          0
FLASHBACK LOG                            0                         0               0          0
FOREIGN ARCHIVED LOG                     0                         0               0          0
AUXILIARY DATAFILE COPY                  0                         0               0          0

Curious what that could be? You will see values other than zero on a Logical Standby Database:

SQL> connect sys/oracle@logst as sysdba
Connected.
SQL> select database_role from v$database;

DATABASE_ROLE
----------------
LOGICAL STANDBY

SQL> select * from v$recovery_area_usage;

FILE_TYPE               PERCENT_SPACE_USED PERCENT_SPACE_RECLAIMABLE NUMBER_OF_FILES     CON_ID
----------------------- ------------------ ------------------------- --------------- ----------
CONTROL FILE                             0                         0               0          0
REDO LOG                                 0                         0               0          0
ARCHIVED LOG                         14.93                         0               9          0
BACKUP PIECE                             0                         0               0          0
IMAGE COPY                               0                         0               0          0
FLASHBACK LOG                            0                         0               0          0
FOREIGN ARCHIVED LOG                  2.03                         0              26          0
AUXILIARY DATAFILE COPY                  0                         0               0          0

In contrast to a Physical Standby Database, this one writes not only into standby logs but also into online logs while being in standby role. That leads to two different kinds of archive logs:
Foreign Archived LogsWhen DML (like insert and update) is done on the primary 1) that leads to redo entries into online logs 2) that are simultaneously shipped to the standby and written there into standby logs 2) also. The online logs on the primary and the standby logs on the standby will be archived 3) eventually. So far that is the same for both physical and logical standby. But now a difference: Logical standby databases do SQL Apply 4) by logmining the standby or the archive logs that came from the primary. That generates similar DML on the standby which in turn leads LGWR there to write redo into online logs 5) that will eventually get archived 6) as well.

A logical standby could do recovery only with its own archive logs (if there was a backup taken before) but not with the foreign archive logs. Therefore, those foreign archive logs can and do get deleted automatically. V$ARCHIVED_LOG and V$FOREIGN_ARCHIVED_LOG can be queried to monitor the two different kinds of logs.

That was one topic of the course Oracle Database 12c: Data Guard Administration that I delivered as an LVC – therefore the picture. Hope you find it useful :-)

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