Usage, Benefits and Limitations of Standby Redo Logs (SRL) [ID 219344.1]

Usage, Benefits and Limitations of Standby Redo Logs (SRL) [ID 219344.1]
		修改時間 10-NOV-2010 型別 REFERENCE 狀態 PUBLISHED

PURPOSE
-------

This document shows the usage, benefit sand limitations for an Oracle 9i/10g
Data Guard Configuration with Standby Redo Logs.


SCOPE & APPLICATION
-------------------

All DBA's and Interested persons who wish to implement Data Guard in
Oracle 9i/10g/11g (9.x-11.x).


1. Standby Redo Logs - what are they and when to use them ??
-------------------------------------------------------------

Starting Oracle 9i you have to opportunity to add Standby Rodo Log Groups to
your Online Redo Log Groups. These Standby Redo Logs then store the information
received from the Primary Database. In case of a Failover situation, you will
have less data loss than without Standby Redo Logs.

Standby Redo Logs are only supported for the Physical Standby Database in
Oracle 9i and as well for Logical Standby Databases in 10g. Standby Redo Logs
are only used if you have the LGWR activated for archival to the Remote Standby
Database.

The great Advantage of Standby Redo Logs is that every Entry written into
the Online RedoLogs of the Primary Database is transfered to the Standby
Site and written into the Standby Redo Logs at the same time; threfore, you
reduce the probability of Data Loss on the Standby Database.

Starting with 10g it is possible to start Real-Time Apply with Physical and
Logical Standby Databases. With Real-Time Apply, the Redo is applied to the
Strandby Database from the Standby RedoLog instead of waiting until an Archive
Log is created. So Standby Redo Logs are required for Real-Time Apply.


2. How Do I create Standby Redo Logs ??
---------------------------------------

Standby Redo Logs are additional Redo Log Groups. If you were to query the
V$LOGFILE view on the Standby Database, the output would typically look
something like this:

   SQL> select * from v$logfile;

   GROUP# STATUS  TYPE    MEMBER
   ------ ------- ------- ------------------------------------
        1          ONLINE C:ORACLEORADATASTANDBYREDO01.LOG
        2          ONLINE C:ORACLEORADATASTANDBYREDO02.LOG
        3          ONLINE C:ORACLEORADATASTANDBYREDO03.LOG

Before you add the Standby Redo Logs to your Standby Database, verify the
number of maximum Logfile Groups and Logfile Members. If you don't remember
these values, you can look at the CREATE CONTROLFILE script. To get such a
script, run the following command:

   SQL> alter database backup controlfile to trace;

This will create a *.TRC-file in your UDUMP Directory. When you edit or view
this newly created file, you will find a script to recreate the Controlfile.
Notice the following relevant entries:

    MAXLOGFILES   8
    MAXLOGMEMBERS 3

In this example you can create a maximum of eight (8) Logfile groups.  Each
group can contain a maximum of three (3) Members.

Now we can add Standby Redo Log files to the Standby Database (of course, the
Standby Database must be in the MOUNT State):

   SQL> alter database add standby logfile group 4
     2  ('C:ORACLEORADATASTANDBYSTBY04.LOG') SIZE 5M;

You can also add any further members to each group:

   SQL> alter database add standby logfile member
     2  'C:ORACLEORADATASTANDBYSTBY14.LOG' to group 4;

Please keep in mind that if the RFS process is able to write into a Standby
Redo Log, the filesize of the Standby Redo Log MUST be equal to the current
Online Redo Log of the Primary Database!!


3. Limitations to Standby Redo Logs
-----------------------------------

In a Oracle 9i/10g Data Guard Environment, the RFS Process on the Standby
Database receives the Data from the Primary and writes it to Disk (either
Standby Redo Logs or Archived Redo Logs).

If you consider using Standby Redo Logs, you must make certain that they are
the same size as the Online Redo Logs. If you have different sizes of Online
Redo Logs, you have to create corresponding Standby Redo Logs. The RFS process
won't attach Standby Redo Logs if they are different from the Online Redo Log.
It is recommended to have at least one more of Standby Redo Log Group as you
have of Online Redo Log Groups per Thread and Size..

Standby Redo Logs are filled with the same information that is written to the
Online Redo Logs of the Primary Database.  Therefore, only the LGWR can provide
this information to the Standby RFS process, so Standby Redo Logs will only
benefit if you set LGWR as the Transmitter on the Primary Database in the
LOG_ARCHIVE_DEST_n Initialization Parameter. Starting with 10.2.0, even the ARCH
is able to write into Standby RedoLogs.

The RFS-Process always tries to allocate the next available Standby RedoLog, so
it is possible that you encounter a Switch between only two Standby RedoLogs,
although you created lots more of them. There's no rota defined here like in
Online RedoLogs. This is reported in Bug 2722195.


4. Differents in the Log Apply Services when using Standby Redo Logs
--------------------------------------------------------------------

In case you do not have Standby Redo Logs, an Archived Redo Log is created
by the RFS process and when it has completed, this Archived Redo Log is applied
to the Standby Database by the MRP (Managed Recovery Process) or the Logical
Apply in Oracle 10g when using Logical Standby. An open (not fully written)
ArchiveLog file cannot be applied on the Standby Database and will not be used
in a Failover situation. This causes a certain data loss.

If you have Standby Redo Logs, the RFS process will write into the Standby Redo
Log as mentioned above and when a log switch occurs, the Archiver Process of the
Standby Database will archive this Standby Redo Log to an Archived Redo Log,
while the MRP process applies the information to the Standby Database.  In a
Failover situation, you will also have access to the information already
written in the Standby Redo Logs, so the information will not be lost.

Starting with Oracle 10g you have also the Option to use Real-Time Apply with
Physical and Logical Standby Apply. When using Real-Time Apply we directly apply
Redo Data from Standby RedoLogs. Real-Time Apply is also not able to apply Redo
from partial filled ArchiveLogs if there are no Standby RedoLogs. So Standby
RedoLogs are mandatory for Real-Time Apply.


RELATED DOCUMENTS
-----------------

Oracle9i  Data Guard Concepts and Administration
Oracle10g Data Guard Concepts and Administration

相關的 產品 Oracle Database Products > Oracle Database > Oracle Database > Oracle Server - Enterprise Edition Oracle Database Products > Oracle Database > Oracle Database > Oracle Server - Enterprise Edition 關鍵字 DATA GUARD; V$LOGFILE; STANDBY REDO LOGS; STANDBY DATABASE; REAL TIME; LOG_ARCHIVE_DEST_N; ARCHIVED LOGS; REDOLOG

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