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Log Shipping When the Standby Database is a Single Node System

Oracle RAC Cluster Tips by Burleson Consulting

This is an excerpt from the bestselling book Oracle Grid & Real Application Clusters.  To get immediate access to the code depot of working RAC scripts, buy it directly from the publisher and save more than 30%.


If the RAC standby configuration is on a single node system, the prerequisites are the same. The difference is that all instances from the primary database have to ship their archive logs to one single system. Due to the SCNs in the archive logs, the managed recovery process (MRP) on the standby database determines the correct order in which to apply the archive logs from the different threads on the standby database. The primary instances in this example are instances AULT1 and AULT2. The single node standby instance is AULT3. This results in the following settings for log_archive_dest:

Instance AULT1 and AULT2:

Instance AULT1:

LOG_ARCHIVE_DEST_1=(location=/usr/backup/ault_rac1/archives1) 
LOG_ARCHIVE_DEST_2=(SERVICE=ault3 LGWR ...)

Instance AULT2:

LOG_ARCHIVE_DEST_1=(location=/usr/backup/ault_rac2/archives2) 
LOG_ARCHIVE_DEST_2=(SERVICE=ault3 LGWR ...)

Different standby redo log threads must also be created on the standby database. Instance AULT3 now receives all the logs and applies them to the standby database.

Instance AULT3:

LOG_ARCHIVE_DEST_1=(location=/usr/backup/ault_rac3/archives3)

Cross Instance Archival

In addition to the parameters shown above, cross instance archival can be configured. This means archive logs can be shipped from one primary RAC instance to another primary RAC instance. This could be helpful in case of gap resolution, if one instance loses network connectivity or archive logs are deleted by default.

To enable cross instance archival, one free log_archive_dest_n parameter is configured for shipping to another primary instance. For cross-instance archival, only the archive process is supported and can be used. If the earlier example were used, the result would be:

Instance AULT1:

LOG_ARCHIVE_DEST_1=(location=/usr/backup/ault_rac1/archives1) 
LOG_ARCHIVE_DEST_2=(SERVICE=ault3 LGWR ...) 
LOG_ARCHIVE_DEST_3=(SERVICE=ault2 ARCH ...)
LOG_ARCHIVE_FORMAT=arc_%s_%t

If cross instance archival is also used on instance AULT2:

LOG_ARCHIVE_DEST_1=(location=/usr/backup/ault_rac2/archives2) 
LOG_ARCHIVE_DEST_2=(SERVICE=ault3 LGWR ...)
LOG_ARCHIVE_DEST_3=(SERVICE=ault1 ARCH ...)
LOG_ARCHIVE_FORMAT=arc_%s_%t

Archive Log Gap Resolution and FAL

There are two modes of gap resolution: 

* The automatic one, which is controlled by the archiver and is turned on automatically. The automatic gap resolution works proactively and will produce some network overhead.

* The Fetch Archive Log (FAL) method, which requests certain logs as required from the remote systems. On the receiving instance, one or more FAL Server is published from which it receives its archive logs. On the sending instance, one or more FAL Client is published, which receive archive logs from a specified instance. The publishing is done by the initialization parameters fal_server and fal_client.

FAL is not supported for logical standby databases. A typical configuration using the current example RAC could be:

Instance AULT1 and AULT2:

FAL_CLIENT=ault3

Instance AULT3 and AULT4:

FAL_SERVER=ault1,ault2

When cross instance archival is configured, instance AULT3 will now receive missing archive logs from instance AULT2, and instance AULT4 from instance AULT1.  

 


This is an excerpt from the bestselling book Oracle Grid & Real Application Clusters, Rampant TechPress, by Mike Ault and Madhu Tumma.

You can buy it direct from the publisher for 30%-off and get instant access to the code depot of Oracle tuning scripts.

http://www.rampant-books.com/book_2004_1_10g_grid.htm


 

 
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