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Wednesday, 11 July 2012

Oracle Cluster Synchronization Services (CSS)

Automatic Storage Management (ASM) requires the use of Oracle Cluster Synchronization Services (CSS), and as such, CSS must be configured and running before attempting to use ASM. The CSS service is required to enable synchronization between an ASM instance and the database instances that rely on it for database file storage.
In a non-RAC environment, the Oracle Universal Installer will configure and start a single-node version of the CSS service. For Oracle Real Application Clusters (RAC) installations, the CSS service is installed with Oracle Cluster Ready Services (CRS) in a separate Oracle home directory (also called the CRS home directory). For single-node installations, the CSS service is installed in and runs from the same Oracle home as the Oracle database.
Because CSS must be running before any ASM instance or database instance starts, Oracle Universal Installer configures it to start automatically when the system starts. For Linux / UNIX platforms, the Oracle Universal Installer writes the CSS configuration tasks to the root.sh which is run by the DBA after the installation process.
With Oracle Database 10g Release 1, CSS was always configured regardless of whether you chose to configure ASM or not. On the Linux / UNIX platform, CSS was installed and configured via the root.sh script. This caused a lot of problems since many did not know what this process was, and for most of them, didn't want the CSS process running since they were not using ASM.
Oracle listened carefully to the concerns (and strongly worded complaints) about the CSS process and in Oracle Database 10g Release 2, will only configure this process when it is absolutely necessary. In Oracle Database 10g Release 2, for example, if you don't choose to configure an ASM stand-alone instance or if you don't choose to configure a database that uses ASM storage, Oracle will not automatically configure CSS in the root.sh script.
In the case where the CSS process is not configured to run on the node (see above), you can make use of the $ORACLE_HOME/bin/localconfig script in Linux / UNIX or %ORACLE_HOME%\bin\localconfig batch file in Windows. For example in Linux, run the following command as root to configure CSS outside of the root.sh script after the fact:
‎$ su
‎# $ORACLE_HOME/bin/localconfig all‎
‎/etc/oracle does not exist. Creating it now.‎
Successfully accumulated necessary OCR keys.‎
Creating OCR keys for user 'root', privgrp 'root'..‎
Operation successful.‎
Configuration for local CSS has been initialized
Adding to inittab
Startup will be queued to init within 90 seconds.‎
Checking the status of new Oracle init process...‎
Expecting the CRS daemons to be up within 600 seconds.‎
CSS is active on these nodes.‎
linux3‎
CSS is active on all nodes.‎
Oracle CSS service is installed and running under init(1M)‎
Note: If you attempt to configure ASM after the fact, the Database Configuration Assistant (DBCA) detects whether CSS is configured on the node. If it does not detect CSS as configured (and running), the installer prompts the user to run 'localconfig add' as necessary
When performing an Oracle Database 10g Custom install, the issue can become a bit more confusing. During a custom install, Oracle will ask for any DB configuration questions during the install itself. It then invokes the DBCAat the end of the install in "custom" mode where the DBCA asks all the questions. As such, at the time Oracle prompts the user to run the root.sh script for a custom install, it does not know whether they will choose to configure ASM or not. Oracle will err on the side of what the majority of people would do. In this case, it means that Oracle will not configure CSS at all in the root.sh script in the case of a custom install since the majority of users will not be using ASM anyway. Here, Oracle relies on the fact that if CSS is not configured, the DBCA will prompt the user to go run 'localconfig add' as root. Once this is done, then CSS will be configured and the DBCA will allow the user to proceed with the configuration of ASM.

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