Before you can install Automatic Storage Management (ASM), Oracle Cluster Synchronization Services (CSS) must be configured and running. In some versions of Oracle, CSS would be installed when you installed the Oracle database software, and thus the service was always there. Oracle Database 10g release 2 has made the install of CSS optional, and thus if you need it, you will have to install it.
If you attempt to create an ASM instance and CSS is not configured, then Oracle will prompt you to install CSS first. The purpose of the service is to provide synchronization between an ASM instance and the database instances that rely on it for database file storage.
CSS can be installed in one of two places, depending on what type of database environment you will be running. If you are running a stand-alone, non clustered (RAC) environment, then the OUI will configure and start a single-node version of the CSS service. The software for that CSS service will reside in your databases ORACLE_HOME. If you are running Oracle Real Application Clusters (RAC) installations, then you will be running CSS as a part of Oracle Cluster Ready Services (CRS). In this case CRS, and thus CSS, will reside in a separate Oracle home directory (also called the CRS home directory).
As a general rule, there is very little reason to run ASM on a single instance node, and thus, very little reason to run CSS. It is an additional burden on your machine in terms of CPU and memory and the returns are not there. However, ASM on a clustered setup is a different story, and in that case you will want CSS installed and running.
To configure CSS on a given database node, you need to run a script called localconfig on UNIX/Linux and localconfig.bat on windows. This script is located in $ORACLE_HOME/bin (%ORACLE_HOME\bin in Windows). Simply run the script and Oracle will configure CSS for you as seen in this example (note that for UNIX installs, this must run as root):
Oracle> su - root
#> $ORACLE_HOME/bin/localconfig all
Successfully accumulated necessary OCR keys.
Creating OCR keys for user 'root', privgrp 'root'..
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.
CSS is active on all nodes.
Oracle CSS service is installed and running under init(1M)