Assume the following situation: Due to some OS or network related problem you are not able to start-up the CRS (this does happen!) before it impacts the database availability and subsequently your business. Your primary goal now is to make the database available as soon as possible irrespective of its mode (RAC or Non-RAC). The reason being you would like to reduce the impact to your business. If you attempt to startup the instance without CRS, you get the following error:
ORA-29702: error occurred in Cluster Group Service operation
This error message indicates that it can't start the instance since CRS is not available. Here is the quick work-around to startup the instance. The option is to remove RAC option from your binaries and start the instance. Once you have resolved the CRS related issues, you can turn-on the RAC option back (of course, your instance needs to be down while to turn-on or turn-off the options). Here is how we turn-off:
make -f ins_rdbms.mk rac_off ioracle
You should now be able to startup the instance without CRS stack being up and running.
SQL> Select * from v$option where parameter like 'Real%';
Real Application Clusters FALSE
Real Application Testing TRUE
Turn-on the RAC option with:
make -f ins_rdbms.mk rac_on ioracle
Alternatively, you can install a non-rac oracle binaries to quickly start the instance.
By the way, you turn off other options in similar way such as Partitioning (part_on/part_off), DB Vault (dv_on/dv_off) etc..