We shall install SPLUNK on new IBM x3650M3 server with 12 SAS HD, 8 GB RAM and 2 E5630 CPU.
OS is Redhat 5.6.
RAID will be 10
In order to get best disk performance, do we have to install the RAID controller driver in LINUX OS?
Splunk will use whatever the operating system sees.
This really isn't the best place for your question -- you'll get much better information by asking it in a RedHat-specific forum, or contacting RedHat directly for support. Be sure to let them know exactly what controller card your server uses, if you have that information.
The operating system will need to recognize the RAID controller somehow, but it's possible that RHEL will include the appropriate kernel module / driver out-of-the box. It looks like it might be a rebadged LSI MegaRAID family card, in which case it would probably work without additional drivers.