Differences between RHEL5 and RHEL6 on SAN Boot installation.
 

We want to install RHEL5.7 OS with multiplepath enabled on our IBM SAN storage for SAN boot. We google out many manuals to start os installation with inputing "linux mpath" on boot line. I wonder if RHEL5 will not load the multiplepath automatically when starting os installation so that we need to type "linux mpath" on boot line for loading mutiplepath driver when starting os installation.

If we do not input "linux mpath" on boot line when starting RHEL5.7 installation, we should see two same disk such as /dev/sda and /dev/sdb which is from SAN side. If inputing "linux mpath" on boot line when starting RHEL5.7 installation, we should see the only one disk such as /dev/mapper/mpath[n]. Is it Right?

In the case of RHEL6.3, we confirmed that we can see the only one disk which is from SAN side such as /dev/mapper/mpath[n] even though we do not input "linux mpath" on boot line. I wonder if multiplepath driver is automatically loaded when starting RHEL6 installation and this is new feature from RHEL6.

And idea, documents and link for SAN boot RHEL5 and RHEL6 installation on IBM storage is appreciated.

Since you've been dealing with Linux for nine years now, you should have a good handle on the basics. However, what you're asking about has MANY variables, based on your hardware (blade servers? stand alone? VM?), and your goals (clustering? stand alone?).

If you need guidance on a complex issue like this, the best people to contact would be both Red Hat and IBM support. IBM can get you SAN related information, and Red Hat for the OS. This will also depend on what HBA's you're using to connect to the SAN, and what hardware they're in. The good news is that you're in a totally supported configuration, so both vendors should be able to get you a solid answer.

That said, unless the system is already booted, it can't load kernel modules and software to perform multipathing, and IBM has documentation on this:
http://www.ibm.com/developerworks/li...oot/index.html

You CAN build a custom initrd image, but to me it's overkill to want to boot with multipathing. Even if one SAN switch is down, you've got a redundant path to your boot disk, and (once its booted), the multipath tools/drivers will be loaded.