I was playing with openfiler and RHEL 6 and managed to setup Red Hat Enterprise Linux on the iSCSI targets provided by openfiler. All partitions (including /boot) was setup on the on the iSCSI devices.

Only after I rebooted, and got a boot failure, and came to realize whatever I did wasn't the recommended way to install Linux on a diskless machine, there has to be some way the OS (RHEL) to know how to access the iSCSI devices, or the OS is to be installed on a locally attached disk media.

I would be glad if you let me know how you are using iSCSI devices, especially diskless machines relying solely on SAN for data storage.

Yes, it's a chicken and egg thing. You can't boot until you have
a filesystem, and you can't read a filesystem until after you've
booted. The only way I know to get around this is to enable pxe
booting and configure dhcp and tftp.

The boot process would start with the device requesting an IP
address and a server to load files from. After pulling down
the kernel and an initrd file, the system will again ask for
an IP address as it boots the kernel in memory. The initrd
file has to be preconfigured to know the scsi targets and to
finish the boot process.

Try searching for "linux boot diskless iscsi" and see if you
can find some instructions. It's more involved than it seems,
but you'll learn a lot in the process. And it's great having
a menu come up when you boot off the net! No more looking for
cds for installations or for troubleshooting!

Just a thought, instead of PXE boot, booting from a read-only CD is another alternative.

True. Or a floppy or a flash drive. But when I think of "diskless,"
I'm thinking no storage devices.

Incidentally, I'd suggest using a virtualization product such as
VirtualBox for setup and testing. The last few times I set up pxe
booting, it saved a lot of time when I tested on virtual hardware.