Why do we need stages ramdisk?
 

There is always a stage2 ramdisk in CD, like stage2.img. Why do we need it? When it will be loaded?

IIGC it is a cramfs image containing Anaconda (the installer), install-time tools and modules, but see for yourself: "mount -o loop -t cramfs stage2.img /mnt/somemountpoint && ls -al /mnt/somemountpoint". If you're gonna change things like modules you'll have to compile against the exact kernel used in the boot image.