What is initrd and why is it necessary

 

the kernel loads initrd (initial ram disk) as its root file system. initrd
inturn load the actual root file system (some thing like chain boot
loaders). On systems using initrd you would notice "umounting initrd"
console message while booting.

Its not absolutely required. The reason for this is that device support can
be added to initrd as module rather than being compiled as part of the
kernel. Most initrd will contain ext3 fs module.

If you want to know what your initrd contains, try this

#zcat initrd.img.gz > /dev/ram
#mount /dev/ram /mnt
#cd /mnt
#ls