I made a diskless node centos.
the pxelinux.cfg/default file :
default centos
label centos
        kernel vmlinuz-2.6.18-194.el5
        append initrd=initrd-2.6.18-194.el5.nfs.img  nfsroot=10.10.10.1:/srv/centos ip=dhcp rw

I can successfully boot-strap the system with no error.
But when i shutdown or reboot it . It will show in the screen :
Unmounting NFS filesystems
INIT:no more process left in this rulevel


and then ,it just hang.

i think it is because it umounted the nfsroot filesystem and can't do the things behind. but I really have no idea about how to resolve that.
Any help is granted. thanks.