ramdisk make :)

(0) prepare rootfilesystem eg.smallroot

(1) Make 24MB Image for ramdisk image 

   sudo dd if=/dev/zero of=./ramdisk bs=1024 count=24000

(2) Filesystem format for ramdisk image

   sudo mke2fs ./ramdisk

(3) Make mount point

  sudo mkdir aa

  sudo mount -o loop ramdisk ./aa

  sudo cp -a smallroot/* ./aa/

  sudo sync

  sudo gzip ramdisk

quite simple :)