Чем заархивирован initrd?

$ file /boot/initrd.img-3.13.0-24-generic 
/boot/initrd.img-3.13.0-24-generic: gzip compressed data, from Unix, last modified: Mon Sep 15 12:51:59 2014

# file system-2.8.img
system-2.8.img: data

# file initrd
initrd: data