extract centos7/rhel7 initramfs

# /usr/lib/dracut/skipcpio /boot/initramfs-3.10.0-327.18.2.el7.x86_64.img | gunzip -c | cpio -dcim
# /usr/lib/dracut/skipcpio /boot/initramfs-3.10.0-327.18.2.el7.x86_64.img | zcat | cpio -idmvc
extract initrd.img resided in isolinux folder.
xz -dc initrd.img|cpio -idmv

http://www.constantstream.com/2016/06/16/extract-initramfs-on-rhelcentos-7/

repack:
xz -z -9 -C crc32 -F xz 1
-C for --check
-F for --format

find . |cpio -c -o |xz -z -9 -C crc32 -F xz > /dev/shm/test.img