[CentOS] How is initrd.img packed and compressed?

Wed Jan 31 16:51:20 UTC 2018
Mircea Husz <mirceahusz at yahoo.com>


In order to work around a known upstream bug I needed to add a udev
rule to pxeboot initrd.img on CentOS 7.

The process is straightforward:
1 - extract the pxeboot initrd.img  to a new directory
2 - add the udev rule needed to fix the bug
3 - pack and compress it back in initrd.img format

The resulting updated image works, it fixes the upstream bug and life
is good. But although it works, it's not 
 the same format as the original initrd.img that ships with the distro.
I would like to know the proper incantation 
used to package initrd.img

Now for the specifics. The original image: http://mirror.steadfast.net/
is extracted: /usr/lib/dracut/skipcpio  initrd.img | xzcat | cpio -i -d
and after adding the needed udev rule, it gets packed and compressed as

find . 2>/dev/null | cpio --quiet -c -o | xz -9 
--format=lzma >"~/patched-initrd.img"

Now for the difference. FIrst the original distro image:
# file initrd.img
initrd.img: xz compressed data

# file patched-initrd.img
patched-initrd.img: LZMA compressed data, streamed
Without the --format=lzma  flag it fails to boot. 

Does anyone know how this is done properly ?