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

[Mircea Husz]

I have already tried, without --format=lzma  it fails to boot.
-Mike
On Wed, 2018-01-31 at 17:10 +0000, Nux! wrote:
> Don't archive as lzma if you don't want lzma, remove the "
> --format=lzma" parameter.
> 
> hth
> Lucian
> 
> --
> Sent from the Delta quadrant using Borg technology!
> 
> Nux!
> www.nux.ro
> 
> ----- Original Message -----
> > From: "Mircea Husz" <mirceahusz at yahoo.com>
> > To: "CentOS mailing list" <centos at centos.org>
> > Sent: Wednesday, 31 January, 2018 16:51:20
> > Subject: [CentOS] How is initrd.img packed and compressed?
> > Hi,
> > 
> > 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/
> > centos/7.4.1708/os/x86_64/isolinux/initrd.img
> > 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
> > follows:
> > 
> > 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 ?
> > 
> > Thanks,
> > -Mike
> > 
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> 
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