Slackware initrd mini HOWTO
by Patrick Volkerding, volkerdi@slackware.com
Sun Mar  7 20:22:02 UTC 2021

This document describes how to create and install an initrd, which may be 
required to use the 4.x kernel. Also see "man mkinitrd".

1. What is an initrd?
2. Why to I need an initrd?
3. How do I build the initrd?
4. Now that I've built an initrd, how do I use it?


1. What is an initrd?

Initrd stands for "initial ramdisk". An initial ramdisk is a very small 
Linux filesystem that is loaded into RAM and mounted as the kernel boots, 
and before the main root filesystem is mounted.

2. Why do I need an initrd?

The usual reason to use an initrd is because you need to load kernel 
modules before mounting the root partition. Usually these modules are 
required to support the filesystem used by the root partition (ext3, ext4,
btrfs, xfs), or perhaps the controller that the hard drive is attached 
to (SCSI, RAID, etc). Essentially, there are so many different options 
available in modern Linux kernels that it isn't practical to try to ship 
many different kernels to try to cover everyone's needs. It's a lot more 
flexible to ship a generic kernel and a set of kernel modules for it.

3. How do I build the initrd?

The easiest way to make the initrd is to use the mkinitrd script included 
in Slackware's mkinitrd package. We'll walk through the process of 
upgrading to the generic 5.10.21 Linux kernel using the packages
found in Slackware's slackware/a/ directory.

First, make sure the kernel, kernel modules, and mkinitrd package are 
installed (the current version numbers might be a little different, so 
this is just an example):

  installpkg kernel-generic-5.10.21-x86_64-1.txz
  installpkg kernel-modules-5.10.21-x86_64-1.txz
  installpkg mkinitrd-1.4.11-x86_64-19.txz

Change into the /boot directory:

  cd /boot

Now you'll want to run "mkinitrd". I'm using ext4 for my root filesystem,
and since the disk controller requires no special support the ext4 module
will be the only one I need to load:

  mkinitrd -c -k 5.10.21 -m ext4

This should do two things. First, it will create a directory
/boot/initrd-tree containing the initrd's filesystem. Then it will
create an initrd (/boot/initrd.gz) from this tree. If you wanted to,
you could make some additional changes in /boot/initrd-tree/ and
then run mkinitrd again without options to rebuild the image. That's
optional, though, and only advanced users will need to think about that.

Here's another example: Build an initrd image using Linux 5.10.21
kernel modules for a system with an ext4 root partition on /dev/sdb3:

  mkinitrd -c -k 5.10.21 -m ext4 -f ext4 -r /dev/sdb3


4. Now that I've built an initrd, how do I use it?

Now that you've got an initrd (/boot/initrd.gz), you'll want to load
it along with the kernel at boot time. If you use LILO for your boot
loader you'll need to edit /etc/lilo.conf and add a line to load the
initrd. Here's an example section of lilo.conf showing how this is
done:

# Linux bootable partition config begins
image = /boot/vmlinuz-generic
  initrd = /boot/initrd.gz
  root = /dev/sda6
  label = Slackware
  read-only
# Linux bootable partition config ends

The initrd is loaded by the "initrd = /boot/initrd.gz" line.
Just add the line right below the line for the kernel image you use.
Save the file, and then run LILO again ('lilo' at the command line).
You'll need to run lilo every time you edit lilo.conf or rebuild the
initrd.

Other bootloaders such as syslinux also support the use of an initrd.
See the documentation for those programs for details on using an
initrd with them.


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Have fun!