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99 lines
3.5 KiB
Text
99 lines
3.5 KiB
Text
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Slackware initrd mini HOWTO
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by Patrick Volkerding, volkerdi@slackware.com
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Mon Oct 24 18:47:20 UTC 2022
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This document describes how to create and install an initrd, which may be
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required to use the 4.x kernel. Also see "man mkinitrd".
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1. What is an initrd?
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2. Why to I need an initrd?
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3. How do I build the initrd?
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4. Now that I've built an initrd, how do I use it?
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1. What is an initrd?
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Initrd stands for "initial ramdisk". An initial ramdisk is a very small
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Linux filesystem that is loaded into RAM and mounted as the kernel boots,
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and before the main root filesystem is mounted.
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2. Why do I need an initrd?
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The usual reason to use an initrd is because you need to load kernel
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modules before mounting the root partition. Usually these modules are
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required to support the filesystem used by the root partition (ext3, ext4,
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btrfs, xfs), or perhaps the controller that the hard drive is attached
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to (SCSI, RAID, etc). Essentially, there are so many different options
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available in modern Linux kernels that it isn't practical to try to ship
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many different kernels to try to cover everyone's needs. It's a lot more
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flexible to ship a generic kernel and a set of kernel modules for it.
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3. How do I build the initrd?
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The easiest way to make the initrd is to use the mkinitrd script included
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in Slackware's mkinitrd package. We'll walk through the process of
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upgrading to the generic 5.19.17 Linux kernel using the packages
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found in Slackware's slackware/a/ directory.
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First, make sure the kernel, kernel modules, and mkinitrd package are
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installed (the current version numbers might be a little different, so
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this is just an example):
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installpkg kernel-generic-5.19.17-x86_64-1.txz
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installpkg kernel-modules-5.19.17-x86_64-1.txz
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installpkg mkinitrd-1.4.11-x86_64-30.txz
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Change into the /boot directory:
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cd /boot
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Now you'll want to run "mkinitrd". I'm using ext4 for my root filesystem,
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and since the disk controller requires no special support the ext4 module
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will be the only one I need to load:
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mkinitrd -c -k 5.19.17 -m ext4
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This should do two things. First, it will create a directory
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/boot/initrd-tree containing the initrd's filesystem. Then it will
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create an initrd (/boot/initrd.gz) from this tree. If you wanted to,
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you could make some additional changes in /boot/initrd-tree/ and
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then run mkinitrd again without options to rebuild the image. That's
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optional, though, and only advanced users will need to think about that.
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Here's another example: Build an initrd image using Linux 5.19.17
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kernel modules for a system with an ext4 root partition on /dev/sdb3:
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mkinitrd -c -k 5.19.17 -m ext4 -f ext4 -r /dev/sdb3
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4. Now that I've built an initrd, how do I use it?
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Now that you've got an initrd (/boot/initrd.gz), you'll want to load
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it along with the kernel at boot time. If you use LILO for your boot
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loader you'll need to edit /etc/lilo.conf and add a line to load the
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initrd. Here's an example section of lilo.conf showing how this is
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done:
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# Linux bootable partition config begins
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image = /boot/vmlinuz-generic
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initrd = /boot/initrd.gz
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root = /dev/sda6
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label = Slackware
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read-only
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# Linux bootable partition config ends
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The initrd is loaded by the "initrd = /boot/initrd.gz" line.
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Just add the line right below the line for the kernel image you use.
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Save the file, and then run LILO again ('lilo' at the command line).
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You'll need to run lilo every time you edit lilo.conf or rebuild the
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initrd.
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Other bootloaders such as syslinux also support the use of an initrd.
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See the documentation for those programs for details on using an
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initrd with them.
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---------
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Have fun!
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