Working with “initramfs” and “initrd” in Linux

In Linux, the initrd (initial RAM disk) and initramfs (initial RAM File System) are different methods we can use to load a temporary root file system to the RAM or system memory for successful booting. Initramfs/initrd is used as the first root filesystem that your machine has access to.

In many situations , you will find yourself needing to:

• Check if a driver or a module exists in the initramfs/initrd or not

• Add a new driver/module to an initramfs/initrd file

• Remove a driver/module from an initramfs/initrd file

• …

I — File name :

The file name changes from a Linux distribution to another and from a version to another, and for the new Linux distribution versions you will generally find the file with these names under the “/boot” directory

• RHEL/CentOS/Fedora/Ubuntu/Debian ..

initramfs-<kernel-version>.img

• SUSE SLES

initrd-<kernel-version>

Note /!\ : SUSE SLES is also using initramfs , however it is stored as /boot/initrd. More details can be found in this link.

For the current kernel version in your server, the file name can be getting using the following expression :

initramfs-$(uname -r).img

initrd-$(uname -r)

The command uname -r is used to get the current kernel name.

Examples :

• RHEL machine :

Example : initramfs-4.18.0–372.9.1.el8.x86_64.img

[root@ip-172-31-39-129 ~]#  ls -la /boot/
total 171228
dr-xr-xr-x.  5 root root     4096 Jun 26 09:32 .
dr-xr-xr-x. 17 root root      224 May  3  2022 ..
-rw-r--r--.  1 root root   195982 Apr 16  2022 config-4.18.0-372.9.1.el8.x86_64
drwxr-xr-x.  3 root root       17 May  3  2022 efi
drwx------.  4 root root       65 May  3  2022 grub2
-rw-------.  1 root root 62262494 May  3  2022 initramfs-0-rescue-ffffffffffffffffffffffffffffffff.img
-rw-------.  1 root root 61798085 May  3  2022 initramfs-4.18.0-372.9.1.el8.x86_64.img
-rw-------.  1 root root 25783296 Jun 26 09:32 initramfs-4.18.0-372.9.1.el8.x86_64kdump.img
drwxr-xr-x.  3 root root       21 May  3  2022 loader
lrwxrwxrwx.  1 root root       49 May  3  2022 symvers-4.18.0-372.9.1.el8.x86_64.gz -> /lib/modules/4.18.0-372.9.1.el8.x86_64/symvers.gz
-rw-------.  1 root root  4359450 Apr 16  2022 System.map-4.18.0-372.9.1.el8.x86_64
-rwxr-xr-x.  1 root root 10460528 May  3  2022 vmlinuz-0-rescue-ffffffffffffffffffffffffffffffff
-rwxr-xr-x.  1 root root 10460528 Apr 16  2022 vmlinuz-4.18.0-372.9.1.el8.x86_64
-rw-r--r--.  1 root root      170 Apr 16  2022 .vmlinuz-4.18.0-372.9.1.el8.x86_64.hmac

[root@ip-172-31-39-129 ~]# uname -r
4.18.0-372.9.1.el8.x86_64

• SUSE machine :

Example : initrd-4.4.180–94.153-default

ip-172-31-29-209:~ # ls -la /boot
total 36320
drwxr-xr-x  3 root root     4096 Jun 26 09:27 .
drwxr-xr-x 23 root root     4096 Jun 26 09:27 ..
-rw-r--r--  1 root root       65 Feb  1  2022 .vmlinuz-4.4.180-94.153-default.hmac
-rw-r--r--  1 root root        0 Mar  3  2022 0x40e2938e
-rw-r--r--  1 root root  3280251 Feb  1  2022 System.map-4.4.180-94.153-default
lrwxrwxrwx  1 root root        1 Jun 26 09:25 boot -> .
-rw-r--r--  1 root root     1725 Nov 15  2018 boot.readme
-rw-r--r--  1 root root   163622 Feb  1  2022 config-4.4.180-94.153-default
drwxr-xr-x  6 root root     4096 Jun 26 09:27 grub2
lrwxrwxrwx  1 root root       29 Mar  3  2022 initrd -> initrd-4.4.180-94.153-default
-rw-------  1 root root  8752104 Jun 26 09:26 initrd-4.4.180-94.141-default
-rw-------  1 root root 10004276 Jun 26 09:26 initrd-4.4.180-94.153-default
-rw-r--r--  1 root root       11 Jun 26 09:27 mbrid
-rw-r--r--  1 root root   323914 Feb  1  2022 symvers-4.4.180-94.153-default.gz
-rw-r--r--  1 root root      377 Feb  1  2022 sysctl.conf-4.4.180-94.153-default
-rw-r--r--  1 root root  1484762 Mar  3  2022 unicode.pf2
-rw-r--r--  1 root root  7025166 Feb  1  2022 vmlinux-4.4.180-94.153-default.gz
lrwxrwxrwx  1 root root       30 Mar  3  2022 vmlinuz -> vmlinuz-4.4.180-94.153-default
-rw-r--r--  1 root root  6115040 Feb  1  2022 vmlinuz-4.4.180-94.153-default
ip-172-31-29-209:~ # 
ip-172-31-29-209:~ # uname -r
4.4.180-94.153-default

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II — List the content of the initramfs/initrd file

— In order to check the content of the initramfs/initrd file , use the command below (this is used for both initramfs and initrd):

# lsinitrd

This command will give you the content of the initramfs/initrd of the current loaded kernel (means the one given by the uname -r command)

— Because the server can have multiple initramfs/initrd files (for each kernel version installed there is a file created for it), and in order to check the content of a specific file, use the command as below :

# lsinitrd  /path/<file-name>

— In order to check if a driver/module exists in the file, use *egrep* :

# lsinitrd  /path/<file-name> | egrep -i "<module-name>"

Let’s take an example :

For EC2 Nitro instances , they need “ena”, “nvme” and “nvme-core” drivers in order to boot correctly. And if the OS is using LVMs (Logical Volumes), then the initramfs/initrd should have the “lvm” module also added . So let’s check them for the following initramfs file :

# lsinitrd  /boot/initramfs-4.18.0-372.9.1.el8.x86_64.img | egrep -i "ena|nvme|lvm"

→ Here we can see that “ena”, “nvme” and “nvme-core” drivers exist in the initramfs file , however the “lvm” module doesn’t exist.

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III — Add a driver/module to an initramfs/initrd file :

Let’s try for example here to add the “lvm” module to the previous initramfs file :

• Let’s first see if the LVM is installed :

[root@ip-172-31-39-129 ~]# rpm -aq | grep lvm
[root@ip-172-31-39-129 ~]#

• The LVM2 package is not installed , let’s install it :

[root@ip-172-31-39-129 ~]# yum install lvm2 -y
[root@ip-172-31-39-129 ~]#
[root@ip-172-31-39-129 ~]# rpm -aq | grep lvm 
lvm2-libs-2.03.14-9.el8.x86_64
lvm2-2.03.14-9.el8.x86_64

The lvm2 package is now installed

• Let’s see if it was added automatically to the initramfs file or not:

# lsinitrd /boot/initramfs-4.18.0-372.9.1.el8.x86_64.img | egrep -i "lvm"

The module will not be added to the initramfs file until we add it manually.

• Okay , let’s now add the lvm module to our initramfs file

1. Create a file under the /etc/dracut.conf.d/ directory and give it a name with .confas suffix

# echo 'add_dracutmodules+=" lvm "'> /etc/dracut.conf.d/lvm.conf

Note /!\ : The module name should be surrounded with white spaces.

2. Re-generate the initramfs for the current kernel (we will see with more details how to regenerate files in the next section):

[root@ip-172-31-39-129 ~]# dracut -f

3. Let’s check again the presence of the lvm module :

# lsinitrd /boot/initramfs-4.18.0-372.9.1.el8.x86_64.img | egrep -i "lvm"

→ The lvm module is now present in our initramfs file :)

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IV — Re-generate the initramfs/initrd file:

Here we will see how to re-gerenerate the initramfs file for the different cases

Note /!  : make sure to backup the initramfs/initrd file before generating the new one if you are not sure about what you are doing!!!

• Create the initramfs for the current kernel:

# dracut -f
or
# dracut initramfs-$(uname -r).img $(uname -r)

• If you need to re-generate it for a specific kernel version (replace the version appropriately):

# dracut -f /boot/initramfs-4.18.0-372.9.1.el8.x86_64.img 4.18.0-372.9.1.el8.x86_64

• If you need to re-generate all initramfs files , use the command :

# dracut -v --force --regenerate-all

Note : the "-f"or “--force” is for overriding the initramfs files

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V — How to remove a module from an initramfs/initrd file :

I’m going to give two options that can be used in order to remove a module from an initramfs/initrd file :

(In this example we will remove the lvm module that we added previously from the initramfs file )

** Option 1 ** : Use the “--omit” option during the regeneration of the initramfs file :

1. Let’s first here moving the initramfs file :

# cd /boot/
# mv initramfs-4.18.0-372.9.1.el8.x86_64.img  initramfs-4.18.0-372.9.1.el8.x86_64.img.bkp

2. Regenerate the initramfs file with the --omit option

# dracut -f /boot/initramfs-4.18.0-372.9.1.el8.x86_64.img 4.18.0-372.9.1.el8.x86_64 --omit lvm

3. Let’s check if the lvm module exists in the new initramfs file :

# lsinitrd  /boot/initramfs-4.18.0-372.9.1.el8.x86_64.img | grep lvm
Arguments: -f --omit 'lvm'
#

→ The module was removed correctly from the initramfs file

** Option 2 ** : Use of the parameter “omit_dracutmodules+=” under the directory /etc/dracut.conf.d/ :

1. Create a config file under the directory /etc/dracut.conf.d/ containing the following :

# echo 'omit_dracutmodules+=" lvm "'> /etc/dracut.conf.d/omit_lvm.conf

2. Regenerate the initramfs file :

# dracut -f

3. Let’s try to check if the lvm module exists in the new initramfs file

# lsinitrd | grep lvm

→ The module was removed correctly :)

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