mkinitrd Command Linux: Complete Guide to Creating Initial RAM Disks

The mkinitrd command is a crucial Linux utility that creates initial RAM disk (initrd) images, which are essential for the boot process in modern Linux systems. This temporary file system loads into memory during boot time, containing necessary drivers and modules required to mount the root filesystem.

What is mkinitrd Command?

The mkinitrd (make initial RAM disk) command generates a compressed filesystem image that the kernel loads into memory during the boot process. This initrd image contains essential drivers, modules, and utilities needed to access the root filesystem before the actual root partition is mounted.

Key Features of mkinitrd:

• Creates bootable RAM disk images
• Includes necessary kernel modules and drivers
• Supports various filesystem types
• Handles device detection and initialization
• Provides emergency boot capabilities

mkinitrd Command Syntax

The basic syntax of the mkinitrd command follows this structure:

mkinitrd [OPTIONS] <initrd_image> <kernel_version>

Common Parameters:

• initrd_image: Path and filename for the output initrd image
• kernel_version: Specific kernel version to build initrd for

Essential mkinitrd Command Options

Basic mkinitrd Command Examples

Example 1: Creating a Basic initrd Image

sudo mkinitrd /boot/initrd-$(uname -r).img $(uname -r)

Output:

Building initrd for kernel version 5.15.0-78-generic
Adding modules: ext4 ahci libahci libata scsi_mod sd_mod
Creating initrd image: /boot/initrd-5.15.0-78-generic.img
Image created successfully (Size: 23.4 MB)

Example 2: Force Overwrite Existing initrd

sudo mkinitrd -f /boot/initrd-custom.img $(uname -r)

Output:

Overwriting existing initrd image: /boot/initrd-custom.img
Building initrd for kernel version 5.15.0-78-generic
Image creation completed

Example 3: Verbose Mode Creation

sudo mkinitrd -v /boot/initrd-verbose.img $(uname -r)

Output:

mkinitrd: version 6.0.3
Using kernel version: 5.15.0-78-generic
Analyzing kernel modules...
Adding module: ext4 (filesystem support)
Adding module: ahci (SATA controller)
Adding module: libahci (AHCI library)
Adding module: usb-storage (USB mass storage)
Creating filesystem structure...
Compressing initrd image...
Final image size: 24.1 MB

Advanced mkinitrd Usage Examples

Example 4: Including Specific Modules

sudo mkinitrd --with=nvidia --with=ext4 /boot/initrd-nvidia.img $(uname -r)

This command creates an initrd image that specifically includes the NVIDIA and ext4 modules.

Example 5: Preloading Network Modules

sudo mkinitrd --preload=e1000e --preload=r8169 /boot/initrd-network.img $(uname -r)

Output:

Preloading modules: e1000e r8169
Building initrd with network support
Creating image: /boot/initrd-network.img
Network modules loaded successfully

Example 6: Omitting SCSI Modules

sudo mkinitrd --omit-scsi-modules /boot/initrd-no-scsi.img $(uname -r)

This creates a smaller initrd image by excluding SCSI-related modules, useful for systems without SCSI devices.

Working with Different Kernel Versions

Example 7: Creating initrd for Specific Kernel

sudo mkinitrd /boot/initrd-5.10.0.img 5.10.0-generic

Example 8: Listing Available Kernels

ls /lib/modules/

Output:

5.10.0-generic
5.15.0-78-generic
5.19.0-42-generic

Troubleshooting mkinitrd Command

Common Error Solutions

Error: “Module not found”

# Check available modules
find /lib/modules/$(uname -r) -name "*.ko" | grep module_name

# Verify module dependencies
modinfo module_name

Error: “No space left on device”

# Check available space in /boot
df -h /boot

# Clean old initrd images
sudo rm /boot/initrd-old-version.img

Example 9: Checking initrd Contents

# Extract and examine initrd contents
mkdir /tmp/initrd_extract
cd /tmp/initrd_extract
zcat /boot/initrd-$(uname -r).img | cpio -i -d

Output:

bin/
dev/
etc/
lib/
proc/
sbin/
sys/
tmp/
var/
init
2847 blocks

Modern Alternatives to mkinitrd

While mkinitrd is still used in some distributions, many modern Linux systems have adopted alternative tools:

dracut Command (RHEL/CentOS/Fedora)

sudo dracut /boot/initramfs-$(uname -r).img $(uname -r)

update-initramfs (Debian/Ubuntu)

sudo update-initramfs -c -k $(uname -r)

Best Practices for mkinitrd Usage

1. Backup Existing initrd Images

sudo cp /boot/initrd-$(uname -r).img /boot/initrd-$(uname -r).img.backup

2. Test New initrd Images

Always test new initrd images in a safe environment before deploying to production systems.

3. Include Essential Modules

Ensure all necessary drivers for your root filesystem and storage devices are included.

4. Regular Maintenance

# Remove old initrd images to save space
sudo find /boot -name "initrd-*" -mtime +30 -delete

Integration with Boot Loaders

GRUB Configuration

After creating a new initrd image, update your GRUB configuration:

sudo update-grub

Or manually edit /boot/grub/grub.cfg to include the new initrd:

menuentry 'Custom Kernel' {
    linux /boot/vmlinuz-custom root=/dev/sda1
    initrd /boot/initrd-custom.img
}

Performance Considerations

Optimizing initrd Size

• Include only necessary modules
• Use compression options effectively
• Remove unused binaries and libraries
• Consider module loading order

Example 10: Creating Minimal initrd

sudo mkinitrd --omit-scsi-modules --omit-raid-modules \
  --builtin=ext4 /boot/initrd-minimal.img $(uname -r)

Security Considerations

When using mkinitrd, consider these security aspects:

• Protect initrd images from unauthorized modification
• Use secure boot when available
• Verify module signatures
• Regular security updates for included modules

Conclusion

The mkinitrd command remains an essential tool for Linux system administrators who need fine-grained control over the initial boot process. While newer alternatives like dracut and update-initramfs are becoming more common, understanding mkinitrd is valuable for working with legacy systems and situations requiring custom initrd creation.

By mastering the various options and techniques demonstrated in this guide, you can create optimized initrd images tailored to your specific system requirements, troubleshoot boot issues effectively, and maintain robust Linux systems.