initrd

Initrd stands for initial RAM disk, a temporary filesystem loaded into RAM during the early stages of the Linux boot process. It provides a minimal environment with kernel modules, drivers, and utilities necessary to initialize hardware and locate and mount the real root filesystem.

During boot, the bootloader loads the kernel and an initrd image. The kernel mounts the initrd as its initial root and executes its /init program. The init script loads essential drivers (such as storage controllers), scans for storage devices, and may assemble software RAID or LVM, as well as decrypt encrypted volumes if present. Once the real root filesystem is located and mounted, the init process uses switch_root or a similar mechanism to replace the initrd’s root with the real root, after which the real system’s init process begins.

Initrd vs initramfs: Historically, initrd referred to a block-based RAM disk image. Modern Linux distributions typically use initramfs, a compressed cpio archive that is unpacked into a ram filesystem and mounted as root. Both serve to provide an early userspace environment, and boot entries often still refer to initrd even when the actual artifact is an initramfs image.

Management and usage: Initrd/initramfs images are created by tooling such as dracut (used by Fedora and others) or mkinitramfs/update-initramfs (used by Debian and derivatives). They are integrated into the boot process by the bootloader (for example, GRUB loads the kernel image and the accompanying initramfs). If mounting the real root fails, the initramfs environment can provide a rescue shell to diagnose and repair boot problems.