initramfs

initramfs is an initial RAM filesystem used by the Linux kernel during the boot process. It is loaded into memory by the bootloader and mounted as a temporary root filesystem. It provides a minimal, early userspace environment in which the system can initialize hardware and locate and mount the real root filesystem. The initramfs is typically a cpio archive compressed with gzip, lzma, or xz, and unpacked into a temporary filesystem in memory.

At boot, the kernel starts with the initramfs as its root, and the first user-space process is /init inside the initramfs. That script loads necessary drivers, detects the root device (disk, LVM, or encrypted partition), may mount other filesystems, and then uses switch_root or pivot_root to switch to the real root filesystem, starting the real init program from the root.

Historically, initramfs evolved from the older initrd approach. Modern Linux distributions rely on an initramfs image that is generated by tooling and loaded by the bootloader. Common tools include dracut, mkinitcpio, and update-initramfs.

The content of an initramfs typically includes a minimal BusyBox set of utilities, a kernel module loader, and scripts to assemble and mount the real root filesystem. It may include drivers and userspace components for handling encryption, LVM, RAID, and various filesystems to ensure root can be mounted regardless of hardware configuration.

Management and size: The initramfs is compressed to reduce boot times and memory usage, and is generated per-kernel and per-boot configuration. It is loaded by the bootloader and can be signed for integrity. Maintenance involves rebuilding it after changes to the kernel, modules, or root configuration.