What is the name of the Linux kernel's initial RAM filesystem?

What is the name of the Linux kernel’s initial RAM filesystem? The answer is `initramfs`.

An initramfs—short for “initial RAM filesystem”—is an archive loaded into memory during Linux boot and unpacked into an early temporary root filesystem. It gives the kernel a small user-space environment before the permanent root filesystem is available. Early-boot tools can then load drivers, discover storage, unlock encryption, assemble RAID or LVM volumes, and mount the real root filesystem.

The design became available with Linux kernel 2.6.13. Unlike the older initrd approach, which commonly used a filesystem image mounted through a virtual block device, initramfs is typically a cpio archive unpacked into `rootfs`, a special in-memory filesystem. The early userspace program—normally `/init`—coordinates the transition to the actual system.

`initrd` is a reasonable historical source of confusion: the terms are related, but they describe different implementation schemes. `tmpfs` is the memory-backed filesystem technology that may support the unpacked environment, not the name of the boot image itself. A “ramdisk” is a broader, older concept rather than the standard Linux term for this mechanism.