Documentation/filesystems/ramfs-rootfs-initramfs.rst

4 Ramfs, rootfs and initramfs
91 What is initramfs?
106   - The old initrd was always a separate file, while the initramfs archive is
112     initramfs archive is a gzipped cpio archive (like tar only simpler,
119     initramfs is not expected to return to the kernel.  (If /init needs to hand
124     umount the ramdisk.  But initramfs is rootfs: you can neither pivot_root
135 Populating initramfs:
138 The 2.6 kernel build process always creates a gzipped cpio format initramfs
144 initramfs archive, which will automatically be incorporated into the
154   file /bin/busybox initramfs/busybox 755 0 0
158   file /init initramfs/init.sh 755 0 0
166 a directory called "initramfs", under the linux-2.6.* directory.  See
212    The cpio man page contains some bad advice that will break your initramfs
217    initramfs.cpio.gz images, it won't work.  The Linux kernel cpio extractor
222 External initramfs images:
227 will autodetect the type (initramfs, not initrd) and extract the external cpio
230 This has the memory efficiency advantages of initramfs (no ramdisk block
232 non-GPL code you'd like to run from initramfs, without conflating it with
235 It can also be used to supplement the kernel's built-in initramfs image.  The
237 the built-in initramfs archive.  Some distributors also prefer to customize
238 a single kernel image with task-specific initramfs images, without recompiling.
240 Contents of initramfs:
243 An initramfs archive is a complete self-contained root filesystem for Linux.
257 myself.  These are LGPL and GPL, respectively.  (A self-contained initramfs
265 A good first step is to get initramfs to run a statically linked "hello world"
281   # Testing external initramfs using the initrd loading mechanism.
285 "init=/bin/sh".  The initramfs equivalent is "rdinit=/bin/sh", and it's
317    various tar archive formats.  The complete initramfs archive format is
319    extracted in init/initramfs.c.  All three together come to less than 26k
341    and, most importantly, designed and implemented the initramfs code.
346 Today (2.6.16), initramfs is always compiled in, but not always used.  The
347 kernel falls back to legacy boot code that is reached only if initramfs does
350 "early userspace" (I.E. initramfs).
362 in userspace.  Both klibc and busybox/uClibc are working on simple initramfs
367 be migrated into a default initramfs, automatically created and used by the