Documentation/filesystems/ramfs-rootfs-initramfs.rst

1 .. SPDX-License-Identifier: GPL-2.0
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16 RAM-based filesystem.
32 The amount of code required to implement ramfs is tiny, because all the
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41 The older "ram disk" mechanism created a synthetic block device out of
42 an area of RAM and used it as backing store for a filesystem.  This block
44 size.  Using a ram disk also required unnecessarily copying memory from the
54 since all file access goes through the page and dentry caches.  The RAM
57 Another reason ramdisks are semi-obsolete is that the introduction of
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80 same reason you can't kill the init process; rather than having special code
87 If CONFIG_TMPFS is enabled, rootfs will use tmpfs instead of ramfs by
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100 archive is extracted into it, the kernel will fall through to the older code
106   - The old initrd was always a separate file, while the initramfs archive is
107     linked into the linux kernel image.  (The directory ``linux-*/usr`` is
110   - The old initrd file was a gzipped filesystem image (in some file format,
113     see cpio(1) and Documentation/driver-api/early-userspace/buffer-format.rst).
114     The kernel's cpio extraction code is not only extremely small, it's also
117   - The program run by the old initrd (which was called /initrd, not /init) did
123   - When switching another root device, initrd would pivot_root and then
126     free up the space (find -xdev / -exec rm '{}' ';'), overmount rootfs
127     with the new root (cd /newmount; mount --move . /; chroot .), attach
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166 a directory called "initramfs", under the linux-2.6.* directory.  See
167 Documentation/driver-api/early-userspace/early_userspace_support.rst for more details.)
174 from usr/gen_init_cpio.c).  The kernel's build-time cpio creation code is
175 entirely self-contained, and the kernel's boot-time extractor is also
176 (obviously) self-contained.
185   cpio -i -d -H newc -F initramfs_data.cpio --no-absolute-filenames
188 use in place of the above config file::
195   if [ $# -ne 2 ]
201   if [ -d "$1" ]
204     (cd "$1"; find . | cpio -o -H newc | gzip) > "$2"
214    of filenames is with the find command; you should give find the -depth
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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.
241 ----------------------
243 An initramfs archive is a complete self-contained root filesystem for Linux.
248 - https://www.tldp.org/HOWTO/Bootdisk-HOWTO/
249 - https://www.tldp.org/HOWTO/From-PowerUp-To-Bash-Prompt-HOWTO.html
250 - http://www.linuxfromscratch.org/lfs/view/stable/
254 code against, along with some related utilities.  It is BSD licensed.
256 I use uClibc (https://www.uclibc.org) and busybox (https://www.busybox.net)
257 myself.  These are LGPL and GPL, respectively.  (A self-contained initramfs
260 In theory you could use glibc, but that's not well suited for small embedded
279   gcc -static hello.c -o init
280   echo init | cpio -o -H newc | gzip > test.cpio.gz
282   qemu -kernel /boot/vmlinuz -initrd test.cpio.gz /dev/zero
289 -------------------------
318    explained in buffer-format.txt, created in usr/gen_init_cpio.c, and
320    total of human-readable text.
338      - http://www.uwsg.iu.edu/hypermail/linux/kernel/0112.2/1550.html
339      - http://www.uwsg.iu.edu/hypermail/linux/kernel/0112.2/1638.html
341    and, most importantly, designed and implemented the initramfs code.
344 ------------------
347 kernel falls back to legacy boot code that is reached only if initramfs does
348 not contain an /init program.  The fallback is legacy code, there to ensure a
358 compressed, encrypted, copy-on-write, loopback mounted, strangely partitioned,
365 The klibc package has now been accepted into Andrew Morton's 2.6.17-mm tree.
366 The kernel's current early boot code (partition detection, etc) will probably