Documentation/filesystems/ramfs-rootfs-initramfs.rst

1 .. SPDX-License-Identifier: GPL-2.0
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15 mechanisms (the page cache and dentry cache) as a dynamically resizable
16 RAM-based filesystem.
28 dentries and page cache as usual, but there's nowhere to write them to.
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45 fake block device into the page cache (and copying changes back out), as well
51 to avoid this copying by playing with the page tables, but they're unpleasantly
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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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).
115     __init text and data that can be discarded during the boot process.
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
195   if [ $# -ne 2 ]
201   if [ -d "$1" ]
204     (cd "$1"; find . | cpio -o -H newc | gzip) > "$2"
212    The cpio man page contains some bad advice that will break your initramfs
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.
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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/
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"
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
284 When debugging a normal root filesystem, it's nice to be able to boot with
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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
344 ------------------
347 kernel falls back to legacy boot code that is reached only if initramfs does
349 smooth transition and allowing early boot functionality to gradually move to
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