€•ñÖ      Œsphinx.addnodes”Œdocument”“”)”}”(Œ	rawsource”Œ ”Œchildren”]”(Œtranslations”ŒLanguagesNode”“”)”}”(hhh]”(h Œpending_xref”“”)”}”(hhh]”Œdocutils.nodes”ŒText”“”ŒChinese (Simplified)”…””}”Œparent”hsbaŒ
attributes”}”(Œids”]”Œclasses”]”Œnames”]”Œdupnames”]”Œbackrefs”]”Œ	refdomain”Œstd”Œreftype”Œdoc”Œ	reftarget”Œ6/translations/zh_CN/filesystems/ramfs-rootfs-initramfs”Œmodname”NŒ	classname”NŒrefexplicit”ˆuŒtagname”hhhubh)”}”(hhh]”hŒChinese (Traditional)”…””}”hh2sbah}”(h]”h ]”h"]”h$]”h&]”Œ	refdomain”h)Œreftype”h+Œ	reftarget”Œ6/translations/zh_TW/filesystems/ramfs-rootfs-initramfs”Œmodname”NŒ	classname”NŒrefexplicit”ˆuh1hhhubh)”}”(hhh]”hŒItalian”…””}”hhFsbah}”(h]”h ]”h"]”h$]”h&]”Œ	refdomain”h)Œreftype”h+Œ	reftarget”Œ6/translations/it_IT/filesystems/ramfs-rootfs-initramfs”Œmodname”NŒ	classname”NŒrefexplicit”ˆuh1hhhubh)”}”(hhh]”hŒJapanese”…””}”hhZsbah}”(h]”h ]”h"]”h$]”h&]”Œ	refdomain”h)Œreftype”h+Œ	reftarget”Œ6/translations/ja_JP/filesystems/ramfs-rootfs-initramfs”Œmodname”NŒ	classname”NŒrefexplicit”ˆuh1hhhubh)”}”(hhh]”hŒKorean”…””}”hhnsbah}”(h]”h ]”h"]”h$]”h&]”Œ	refdomain”h)Œreftype”h+Œ	reftarget”Œ6/translations/ko_KR/filesystems/ramfs-rootfs-initramfs”Œmodname”NŒ	classname”NŒrefexplicit”ˆuh1hhhubh)”}”(hhh]”hŒSpanish”…””}”hh‚sbah}”(h]”h ]”h"]”h$]”h&]”Œ	refdomain”h)Œreftype”h+Œ	reftarget”Œ6/translations/sp_SP/filesystems/ramfs-rootfs-initramfs”Œmodname”NŒ	classname”NŒrefexplicit”ˆuh1hhhubeh}”(h]”h ]”h"]”h$]”h&]”Œcurrent_language”ŒEnglish”uh1h
hhŒ	_document”hŒsource”NŒline”NubhŒcomment”“”)”}”(hŒ SPDX-License-Identifier: GPL-2.0”h]”hŒ SPDX-License-Identifier: GPL-2.0”…””}”hh£sbah}”(h]”h ]”h"]”h$]”h&]”Œ	xml:space”Œpreserve”uh1h¡hhhžhhŸŒP/var/lib/git/docbuild/linux/Documentation/filesystems/ramfs-rootfs-initramfs.rst”h KubhŒsection”“”)”}”(hhh]”(hŒtitle”“”)”}”(hŒRamfs, rootfs and initramfs”h]”hŒRamfs, rootfs and initramfs”…””}”(hh»hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h¹hh¶hžhhŸh³h KubhŒ	paragraph”“”)”}”(hŒOctober 17, 2005”h]”hŒOctober 17, 2005”…””}”(hhËhžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhŸh³h Khh¶hžhubhŒ
field_list”“”)”}”(hhh]”hŒfield”“”)”}”(hhh]”(hŒ
field_name”“”)”}”(hŒAuthor”h]”hŒAuthor”…””}”(hhåhžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hãhhàhŸh³h K ubhŒ
field_body”“”)”}”(hŒRob Landley <rob@landley.net>
”h]”hÊ)”}”(hŒRob Landley <rob@landley.net>”h]”(hŒRob Landley <”…””}”(hhùhžhhŸNh NubhŒ	reference”“”)”}”(hŒrob@landley.net”h]”hŒrob@landley.net”…””}”(hj  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”Œrefuri”Œmailto:rob@landley.net”uh1j  hhùubhŒ>”…””}”(hhùhžhhŸNh Nubeh}”(h]”h ]”h"]”h$]”h&]”uh1hÉhŸh³h K	hhõubah}”(h]”h ]”h"]”h$]”h&]”uh1hóhhàubeh}”(h]”h ]”h"]”h$]”h&]”uh1hÞhŸh³h K	hhÛhžhubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙhh¶hžhhŸh³h K	ubhµ)”}”(hhh]”(hº)”}”(hŒWhat is ramfs?”h]”hŒWhat is ramfs?”…””}”(hj2  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h¹hj/  hžhhŸh³h KubhÊ)”}”(hŒ¡Ramfs is a very simple filesystem that exports Linux's disk caching
mechanisms (the page cache and dentry cache) as a dynamically resizable
RAM-based filesystem.”h]”hŒ£Ramfs is a very simple filesystem that exports Linuxâ€™s disk caching
mechanisms (the page cache and dentry cache) as a dynamically resizable
RAM-based filesystem.”…””}”(hj@  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhŸh³h Khj/  hžhubhÊ)”}”(hX  Normally all files are cached in memory by Linux.  Pages of data read from
backing store (usually the block device the filesystem is mounted on) are kept
around in case it's needed again, but marked as clean (freeable) in case the
Virtual Memory system needs the memory for something else.  Similarly, data
written to files is marked clean as soon as it has been written to backing
store, but kept around for caching purposes until the VM reallocates the
memory.  A similar mechanism (the dentry cache) greatly speeds up access to
directories.”h]”hX!  Normally all files are cached in memory by Linux.  Pages of data read from
backing store (usually the block device the filesystem is mounted on) are kept
around in case itâ€™s needed again, but marked as clean (freeable) in case the
Virtual Memory system needs the memory for something else.  Similarly, data
written to files is marked clean as soon as it has been written to backing
store, but kept around for caching purposes until the VM reallocates the
memory.  A similar mechanism (the dentry cache) greatly speeds up access to
directories.”…””}”(hjN  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhŸh³h Khj/  hžhubhÊ)”}”(hX  With ramfs, there is no backing store.  Files written into ramfs allocate
dentries and page cache as usual, but there's nowhere to write them to.
This means the pages are never marked clean, so they can't be freed by the
VM when it's looking to recycle memory.”h]”hX
  With ramfs, there is no backing store.  Files written into ramfs allocate
dentries and page cache as usual, but thereâ€™s nowhere to write them to.
This means the pages are never marked clean, so they canâ€™t be freed by the
VM when itâ€™s looking to recycle memory.”…””}”(hj\  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhŸh³h Khj/  hžhubhÊ)”}”(hX<  The amount of code required to implement ramfs is tiny, because all the
work is done by the existing Linux caching infrastructure.  Basically,
you're mounting the disk cache as a filesystem.  Because of this, ramfs is not
an optional component removable via menuconfig, since there would be negligible
space savings.”h]”hX>  The amount of code required to implement ramfs is tiny, because all the
work is done by the existing Linux caching infrastructure.  Basically,
youâ€™re mounting the disk cache as a filesystem.  Because of this, ramfs is not
an optional component removable via menuconfig, since there would be negligible
space savings.”…””}”(hjj  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhŸh³h K hj/  hžhubeh}”(h]”Œwhat-is-ramfs”ah ]”h"]”Œwhat is ramfs?”ah$]”h&]”uh1h´hh¶hžhhŸh³h Kubhµ)”}”(hhh]”(hº)”}”(hŒramfs and ramdisk:”h]”hŒramfs and ramdisk:”…””}”(hjƒ  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h¹hj€  hžhhŸh³h K'ubhÊ)”}”(hXì  The older "ram disk" mechanism created a synthetic block device out of
an area of RAM and used it as backing store for a filesystem.  This block
device was of fixed size, so the filesystem mounted on it was of fixed
size.  Using a ram disk also required unnecessarily copying memory from the
fake block device into the page cache (and copying changes back out), as well
as creating and destroying dentries.  Plus it needed a filesystem driver
(such as ext2) to format and interpret this data.”h]”hXð  The older â€œram diskâ€ mechanism created a synthetic block device out of
an area of RAM and used it as backing store for a filesystem.  This block
device was of fixed size, so the filesystem mounted on it was of fixed
size.  Using a ram disk also required unnecessarily copying memory from the
fake block device into the page cache (and copying changes back out), as well
as creating and destroying dentries.  Plus it needed a filesystem driver
(such as ext2) to format and interpret this data.”…””}”(hj‘  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhŸh³h K)hj€  hžhubhÊ)”}”(hXþ  Compared to ramfs, this wastes memory (and memory bus bandwidth), creates
unnecessary work for the CPU, and pollutes the CPU caches.  (There are tricks
to avoid this copying by playing with the page tables, but they're unpleasantly
complicated and turn out to be about as expensive as the copying anyway.)
More to the point, all the work ramfs is doing has to happen _anyway_,
since all file access goes through the page and dentry caches.  The RAM
disk is simply unnecessary; ramfs is internally much simpler.”h]”hX   Compared to ramfs, this wastes memory (and memory bus bandwidth), creates
unnecessary work for the CPU, and pollutes the CPU caches.  (There are tricks
to avoid this copying by playing with the page tables, but theyâ€™re unpleasantly
complicated and turn out to be about as expensive as the copying anyway.)
More to the point, all the work ramfs is doing has to happen _anyway_,
since all file access goes through the page and dentry caches.  The RAM
disk is simply unnecessary; ramfs is internally much simpler.”…””}”(hjŸ  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhŸh³h K1hj€  hžhubhÊ)”}”(hŒòAnother reason ramdisks are semi-obsolete is that the introduction of
loopback devices offered a more flexible and convenient way to create
synthetic block devices, now from files instead of from chunks of memory.
See losetup (8) for details.”h]”hŒòAnother reason ramdisks are semi-obsolete is that the introduction of
loopback devices offered a more flexible and convenient way to create
synthetic block devices, now from files instead of from chunks of memory.
See losetup (8) for details.”…””}”(hj­  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhŸh³h K9hj€  hžhubeh}”(h]”Œramfs-and-ramdisk”ah ]”h"]”Œramfs and ramdisk:”ah$]”h&]”uh1h´hh¶hžhhŸh³h K'ubhµ)”}”(hhh]”(hº)”}”(hŒramfs and tmpfs:”h]”hŒramfs and tmpfs:”…””}”(hjÆ  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h¹hjÃ  hžhhŸh³h K?ubhÊ)”}”(hXY  One downside of ramfs is you can keep writing data into it until you fill
up all memory, and the VM can't free it because the VM thinks that files
should get written to backing store (rather than swap space), but ramfs hasn't
got any backing store.  Because of this, only root (or a trusted user) should
be allowed write access to a ramfs mount.”h]”hX]  One downside of ramfs is you can keep writing data into it until you fill
up all memory, and the VM canâ€™t free it because the VM thinks that files
should get written to backing store (rather than swap space), but ramfs hasnâ€™t
got any backing store.  Because of this, only root (or a trusted user) should
be allowed write access to a ramfs mount.”…””}”(hjÔ  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhŸh³h KAhjÃ  hžhubhÊ)”}”(hŒêA ramfs derivative called tmpfs was created to add size limits, and the ability
to write the data to swap space.  Normal users can be allowed write access to
tmpfs mounts.  See Documentation/filesystems/tmpfs.rst for more information.”h]”hŒêA ramfs derivative called tmpfs was created to add size limits, and the ability
to write the data to swap space.  Normal users can be allowed write access to
tmpfs mounts.  See Documentation/filesystems/tmpfs.rst for more information.”…””}”(hjâ  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhŸh³h KGhjÃ  hžhubeh}”(h]”Œramfs-and-tmpfs”ah ]”h"]”Œramfs and tmpfs:”ah$]”h&]”uh1h´hh¶hžhhŸh³h K?ubhµ)”}”(hhh]”(hº)”}”(hŒWhat is rootfs?”h]”hŒWhat is rootfs?”…””}”(hjû  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h¹hjø  hžhhŸh³h KLubhÊ)”}”(hXl  Rootfs is a special instance of ramfs (or tmpfs, if that's enabled), which is
always present in 2.6 systems.  You can't unmount rootfs for approximately the
same reason you can't kill the init process; rather than having special code
to check for and handle an empty list, it's smaller and simpler for the kernel
to just make sure certain lists can't become empty.”h]”hXv  Rootfs is a special instance of ramfs (or tmpfs, if thatâ€™s enabled), which is
always present in 2.6 systems.  You canâ€™t unmount rootfs for approximately the
same reason you canâ€™t kill the init process; rather than having special code
to check for and handle an empty list, itâ€™s smaller and simpler for the kernel
to just make sure certain lists canâ€™t become empty.”…””}”(hj	  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhŸh³h KNhjø  hžhubhÊ)”}”(hŒ‡Most systems just mount another filesystem over rootfs and ignore it.  The
amount of space an empty instance of ramfs takes up is tiny.”h]”hŒ‡Most systems just mount another filesystem over rootfs and ignore it.  The
amount of space an empty instance of ramfs takes up is tiny.”…””}”(hj  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhŸh³h KThjø  hžhubhÊ)”}”(hŒ’If CONFIG_TMPFS is enabled, rootfs will use tmpfs instead of ramfs by
default.  To force ramfs, add "rootfstype=ramfs" to the kernel command
line.”h]”hŒ–If CONFIG_TMPFS is enabled, rootfs will use tmpfs instead of ramfs by
default.  To force ramfs, add â€œrootfstype=ramfsâ€ to the kernel command
line.”…””}”(hj%  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhŸh³h KWhjø  hžhubeh}”(h]”Œwhat-is-rootfs”ah ]”h"]”Œwhat is rootfs?”ah$]”h&]”uh1h´hh¶hžhhŸh³h KLubhµ)”}”(hhh]”(hº)”}”(hŒWhat is initramfs?”h]”hŒWhat is initramfs?”…””}”(hj>  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h¹hj;  hžhhŸh³h K\ubhÊ)”}”(hXl  All 2.6 Linux kernels contain a gzipped "cpio" format archive, which is
extracted into rootfs when the kernel boots up.  After extracting, the kernel
checks to see if rootfs contains a file "init", and if so it executes it as PID
1.  If found, this init process is responsible for bringing the system the
rest of the way up, including locating and mounting the real root device (if
any).  If rootfs does not contain an init program after the embedded cpio
archive is extracted into it, the kernel will fall through to the older code
to locate and mount a root partition, then exec some variant of /sbin/init
out of that.”h]”hXt  All 2.6 Linux kernels contain a gzipped â€œcpioâ€ format archive, which is
extracted into rootfs when the kernel boots up.  After extracting, the kernel
checks to see if rootfs contains a file â€œinitâ€, and if so it executes it as PID
1.  If found, this init process is responsible for bringing the system the
rest of the way up, including locating and mounting the real root device (if
any).  If rootfs does not contain an init program after the embedded cpio
archive is extracted into it, the kernel will fall through to the older code
to locate and mount a root partition, then exec some variant of /sbin/init
out of that.”…””}”(hjL  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhŸh³h K^hj;  hžhubhÊ)”}”(hŒ5All this differs from the old initrd in several ways:”h]”hŒ5All this differs from the old initrd in several ways:”…””}”(hjZ  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhŸh³h Khhj;  hžhubhŒblock_quote”“”)”}”(hX¶  - The old initrd was always a separate file, while the initramfs archive is
  linked into the linux kernel image.  (The directory ``linux-*/usr`` is
  devoted to generating this archive during the build.)

- The old initrd file was a gzipped filesystem image (in some file format,
  such as ext2, that needed a driver built into the kernel), while the new
  initramfs archive is a gzipped cpio archive (like tar only simpler,
  see cpio(1) and Documentation/driver-api/early-userspace/buffer-format.rst).
  The kernel's cpio extraction code is not only extremely small, it's also
  __init text and data that can be discarded during the boot process.

- The program run by the old initrd (which was called /initrd, not /init) did
  some setup and then returned to the kernel, while the init program from
  initramfs is not expected to return to the kernel.  (If /init needs to hand
  off control it can overmount / with a new root device and exec another init
  program.  See the switch_root utility, below.)

- When switching another root device, initrd would pivot_root and then
  umount the ramdisk.  But initramfs is rootfs: you can neither pivot_root
  rootfs, nor unmount it.  Instead delete everything out of rootfs to
  free up the space (find -xdev / -exec rm '{}' ';'), overmount rootfs
  with the new root (cd /newmount; mount --move . /; chroot .), attach
  stdin/stdout/stderr to the new /dev/console, and exec the new init.

  Since this is a remarkably persnickety process (and involves deleting
  commands before you can run them), the klibc package introduced a helper
  program (utils/run_init.c) to do all this for you.  Most other packages
  (such as busybox) have named this command "switch_root".
”h]”hŒbullet_list”“”)”}”(hhh]”(hŒ	list_item”“”)”}”(hŒÇThe old initrd was always a separate file, while the initramfs archive is
linked into the linux kernel image.  (The directory ``linux-*/usr`` is
devoted to generating this archive during the build.)
”h]”hÊ)”}”(hŒÆThe old initrd was always a separate file, while the initramfs archive is
linked into the linux kernel image.  (The directory ``linux-*/usr`` is
devoted to generating this archive during the build.)”h]”(hŒ~The old initrd was always a separate file, while the initramfs archive is
linked into the linux kernel image.  (The directory ”…””}”(hjy  hžhhŸNh NubhŒliteral”“”)”}”(hŒ``linux-*/usr``”h]”hŒlinux-*/usr”…””}”(hjƒ  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1j  hjy  ubhŒ9 is
devoted to generating this archive during the build.)”…””}”(hjy  hžhhŸNh Nubeh}”(h]”h ]”h"]”h$]”h&]”uh1hÉhŸh³h Kjhju  ubah}”(h]”h ]”h"]”h$]”h&]”uh1js  hjp  ubjt  )”}”(hX°  The old initrd file was a gzipped filesystem image (in some file format,
such as ext2, that needed a driver built into the kernel), while the new
initramfs archive is a gzipped cpio archive (like tar only simpler,
see cpio(1) and Documentation/driver-api/early-userspace/buffer-format.rst).
The kernel's cpio extraction code is not only extremely small, it's also
__init text and data that can be discarded during the boot process.
”h]”hÊ)”}”(hX¯  The old initrd file was a gzipped filesystem image (in some file format,
such as ext2, that needed a driver built into the kernel), while the new
initramfs archive is a gzipped cpio archive (like tar only simpler,
see cpio(1) and Documentation/driver-api/early-userspace/buffer-format.rst).
The kernel's cpio extraction code is not only extremely small, it's also
__init text and data that can be discarded during the boot process.”h]”hX³  The old initrd file was a gzipped filesystem image (in some file format,
such as ext2, that needed a driver built into the kernel), while the new
initramfs archive is a gzipped cpio archive (like tar only simpler,
see cpio(1) and Documentation/driver-api/early-userspace/buffer-format.rst).
The kernelâ€™s cpio extraction code is not only extremely small, itâ€™s also
__init text and data that can be discarded during the boot process.”…””}”(hj¥  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhŸh³h Knhj¡  ubah}”(h]”h ]”h"]”h$]”h&]”uh1js  hjp  ubjt  )”}”(hX[  The program run by the old initrd (which was called /initrd, not /init) did
some setup and then returned to the kernel, while the init program from
initramfs is not expected to return to the kernel.  (If /init needs to hand
off control it can overmount / with a new root device and exec another init
program.  See the switch_root utility, below.)
”h]”hÊ)”}”(hXZ  The program run by the old initrd (which was called /initrd, not /init) did
some setup and then returned to the kernel, while the init program from
initramfs is not expected to return to the kernel.  (If /init needs to hand
off control it can overmount / with a new root device and exec another init
program.  See the switch_root utility, below.)”h]”hXZ  The program run by the old initrd (which was called /initrd, not /init) did
some setup and then returned to the kernel, while the init program from
initramfs is not expected to return to the kernel.  (If /init needs to hand
off control it can overmount / with a new root device and exec another init
program.  See the switch_root utility, below.)”…””}”(hj½  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhŸh³h Kuhj¹  ubah}”(h]”h ]”h"]”h$]”h&]”uh1js  hjp  ubjt  )”}”(hX±  When switching another root device, initrd would pivot_root and then
umount the ramdisk.  But initramfs is rootfs: you can neither pivot_root
rootfs, nor unmount it.  Instead delete everything out of rootfs to
free up the space (find -xdev / -exec rm '{}' ';'), overmount rootfs
with the new root (cd /newmount; mount --move . /; chroot .), attach
stdin/stdout/stderr to the new /dev/console, and exec the new init.

Since this is a remarkably persnickety process (and involves deleting
commands before you can run them), the klibc package introduced a helper
program (utils/run_init.c) to do all this for you.  Most other packages
(such as busybox) have named this command "switch_root".
”h]”(hÊ)”}”(hXŸ  When switching another root device, initrd would pivot_root and then
umount the ramdisk.  But initramfs is rootfs: you can neither pivot_root
rootfs, nor unmount it.  Instead delete everything out of rootfs to
free up the space (find -xdev / -exec rm '{}' ';'), overmount rootfs
with the new root (cd /newmount; mount --move . /; chroot .), attach
stdin/stdout/stderr to the new /dev/console, and exec the new init.”h]”hX§  When switching another root device, initrd would pivot_root and then
umount the ramdisk.  But initramfs is rootfs: you can neither pivot_root
rootfs, nor unmount it.  Instead delete everything out of rootfs to
free up the space (find -xdev / -exec rm â€˜{}â€™ â€˜;â€™), overmount rootfs
with the new root (cd /newmount; mount --move . /; chroot .), attach
stdin/stdout/stderr to the new /dev/console, and exec the new init.”…””}”(hjÕ  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhŸh³h K{hjÑ  ubhÊ)”}”(hX  Since this is a remarkably persnickety process (and involves deleting
commands before you can run them), the klibc package introduced a helper
program (utils/run_init.c) to do all this for you.  Most other packages
(such as busybox) have named this command "switch_root".”h]”hX  Since this is a remarkably persnickety process (and involves deleting
commands before you can run them), the klibc package introduced a helper
program (utils/run_init.c) to do all this for you.  Most other packages
(such as busybox) have named this command â€œswitch_rootâ€.”…””}”(hjã  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhŸh³h K‚hjÑ  ubeh}”(h]”h ]”h"]”h$]”h&]”uh1js  hjp  ubeh}”(h]”h ]”h"]”h$]”h&]”Œbullet”Œ-”uh1jn  hŸh³h Kjhjj  ubah}”(h]”h ]”h"]”h$]”h&]”uh1jh  hŸh³h Kjhj;  hžhubeh}”(h]”Œwhat-is-initramfs”ah ]”h"]”Œwhat is initramfs?”ah$]”h&]”uh1h´hh¶hžhhŸh³h K\ubhµ)”}”(hhh]”(hº)”}”(hŒPopulating initramfs:”h]”hŒPopulating initramfs:”…””}”(hj  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h¹hj  hžhhŸh³h KˆubhÊ)”}”(hŒÃThe 2.6 kernel build process always creates a gzipped cpio format initramfs
archive and links it into the resulting kernel binary.  By default, this
archive is empty (consuming 134 bytes on x86).”h]”hŒÃThe 2.6 kernel build process always creates a gzipped cpio format initramfs
archive and links it into the resulting kernel binary.  By default, this
archive is empty (consuming 134 bytes on x86).”…””}”(hj  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhŸh³h KŠhj  hžhubhÊ)”}”(hXŠ  The config option CONFIG_INITRAMFS_SOURCE (in General Setup in menuconfig,
and living in usr/Kconfig) can be used to specify a source for the
initramfs archive, which will automatically be incorporated into the
resulting binary.  This option can point to an existing gzipped cpio
archive, a directory containing files to be archived, or a text file
specification such as the following example::”h]”hX‰  The config option CONFIG_INITRAMFS_SOURCE (in General Setup in menuconfig,
and living in usr/Kconfig) can be used to specify a source for the
initramfs archive, which will automatically be incorporated into the
resulting binary.  This option can point to an existing gzipped cpio
archive, a directory containing files to be archived, or a text file
specification such as the following example:”…””}”(hj,  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhŸh³h KŽhj  hžhubhŒliteral_block”“”)”}”(hX  dir /dev 755 0 0
nod /dev/console 644 0 0 c 5 1
nod /dev/loop0 644 0 0 b 7 0
dir /bin 755 1000 1000
slink /bin/sh busybox 777 0 0
file /bin/busybox initramfs/busybox 755 0 0
dir /proc 755 0 0
dir /sys 755 0 0
dir /mnt 755 0 0
file /init initramfs/init.sh 755 0 0”h]”hX  dir /dev 755 0 0
nod /dev/console 644 0 0 c 5 1
nod /dev/loop0 644 0 0 b 7 0
dir /bin 755 1000 1000
slink /bin/sh busybox 777 0 0
file /bin/busybox initramfs/busybox 755 0 0
dir /proc 755 0 0
dir /sys 755 0 0
dir /mnt 755 0 0
file /init initramfs/init.sh 755 0 0”…””}”hj<  sbah}”(h]”h ]”h"]”h$]”h&]”h±h²uh1j:  hŸh³h K•hj  hžhubhÊ)”}”(hŒjRun "usr/gen_init_cpio" (after the kernel build) to get a usage message
documenting the above file format.”h]”hŒnRun â€œusr/gen_init_cpioâ€ (after the kernel build) to get a usage message
documenting the above file format.”…””}”(hjJ  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhŸh³h K hj  hžhubhÊ)”}”(hXŠ  One advantage of the configuration file is that root access is not required to
set permissions or create device nodes in the new archive.  (Note that those
two example "file" entries expect to find files named "init.sh" and "busybox" in
a directory called "initramfs", under the linux-2.6.* directory.  See
Documentation/driver-api/early-userspace/early_userspace_support.rst for more details.)”h]”hXš  One advantage of the configuration file is that root access is not required to
set permissions or create device nodes in the new archive.  (Note that those
two example â€œfileâ€ entries expect to find files named â€œinit.shâ€ and â€œbusyboxâ€ in
a directory called â€œinitramfsâ€, under the linux-2.6.* directory.  See
Documentation/driver-api/early-userspace/early_userspace_support.rst for more details.)”…””}”(hjX  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhŸh³h K£hj  hžhubhÊ)”}”(hX  The kernel does not depend on external cpio tools.  If you specify a
directory instead of a configuration file, the kernel's build infrastructure
creates a configuration file from that directory (usr/Makefile calls
usr/gen_initramfs.sh), and proceeds to package up that directory
using the config file (by feeding it to usr/gen_init_cpio, which is created
from usr/gen_init_cpio.c).  The kernel's build-time cpio creation code is
entirely self-contained, and the kernel's boot-time extractor is also
(obviously) self-contained.”h]”hX  The kernel does not depend on external cpio tools.  If you specify a
directory instead of a configuration file, the kernelâ€™s build infrastructure
creates a configuration file from that directory (usr/Makefile calls
usr/gen_initramfs.sh), and proceeds to package up that directory
using the config file (by feeding it to usr/gen_init_cpio, which is created
from usr/gen_init_cpio.c).  The kernelâ€™s build-time cpio creation code is
entirely self-contained, and the kernelâ€™s boot-time extractor is also
(obviously) self-contained.”…””}”(hjf  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhŸh³h K©hj  hžhubhÊ)”}”(hŒÁThe one thing you might need external cpio utilities installed for is creating
or extracting your own preprepared cpio files to feed to the kernel build
(instead of a config file or directory).”h]”hŒÁThe one thing you might need external cpio utilities installed for is creating
or extracting your own preprepared cpio files to feed to the kernel build
(instead of a config file or directory).”…””}”(hjt  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhŸh³h K²hj  hžhubhÊ)”}”(hŒ‡The following command line can extract a cpio image (either by the above script
or by the kernel build) back into its component files::”h]”hŒ†The following command line can extract a cpio image (either by the above script
or by the kernel build) back into its component files:”…””}”(hj‚  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhŸh³h K¶hj  hžhubj;  )”}”(hŒAcpio -i -d -H newc -F initramfs_data.cpio --no-absolute-filenames”h]”hŒAcpio -i -d -H newc -F initramfs_data.cpio --no-absolute-filenames”…””}”hj  sbah}”(h]”h ]”h"]”h$]”h&]”h±h²uh1j:  hŸh³h K¹hj  hžhubhÊ)”}”(hŒmThe following shell script can create a prebuilt cpio archive you can
use in place of the above config file::”h]”hŒlThe following shell script can create a prebuilt cpio archive you can
use in place of the above config file:”…””}”(hjž  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhŸh³h K»hj  hžhubj;  )”}”(hXn  #!/bin/sh

# Copyright 2006 Rob Landley <rob@landley.net> and TimeSys Corporation.
# Licensed under GPL version 2

if [ $# -ne 2 ]
then
  echo "usage: mkinitramfs directory imagename.cpio.gz"
  exit 1
fi

if [ -d "$1" ]
then
  echo "creating $2 from $1"
  (cd "$1"; find . | cpio -o -H newc | gzip) > "$2"
else
  echo "First argument must be a directory"
  exit 1
fi”h]”hXn  #!/bin/sh

# Copyright 2006 Rob Landley <rob@landley.net> and TimeSys Corporation.
# Licensed under GPL version 2

if [ $# -ne 2 ]
then
  echo "usage: mkinitramfs directory imagename.cpio.gz"
  exit 1
fi

if [ -d "$1" ]
then
  echo "creating $2 from $1"
  (cd "$1"; find . | cpio -o -H newc | gzip) > "$2"
else
  echo "First argument must be a directory"
  exit 1
fi”…””}”hj¬  sbah}”(h]”h ]”h"]”h$]”h&]”h±h²uh1j:  hŸh³h K¾hj  hžhubhŒnote”“”)”}”(hXW  The cpio man page contains some bad advice that will break your initramfs
archive if you follow it.  It says "A typical way to generate the list
of filenames is with the find command; you should give find the -depth
option to minimize problems with permissions on directories that are
unwritable or not searchable."  Don't do this when creating
initramfs.cpio.gz images, it won't work.  The Linux kernel cpio extractor
won't create files in a directory that doesn't exist, so the directory
entries must go before the files that go in those directories.
The above script gets them in the right order.”h]”hÊ)”}”(hXW  The cpio man page contains some bad advice that will break your initramfs
archive if you follow it.  It says "A typical way to generate the list
of filenames is with the find command; you should give find the -depth
option to minimize problems with permissions on directories that are
unwritable or not searchable."  Don't do this when creating
initramfs.cpio.gz images, it won't work.  The Linux kernel cpio extractor
won't create files in a directory that doesn't exist, so the directory
entries must go before the files that go in those directories.
The above script gets them in the right order.”h]”hXc  The cpio man page contains some bad advice that will break your initramfs
archive if you follow it.  It says â€œA typical way to generate the list
of filenames is with the find command; you should give find the -depth
option to minimize problems with permissions on directories that are
unwritable or not searchable.â€  Donâ€™t do this when creating
initramfs.cpio.gz images, it wonâ€™t work.  The Linux kernel cpio extractor
wonâ€™t create files in a directory that doesnâ€™t exist, so the directory
entries must go before the files that go in those directories.
The above script gets them in the right order.”…””}”(hjÀ  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhŸh³h KÔhj¼  ubah}”(h]”h ]”h"]”h$]”h&]”uh1jº  hj  hžhhŸh³h Nubeh}”(h]”Œpopulating-initramfs”ah ]”h"]”Œpopulating initramfs:”ah$]”h&]”uh1h´hh¶hžhhŸh³h Kˆubhµ)”}”(hhh]”(hº)”}”(hŒExternal initramfs images:”h]”hŒExternal initramfs images:”…””}”(hjß  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h¹hjÜ  hžhhŸh³h KßubhÊ)”}”(hX  If the kernel has initrd support enabled, an external cpio.gz archive can also
be passed into a 2.6 kernel in place of an initrd.  In this case, the kernel
will autodetect the type (initramfs, not initrd) and extract the external cpio
archive into rootfs before trying to run /init.”h]”hX  If the kernel has initrd support enabled, an external cpio.gz archive can also
be passed into a 2.6 kernel in place of an initrd.  In this case, the kernel
will autodetect the type (initramfs, not initrd) and extract the external cpio
archive into rootfs before trying to run /init.”…””}”(hjí  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhŸh³h KáhjÜ  hžhubhÊ)”}”(hX  This has the memory efficiency advantages of initramfs (no ramdisk block
device) but the separate packaging of initrd (which is nice if you have
non-GPL code you'd like to run from initramfs, without conflating it with
the GPL licensed Linux kernel binary).”h]”hX  This has the memory efficiency advantages of initramfs (no ramdisk block
device) but the separate packaging of initrd (which is nice if you have
non-GPL code youâ€™d like to run from initramfs, without conflating it with
the GPL licensed Linux kernel binary).”…””}”(hjû  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhŸh³h KæhjÜ  hžhubhÊ)”}”(hX/  It can also be used to supplement the kernel's built-in initramfs image.  The
files in the external archive will overwrite any conflicting files in
the built-in initramfs archive.  Some distributors also prefer to customize
a single kernel image with task-specific initramfs images, without recompiling.”h]”hX1  It can also be used to supplement the kernelâ€™s built-in initramfs image.  The
files in the external archive will overwrite any conflicting files in
the built-in initramfs archive.  Some distributors also prefer to customize
a single kernel image with task-specific initramfs images, without recompiling.”…””}”(hj	  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhŸh³h KëhjÜ  hžhubeh}”(h]”Œexternal-initramfs-images”ah ]”h"]”Œexternal initramfs images:”ah$]”h&]”uh1h´hh¶hžhhŸh³h Kßubhµ)”}”(hhh]”(hº)”}”(hŒContents of initramfs:”h]”hŒContents of initramfs:”…””}”(hj"  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h¹hj  hžhhŸh³h KñubhÊ)”}”(hŒêAn initramfs archive is a complete self-contained root filesystem for Linux.
If you don't already understand what shared libraries, devices, and paths
you need to get a minimal root filesystem up and running, here are some
references:”h]”hŒìAn initramfs archive is a complete self-contained root filesystem for Linux.
If you donâ€™t already understand what shared libraries, devices, and paths
you need to get a minimal root filesystem up and running, here are some
references:”…””}”(hj0  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhŸh³h Kóhj  hžhubjo  )”}”(hhh]”(jt  )”}”(hŒ*https://www.tldp.org/HOWTO/Bootdisk-HOWTO/”h]”hÊ)”}”(hjC  h]”j  )”}”(hjC  h]”hŒ*https://www.tldp.org/HOWTO/Bootdisk-HOWTO/”…””}”(hjH  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”Œrefuri”jC  uh1j  hjE  ubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhŸh³h KøhjA  ubah}”(h]”h ]”h"]”h$]”h&]”uh1js  hj>  hžhhŸh³h Nubjt  )”}”(hŒAhttps://www.tldp.org/HOWTO/From-PowerUp-To-Bash-Prompt-HOWTO.html”h]”hÊ)”}”(hjd  h]”j  )”}”(hjd  h]”hŒAhttps://www.tldp.org/HOWTO/From-PowerUp-To-Bash-Prompt-HOWTO.html”…””}”(hji  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”Œrefuri”jd  uh1j  hjf  ubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhŸh³h Kùhjb  ubah}”(h]”h ]”h"]”h$]”h&]”uh1js  hj>  hžhhŸh³h Nubjt  )”}”(hŒ1http://www.linuxfromscratch.org/lfs/view/stable/
”h]”hÊ)”}”(hŒ0http://www.linuxfromscratch.org/lfs/view/stable/”h]”j  )”}”(hj‰  h]”hŒ0http://www.linuxfromscratch.org/lfs/view/stable/”…””}”(hj‹  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”Œrefuri”j‰  uh1j  hj‡  ubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhŸh³h Kúhjƒ  ubah}”(h]”h ]”h"]”h$]”h&]”uh1js  hj>  hžhhŸh³h Nubeh}”(h]”h ]”h"]”h$]”h&]”jý  jþ  uh1jn  hŸh³h Køhj  hžhubhÊ)”}”(hŒÍThe "klibc" package (https://www.kernel.org/pub/linux/libs/klibc) is
designed to be a tiny C library to statically link early userspace
code against, along with some related utilities.  It is BSD licensed.”h]”(hŒThe â€œklibcâ€ package (”…””}”(hj«  hžhhŸNh Nubj  )”}”(hŒ+https://www.kernel.org/pub/linux/libs/klibc”h]”hŒ+https://www.kernel.org/pub/linux/libs/klibc”…””}”(hj³  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”Œrefuri”jµ  uh1j  hj«  ubhŒ) is
designed to be a tiny C library to statically link early userspace
code against, along with some related utilities.  It is BSD licensed.”…””}”(hj«  hžhhŸNh Nubeh}”(h]”h ]”h"]”h$]”h&]”uh1hÉhŸh³h Kühj  hžhubhÊ)”}”(hŒÈI use uClibc (https://www.uclibc.org) and busybox (https://www.busybox.net)
myself.  These are LGPL and GPL, respectively.  (A self-contained initramfs
package is planned for the busybox 1.3 release.)”h]”(hŒI use uClibc (”…””}”(hjÌ  hžhhŸNh Nubj  )”}”(hŒhttps://www.uclibc.org”h]”hŒhttps://www.uclibc.org”…””}”(hjÔ  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”Œrefuri”jÖ  uh1j  hjÌ  ubhŒ) and busybox (”…””}”(hjÌ  hžhhŸNh Nubj  )”}”(hŒhttps://www.busybox.net”h]”hŒhttps://www.busybox.net”…””}”(hjç  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”Œrefuri”jé  uh1j  hjÌ  ubhŒ~)
myself.  These are LGPL and GPL, respectively.  (A self-contained initramfs
package is planned for the busybox 1.3 release.)”…””}”(hjÌ  hžhhŸNh Nubeh}”(h]”h ]”h"]”h$]”h&]”uh1hÉhŸh³h M hj  hžhubhÊ)”}”(hX  In theory you could use glibc, but that's not well suited for small embedded
uses like this.  (A "hello world" program statically linked against glibc is
over 400k.  With uClibc it's 7k.  Also note that glibc dlopens libnss to do
name lookups, even when otherwise statically linked.)”h]”hX#  In theory you could use glibc, but thatâ€™s not well suited for small embedded
uses like this.  (A â€œhello worldâ€ program statically linked against glibc is
over 400k.  With uClibc itâ€™s 7k.  Also note that glibc dlopens libnss to do
name lookups, even when otherwise statically linked.)”…””}”(hj   hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhŸh³h Mhj  hžhubhÊ)”}”(hŒ´A good first step is to get initramfs to run a statically linked "hello world"
program as init, and test it under an emulator like qemu (www.qemu.org) or
User Mode Linux, like so::”h]”hŒ·A good first step is to get initramfs to run a statically linked â€œhello worldâ€
program as init, and test it under an emulator like qemu (www.qemu.org) or
User Mode Linux, like so:”…””}”(hj  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhŸh³h M	hj  hžhubj;  )”}”(hX^  cat > hello.c << EOF
#include <stdio.h>
#include <unistd.h>

int main(int argc, char *argv[])
{
  printf("Hello world!\n");
  sleep(999999999);
}
EOF
gcc -static hello.c -o init
echo init | cpio -o -H newc | gzip > test.cpio.gz
# Testing external initramfs using the initrd loading mechanism.
qemu -kernel /boot/vmlinuz -initrd test.cpio.gz /dev/zero”h]”hX^  cat > hello.c << EOF
#include <stdio.h>
#include <unistd.h>

int main(int argc, char *argv[])
{
  printf("Hello world!\n");
  sleep(999999999);
}
EOF
gcc -static hello.c -o init
echo init | cpio -o -H newc | gzip > test.cpio.gz
# Testing external initramfs using the initrd loading mechanism.
qemu -kernel /boot/vmlinuz -initrd test.cpio.gz /dev/zero”…””}”hj  sbah}”(h]”h ]”h"]”h$]”h&]”h±h²uh1j:  hŸh³h Mhj  hžhubhÊ)”}”(hŒ¢When debugging a normal root filesystem, it's nice to be able to boot with
"init=/bin/sh".  The initramfs equivalent is "rdinit=/bin/sh", and it's
just as useful.”h]”hŒ®When debugging a normal root filesystem, itâ€™s nice to be able to boot with
â€œinit=/bin/shâ€.  The initramfs equivalent is â€œrdinit=/bin/shâ€, and itâ€™s
just as useful.”…””}”(hj*  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhŸh³h Mhj  hžhubeh}”(h]”Œcontents-of-initramfs”ah ]”h"]”Œcontents of initramfs:”ah$]”h&]”uh1h´hh¶hžhhŸh³h Kñubhµ)”}”(hhh]”(hº)”}”(hŒWhy cpio rather than tar?”h]”hŒWhy cpio rather than tar?”…””}”(hjC  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h¹hj@  hžhhŸh³h M!ubhÊ)”}”(hŒLThis decision was made back in December, 2001.  The discussion started here:”h]”hŒLThis decision was made back in December, 2001.  The discussion started here:”…””}”(hjQ  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhŸh³h M#hj@  hžhubji  )”}”(hŒ?http://www.uwsg.iu.edu/hypermail/linux/kernel/0112.2/1538.html
”h]”hÊ)”}”(hŒ>http://www.uwsg.iu.edu/hypermail/linux/kernel/0112.2/1538.html”h]”j  )”}”(hje  h]”hŒ>http://www.uwsg.iu.edu/hypermail/linux/kernel/0112.2/1538.html”…””}”(hjg  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”Œrefuri”je  uh1j  hjc  ubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhŸh³h M%hj_  ubah}”(h]”h ]”h"]”h$]”h&]”uh1jh  hŸh³h M%hj@  hžhubhÊ)”}”(hŒIAnd spawned a second thread (specifically on tar vs cpio), starting here:”h]”hŒIAnd spawned a second thread (specifically on tar vs cpio), starting here:”…””}”(hj  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhŸh³h M'hj@  hžhubji  )”}”(hŒ?http://www.uwsg.iu.edu/hypermail/linux/kernel/0112.2/1587.html
”h]”hÊ)”}”(hŒ>http://www.uwsg.iu.edu/hypermail/linux/kernel/0112.2/1587.html”h]”j  )”}”(hj•  h]”hŒ>http://www.uwsg.iu.edu/hypermail/linux/kernel/0112.2/1587.html”…””}”(hj—  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”Œrefuri”j•  uh1j  hj“  ubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhŸh³h M)hj  ubah}”(h]”h ]”h"]”h$]”h&]”uh1jh  hŸh³h M)hj@  hžhubhÊ)”}”(hŒ^The quick and dirty summary version (which is no substitute for reading
the above threads) is:”h]”hŒ^The quick and dirty summary version (which is no substitute for reading
the above threads) is:”…””}”(hj±  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhŸh³h M+hj@  hžhubhŒenumerated_list”“”)”}”(hhh]”(jt  )”}”(hXò  cpio is a standard.  It's decades old (from the AT&T days), and already
widely used on Linux (inside RPM, Red Hat's device driver disks).  Here's
a Linux Journal article about it from 1996:

   http://www.linuxjournal.com/article/1213

It's not as popular as tar because the traditional cpio command line tools
require _truly_hideous_ command line arguments.  But that says nothing
either way about the archive format, and there are alternative tools,
such as:

  http://freecode.com/projects/afio
”h]”(hÊ)”}”(hŒ½cpio is a standard.  It's decades old (from the AT&T days), and already
widely used on Linux (inside RPM, Red Hat's device driver disks).  Here's
a Linux Journal article about it from 1996:”h]”hŒÃcpio is a standard.  Itâ€™s decades old (from the AT&T days), and already
widely used on Linux (inside RPM, Red Hatâ€™s device driver disks).  Hereâ€™s
a Linux Journal article about it from 1996:”…””}”(hjÈ  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhŸh³h M.hjÄ  ubji  )”}”(hŒ)http://www.linuxjournal.com/article/1213
”h]”hÊ)”}”(hŒ(http://www.linuxjournal.com/article/1213”h]”j  )”}”(hjÜ  h]”hŒ(http://www.linuxjournal.com/article/1213”…””}”(hjÞ  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”Œrefuri”jÜ  uh1j  hjÚ  ubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhŸh³h M2hjÖ  ubah}”(h]”h ]”h"]”h$]”h&]”uh1jh  hŸh³h M2hjÄ  ubhÊ)”}”(hŒàIt's not as popular as tar because the traditional cpio command line tools
require _truly_hideous_ command line arguments.  But that says nothing
either way about the archive format, and there are alternative tools,
such as:”h]”hŒâItâ€™s not as popular as tar because the traditional cpio command line tools
require _truly_hideous_ command line arguments.  But that says nothing
either way about the archive format, and there are alternative tools,
such as:”…””}”(hjø  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhŸh³h M4hjÄ  ubji  )”}”(hŒ"http://freecode.com/projects/afio
”h]”hÊ)”}”(hŒ!http://freecode.com/projects/afio”h]”j  )”}”(hj  h]”hŒ!http://freecode.com/projects/afio”…””}”(hj  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”Œrefuri”j  uh1j  hj
  ubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhŸh³h M9hj  ubah}”(h]”h ]”h"]”h$]”h&]”uh1jh  hŸh³h M9hjÄ  ubeh}”(h]”h ]”h"]”h$]”h&]”uh1js  hjÁ  hžhhŸh³h Nubjt  )”}”(hX‚  The cpio archive format chosen by the kernel is simpler and cleaner (and
thus easier to create and parse) than any of the (literally dozens of)
various tar archive formats.  The complete initramfs archive format is
explained in buffer-format.rst, created in usr/gen_init_cpio.c, and
extracted in init/initramfs.c.  All three together come to less than 26k
total of human-readable text.
”h]”hÊ)”}”(hX  The cpio archive format chosen by the kernel is simpler and cleaner (and
thus easier to create and parse) than any of the (literally dozens of)
various tar archive formats.  The complete initramfs archive format is
explained in buffer-format.rst, created in usr/gen_init_cpio.c, and
extracted in init/initramfs.c.  All three together come to less than 26k
total of human-readable text.”h]”hX  The cpio archive format chosen by the kernel is simpler and cleaner (and
thus easier to create and parse) than any of the (literally dozens of)
various tar archive formats.  The complete initramfs archive format is
explained in buffer-format.rst, created in usr/gen_init_cpio.c, and
extracted in init/initramfs.c.  All three together come to less than 26k
total of human-readable text.”…””}”(hj2  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhŸh³h M;hj.  ubah}”(h]”h ]”h"]”h$]”h&]”uh1js  hjÁ  hžhhŸh³h Nubjt  )”}”(hŒ²The GNU project standardizing on tar is approximately as relevant as
Windows standardizing on zip.  Linux is not part of either, and is free
to make its own technical decisions.
”h]”hÊ)”}”(hŒ±The GNU project standardizing on tar is approximately as relevant as
Windows standardizing on zip.  Linux is not part of either, and is free
to make its own technical decisions.”h]”hŒ±The GNU project standardizing on tar is approximately as relevant as
Windows standardizing on zip.  Linux is not part of either, and is free
to make its own technical decisions.”…””}”(hjJ  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhŸh³h MBhjF  ubah}”(h]”h ]”h"]”h$]”h&]”uh1js  hjÁ  hžhhŸh³h Nubjt  )”}”(hŒãSince this is a kernel internal format, it could easily have been
something brand new.  The kernel provides its own tools to create and
extract this format anyway.  Using an existing standard was preferable,
but not essential.
”h]”hÊ)”}”(hŒâSince this is a kernel internal format, it could easily have been
something brand new.  The kernel provides its own tools to create and
extract this format anyway.  Using an existing standard was preferable,
but not essential.”h]”hŒâSince this is a kernel internal format, it could easily have been
something brand new.  The kernel provides its own tools to create and
extract this format anyway.  Using an existing standard was preferable,
but not essential.”…””}”(hjb  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhŸh³h MFhj^  ubah}”(h]”h ]”h"]”h$]”h&]”uh1js  hjÁ  hžhhŸh³h Nubjt  )”}”(hX”  Al Viro made the decision (quote: "tar is ugly as hell and not going to be
supported on the kernel side"):

   http://www.uwsg.iu.edu/hypermail/linux/kernel/0112.2/1540.html

explained his reasoning:

  - http://www.uwsg.iu.edu/hypermail/linux/kernel/0112.2/1550.html
  - http://www.uwsg.iu.edu/hypermail/linux/kernel/0112.2/1638.html

and, most importantly, designed and implemented the initramfs code.
”h]”(hÊ)”}”(hŒjAl Viro made the decision (quote: "tar is ugly as hell and not going to be
supported on the kernel side"):”h]”hŒnAl Viro made the decision (quote: â€œtar is ugly as hell and not going to be
supported on the kernel sideâ€):”…””}”(hjz  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhŸh³h MKhjv  ubji  )”}”(hŒ?http://www.uwsg.iu.edu/hypermail/linux/kernel/0112.2/1540.html
”h]”hÊ)”}”(hŒ>http://www.uwsg.iu.edu/hypermail/linux/kernel/0112.2/1540.html”h]”j  )”}”(hjŽ  h]”hŒ>http://www.uwsg.iu.edu/hypermail/linux/kernel/0112.2/1540.html”…””}”(hj  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”Œrefuri”jŽ  uh1j  hjŒ  ubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhŸh³h MNhjˆ  ubah}”(h]”h ]”h"]”h$]”h&]”uh1jh  hŸh³h MNhjv  ubhÊ)”}”(hŒexplained his reasoning:”h]”hŒexplained his reasoning:”…””}”(hjª  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhŸh³h MPhjv  ubji  )”}”(hŒ‚- http://www.uwsg.iu.edu/hypermail/linux/kernel/0112.2/1550.html
- http://www.uwsg.iu.edu/hypermail/linux/kernel/0112.2/1638.html
”h]”jo  )”}”(hhh]”(jt  )”}”(hŒ>http://www.uwsg.iu.edu/hypermail/linux/kernel/0112.2/1550.html”h]”hÊ)”}”(hjÁ  h]”j  )”}”(hjÁ  h]”hŒ>http://www.uwsg.iu.edu/hypermail/linux/kernel/0112.2/1550.html”…””}”(hjÆ  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”Œrefuri”jÁ  uh1j  hjÃ  ubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhŸh³h MRhj¿  ubah}”(h]”h ]”h"]”h$]”h&]”uh1js  hj¼  ubjt  )”}”(hŒ?http://www.uwsg.iu.edu/hypermail/linux/kernel/0112.2/1638.html
”h]”hÊ)”}”(hŒ>http://www.uwsg.iu.edu/hypermail/linux/kernel/0112.2/1638.html”h]”j  )”}”(hjæ  h]”hŒ>http://www.uwsg.iu.edu/hypermail/linux/kernel/0112.2/1638.html”…””}”(hjè  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”Œrefuri”jæ  uh1j  hjä  ubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhŸh³h MShjà  ubah}”(h]”h ]”h"]”h$]”h&]”uh1js  hj¼  ubeh}”(h]”h ]”h"]”h$]”h&]”jý  jþ  uh1jn  hŸh³h MRhj¸  ubah}”(h]”h ]”h"]”h$]”h&]”uh1jh  hŸh³h MRhjv  ubhÊ)”}”(hŒCand, most importantly, designed and implemented the initramfs code.”h]”hŒCand, most importantly, designed and implemented the initramfs code.”…””}”(hj  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhŸh³h MUhjv  ubeh}”(h]”h ]”h"]”h$]”h&]”uh1js  hjÁ  hžhhŸh³h Nubeh}”(h]”h ]”h"]”h$]”h&]”Œenumtype”Œarabic”Œprefix”hŒsuffix”Œ)”uh1j¿  hj@  hžhhŸh³h M.ubeh}”(h]”Œwhy-cpio-rather-than-tar”ah ]”h"]”Œwhy cpio rather than tar?”ah$]”h&]”uh1h´hh¶hžhhŸh³h M!ubhµ)”}”(hhh]”(hº)”}”(hŒFuture directions:”h]”hŒFuture directions:”…””}”(hj8  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h¹hj5  hžhhŸh³h MXubhÊ)”}”(hXV  Today (2.6.16), initramfs is always compiled in, but not always used.  The
kernel falls back to legacy boot code that is reached only if initramfs does
not contain an /init program.  The fallback is legacy code, there to ensure a
smooth transition and allowing early boot functionality to gradually move to
"early userspace" (I.E. initramfs).”h]”hXZ  Today (2.6.16), initramfs is always compiled in, but not always used.  The
kernel falls back to legacy boot code that is reached only if initramfs does
not contain an /init program.  The fallback is legacy code, there to ensure a
smooth transition and allowing early boot functionality to gradually move to
â€œearly userspaceâ€ (I.E. initramfs).”…””}”(hjF  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhŸh³h MZhj5  hžhubhÊ)”}”(hX#  The move to early userspace is necessary because finding and mounting the real
root device is complex.  Root partitions can span multiple devices (raid or
separate journal).  They can be out on the network (requiring dhcp, setting a
specific MAC address, logging into a server, etc).  They can live on removable
media, with dynamically allocated major/minor numbers and persistent naming
issues requiring a full udev implementation to sort out.  They can be
compressed, encrypted, copy-on-write, loopback mounted, strangely partitioned,
and so on.”h]”hX#  The move to early userspace is necessary because finding and mounting the real
root device is complex.  Root partitions can span multiple devices (raid or
separate journal).  They can be out on the network (requiring dhcp, setting a
specific MAC address, logging into a server, etc).  They can live on removable
media, with dynamically allocated major/minor numbers and persistent naming
issues requiring a full udev implementation to sort out.  They can be
compressed, encrypted, copy-on-write, loopback mounted, strangely partitioned,
and so on.”…””}”(hjT  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhŸh³h M`hj5  hžhubhÊ)”}”(hŒÀThis kind of complexity (which inevitably includes policy) is rightly handled
in userspace.  Both klibc and busybox/uClibc are working on simple initramfs
packages to drop into a kernel build.”h]”hŒÀThis kind of complexity (which inevitably includes policy) is rightly handled
in userspace.  Both klibc and busybox/uClibc are working on simple initramfs
packages to drop into a kernel build.”…””}”(hjb  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhŸh³h Mihj5  hžhubhÊ)”}”(hŒôThe klibc package has now been accepted into Andrew Morton's 2.6.17-mm tree.
The kernel's current early boot code (partition detection, etc) will probably
be migrated into a default initramfs, automatically created and used by the
kernel build.”h]”hŒøThe klibc package has now been accepted into Andrew Mortonâ€™s 2.6.17-mm tree.
The kernelâ€™s current early boot code (partition detection, etc) will probably
be migrated into a default initramfs, automatically created and used by the
kernel build.”…””}”(hjp  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhŸh³h Mmhj5  hžhubeh}”(h]”Œfuture-directions”ah ]”h"]”Œfuture directions:”ah$]”h&]”uh1h´hh¶hžhhŸh³h MXubeh}”(h]”Œramfs-rootfs-and-initramfs”ah ]”h"]”Œramfs, rootfs and initramfs”ah$]”h&]”uh1h´hhhžhhŸh³h Kubeh}”(h]”h ]”h"]”h$]”h&]”Œsource”h³uh1hŒcurrent_source”NŒcurrent_line”NŒsettings”Œdocutils.frontend”ŒValues”“”)”}”(h¹NŒ	generator”NŒ	datestamp”NŒsource_link”NŒ
source_url”NŒtoc_backlinks”Œentry”Œfootnote_backlinks”KŒsectnum_xform”KŒstrip_comments”NŒstrip_elements_with_classes”NŒstrip_classes”NŒreport_level”KŒ
halt_level”KŒexit_status_level”KŒdebug”NŒwarning_stream”NŒ	traceback”ˆŒinput_encoding”Œ	utf-8-sig”Œinput_encoding_error_handler”Œstrict”Œoutput_encoding”Œutf-8”Œoutput_encoding_error_handler”j±  Œerror_encoding”Œutf-8”Œerror_encoding_error_handler”Œbackslashreplace”Œlanguage_code”Œen”Œrecord_dependencies”NŒconfig”NŒ	id_prefix”hŒauto_id_prefix”Œid”Œdump_settings”NŒdump_internals”NŒdump_transforms”NŒdump_pseudo_xml”NŒexpose_internals”NŒstrict_visitor”NŒ_disable_config”NŒ_source”h³Œ_destination”NŒ_config_files”]”Œ7/var/lib/git/docbuild/linux/Documentation/docutils.conf”aŒfile_insertion_enabled”ˆŒraw_enabled”KŒline_length_limit”M'Œpep_references”NŒpep_base_url”Œhttps://peps.python.org/”Œpep_file_url_template”Œpep-%04d”Œrfc_references”NŒrfc_base_url”Œ&https://datatracker.ietf.org/doc/html/”Œ	tab_width”KŒtrim_footnote_reference_space”‰Œsyntax_highlight”Œlong”Œsmart_quotes”ˆŒsmartquotes_locales”]”Œcharacter_level_inline_markup”‰Œdoctitle_xform”‰Œdocinfo_xform”KŒsectsubtitle_xform”‰Œimage_loading”Œlink”Œembed_stylesheet”‰Œcloak_email_addresses”ˆŒsection_self_link”‰Œenv”NubŒreporter”NŒindirect_targets”]”Œsubstitution_defs”}”Œsubstitution_names”}”Œrefnames”}”Œrefids”}”Œnameids”}”(j‹  jˆ  j}  jz  jÀ  j½  jõ  jò  j8  j5  j
  j  jÙ  jÖ  j  j  j=  j:  j2  j/  jƒ  j€  uŒ	nametypes”}”(j‹  ‰j}  ‰jÀ  ‰jõ  ‰j8  ‰j
  ‰jÙ  ‰j  ‰j=  ‰j2  ‰jƒ  ‰uh}”(jˆ  h¶jz  j/  j½  j€  jò  jÃ  j5  jø  j  j;  jÖ  j  j  jÜ  j:  j  j/  j@  j€  j5  uŒfootnote_refs”}”Œcitation_refs”}”Œautofootnotes”]”Œautofootnote_refs”]”Œsymbol_footnotes”]”Œsymbol_footnote_refs”]”Œ	footnotes”]”Œ	citations”]”Œautofootnote_start”KŒsymbol_footnote_start”K Œ
id_counter”Œcollections”ŒCounter”“”}”…”R”Œparse_messages”]”Œtransform_messages”]”Œtransformer”NŒinclude_log”]”Œ
decoration”Nhžhub.