[PATCH] ramfs, rootfs, and initramfs docs
Docs for ramfs, rootfs, and initramfs. Signed-off-by: Rob Landley <rob@landley.net> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
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Rob Landley
Linus Torvalds
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Documentation/filesystems/ramfs-rootfs-initramfs.txt
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ramfs, rootfs and initramfs
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October 17, 2005
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Rob Landley <rob@landley.net>
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=============================
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What is ramfs?
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--------------
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Ramfs is a very simple filesystem that exports Linux's disk caching
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mechanisms (the page cache and dentry cache) as a dynamically resizable
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ram-based filesystem.
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Normally all files are cached in memory by Linux. Pages of data read from
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backing store (usually the block device the filesystem is mounted on) are kept
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around in case it's needed again, but marked as clean (freeable) in case the
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Virtual Memory system needs the memory for something else. Similarly, data
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written to files is marked clean as soon as it has been written to backing
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store, but kept around for caching purposes until the VM reallocates the
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memory. A similar mechanism (the dentry cache) greatly speeds up access to
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directories.
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With ramfs, there is no backing store. Files written into ramfs allocate
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dentries and page cache as usual, but there's nowhere to write them to.
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This means the pages are never marked clean, so they can't be freed by the
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VM when it's looking to recycle memory.
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The amount of code required to implement ramfs is tiny, because all the
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work is done by the existing Linux caching infrastructure. Basically,
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you're mounting the disk cache as a filesystem. Because of this, ramfs is not
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an optional component removable via menuconfig, since there would be negligible
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space savings.
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ramfs and ramdisk:
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------------------
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The older "ram disk" mechanism created a synthetic block device out of
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an area of ram and used it as backing store for a filesystem. This block
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device was of fixed size, so the filesystem mounted on it was of fixed
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size. Using a ram disk also required unnecessarily copying memory from the
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fake block device into the page cache (and copying changes back out), as well
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as creating and destroying dentries. Plus it needed a filesystem driver
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(such as ext2) to format and interpret this data.
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Compared to ramfs, this wastes memory (and memory bus bandwidth), creates
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unnecessary work for the CPU, and pollutes the CPU caches. (There are tricks
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to avoid this copying by playing with the page tables, but they're unpleasantly
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complicated and turn out to be about as expensive as the copying anyway.)
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More to the point, all the work ramfs is doing has to happen _anyway_,
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since all file access goes through the page and dentry caches. The ram
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disk is simply unnecessary, ramfs is internally much simpler.
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Another reason ramdisks are semi-obsolete is that the introduction of
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loopback devices offered a more flexible and convenient way to create
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synthetic block devices, now from files instead of from chunks of memory.
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See losetup (8) for details.
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ramfs and tmpfs:
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----------------
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One downside of ramfs is you can keep writing data into it until you fill
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up all memory, and the VM can't free it because the VM thinks that files
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should get written to backing store (rather than swap space), but ramfs hasn't
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got any backing store. Because of this, only root (or a trusted user) should
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be allowed write access to a ramfs mount.
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A ramfs derivative called tmpfs was created to add size limits, and the ability
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to write the data to swap space. Normal users can be allowed write access to
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tmpfs mounts. See Documentation/filesystems/tmpfs.txt for more information.
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What is rootfs?
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---------------
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Rootfs is a special instance of ramfs, which is always present in 2.6 systems.
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(It's used internally as the starting and stopping point for searches of the
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kernel's doubly-linked list of mount points.)
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Most systems just mount another filesystem over it and ignore it. The
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amount of space an empty instance of ramfs takes up is tiny.
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What is initramfs?
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------------------
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All 2.6 Linux kernels contain a gzipped "cpio" format archive, which is
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extracted into rootfs when the kernel boots up. After extracting, the kernel
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checks to see if rootfs contains a file "init", and if so it executes it as PID
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1. If found, this init process is responsible for bringing the system the
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rest of the way up, including locating and mounting the real root device (if
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any). If rootfs does not contain an init program after the embedded cpio
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archive is extracted into it, the kernel will fall through to the older code
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to locate and mount a root partition, then exec some variant of /sbin/init
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out of that.
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All this differs from the old initrd in several ways:
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- The old initrd was a separate file, while the initramfs archive is linked
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into the linux kernel image. (The directory linux-*/usr is devoted to
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generating this archive during the build.)
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- The old initrd file was a gzipped filesystem image (in some file format,
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such as ext2, that had to be built into the kernel), while the new
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initramfs archive is a gzipped cpio archive (like tar only simpler,
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see cpio(1) and Documentation/early-userspace/buffer-format.txt).
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- The program run by the old initrd (which was called /initrd, not /init) did
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some setup and then returned to the kernel, while the init program from
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initramfs is not expected to return to the kernel. (If /init needs to hand
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off control it can overmount / with a new root device and exec another init
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program. See the switch_root utility, below.)
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- When switching another root device, initrd would pivot_root and then
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umount the ramdisk. But initramfs is rootfs: you can neither pivot_root
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rootfs, nor unmount it. Instead delete everything out of rootfs to
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free up the space (find -xdev / -exec rm '{}' ';'), overmount rootfs
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with the new root (cd /newmount; mount --move . /; chroot .), attach
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stdin/stdout/stderr to the new /dev/console, and exec the new init.
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Since this is a remarkably persnickity process (and involves deleting
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commands before you can run them), the klibc package introduced a helper
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program (utils/run_init.c) to do all this for you. Most other packages
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(such as busybox) have named this command "switch_root".
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Populating initramfs:
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---------------------
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The 2.6 kernel build process always creates a gzipped cpio format initramfs
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archive and links it into the resulting kernel binary. By default, this
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archive is empty (consuming 134 bytes on x86). The config option
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CONFIG_INITRAMFS_SOURCE (for some reason buried under devices->block devices
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in menuconfig, and living in usr/Kconfig) can be used to specify a source for
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the initramfs archive, which will automatically be incorporated into the
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resulting binary. This option can point to an existing gzipped cpio archive, a
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directory containing files to be archived, or a text file specification such
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as the following example:
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dir /dev 755 0 0
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nod /dev/console 644 0 0 c 5 1
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nod /dev/loop0 644 0 0 b 7 0
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dir /bin 755 1000 1000
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slink /bin/sh busybox 777 0 0
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file /bin/busybox initramfs/busybox 755 0 0
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dir /proc 755 0 0
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dir /sys 755 0 0
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dir /mnt 755 0 0
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file /init initramfs/init.sh 755 0 0
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One advantage of the text file is that root access is not required to
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set permissions or create device nodes in the new archive. (Note that those
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two example "file" entries expect to find files named "init.sh" and "busybox" in
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a directory called "initramfs", under the linux-2.6.* directory. See
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Documentation/early-userspace/README for more details.)
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If you don't already understand what shared libraries, devices, and paths
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you need to get a minimal root filesystem up and running, here are some
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references:
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http://www.tldp.org/HOWTO/Bootdisk-HOWTO/
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http://www.tldp.org/HOWTO/From-PowerUp-To-Bash-Prompt-HOWTO.html
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http://www.linuxfromscratch.org/lfs/view/stable/
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The "klibc" package (http://www.kernel.org/pub/linux/libs/klibc) is
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designed to be a tiny C library to statically link early userspace
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code against, along with some related utilities. It is BSD licensed.
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I use uClibc (http://www.uclibc.org) and busybox (http://www.busybox.net)
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myself. These are LGPL and GPL, respectively.
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In theory you could use glibc, but that's not well suited for small embedded
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uses like this. (A "hello world" program statically linked against glibc is
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over 400k. With uClibc it's 7k. Also note that glibc dlopens libnss to do
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name lookups, even when otherwise statically linked.)
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Future directions:
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------------------
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Today (2.6.14), initramfs is always compiled in, but not always used. The
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kernel falls back to legacy boot code that is reached only if initramfs does
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not contain an /init program. The fallback is legacy code, there to ensure a
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smooth transition and allowing early boot functionality to gradually move to
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"early userspace" (I.E. initramfs).
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The move to early userspace is necessary because finding and mounting the real
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root device is complex. Root partitions can span multiple devices (raid or
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separate journal). They can be out on the network (requiring dhcp, setting a
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specific mac address, logging into a server, etc). They can live on removable
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media, with dynamically allocated major/minor numbers and persistent naming
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issues requiring a full udev implementation to sort out. They can be
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compressed, encrypted, copy-on-write, loopback mounted, strangely partitioned,
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and so on.
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This kind of complexity (which inevitably includes policy) is rightly handled
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in userspace. Both klibc and busybox/uClibc are working on simple initramfs
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packages to drop into a kernel build, and when standard solutions are ready
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and widely deployed, the kernel's legacy early boot code will become obsolete
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and a candidate for the feature removal schedule.
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But that's a while off yet.
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