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| -rw-r--r-- | Documentation/filesystems/ramfs-rootfs-initramfs.txt | 195 |
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diff --git a/Documentation/filesystems/ramfs-rootfs-initramfs.txt b/Documentation/filesystems/ramfs-rootfs-initramfs.txt new file mode 100644 index 0000000..b3404a0 --- /dev/null +++ b/Documentation/filesystems/ramfs-rootfs-initramfs.txt @@ -0,0 +1,195 @@ +ramfs, rootfs and initramfs +October 17, 2005 +Rob Landley <rob@landley.net> +============================= + +What is ramfs? +-------------- + +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. + +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. + +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. + +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. + +ramfs and ramdisk: +------------------ + +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. + +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. + +Another 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. + +ramfs and tmpfs: +---------------- + +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. + +A 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.txt for more information. + +What is rootfs? +--------------- + +Rootfs is a special instance of ramfs, which is always present in 2.6 systems. +(It's used internally as the starting and stopping point for searches of the +kernel's doubly-linked list of mount points.) + +Most systems just mount another filesystem over it and ignore it. The +amount of space an empty instance of ramfs takes up is tiny. + +What is initramfs? +------------------ + +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. + +All this differs from the old initrd in several ways: + + - The old initrd was 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 had to be built into the kernel), while the new + initramfs archive is a gzipped cpio archive (like tar only simpler, + see cpio(1) and Documentation/early-userspace/buffer-format.txt). + + - 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 persnickity 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". + +Populating initramfs: +--------------------- + +The 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). The config option +CONFIG_INITRAMFS_SOURCE (for some reason buried under devices->block devices +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: + + 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 + +One advantage of the text 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/early-userspace/README for more details.) + +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: +http://www.tldp.org/HOWTO/Bootdisk-HOWTO/ +http://www.tldp.org/HOWTO/From-PowerUp-To-Bash-Prompt-HOWTO.html +http://www.linuxfromscratch.org/lfs/view/stable/ + +The "klibc" package (http://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. + +I use uClibc (http://www.uclibc.org) and busybox (http://www.busybox.net) +myself. These are LGPL and GPL, respectively. + +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.) + +Future directions: +------------------ + +Today (2.6.14), 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). + +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. + +This 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, and when standard solutions are ready +and widely deployed, the kernel's legacy early boot code will become obsolete +and a candidate for the feature removal schedule. + +But that's a while off yet. |
