Traditionally, UNIX platforms called the kernel image /unix. With the development of virtual memory, kernels that supported this feature were given the vm- prefix to differentiate them. The name vmlinux is a mutation of vmunix, while in vmlinuz the letter z at the end denotes that it is compressed (for example gzipped).[1]
Location
Traditionally, the kernel was located in the root directory of the filesystem hierarchy; however, as the bootloader must use BIOS drivers to access the hard disk, limitations on some i386 systems meant only the first 1024 cylinders of the hard disk were addressable.
To overcome this, Linux distributors encouraged users to create a partition at the beginning of their drives specifically for storing bootloader and kernel-related files. GRUB, LILO and SYSLINUX are common bootloaders.
By convention, this partition is mounted on the filesystem hierarchy as /boot. This was later standardised by the Filesystem Hierarchy Standard (FHS), which now requires the Linux kernel image to be located in either / or /boot, although there is no technical restriction enforcing this.[2]
Compression
Traditionally, when creating a bootable kernel image, the kernel is also compressed using gzip, or, since Linux 2.6.30,[3] using LZMA or bzip2, which requires a very small decompression stub to be included in the resulting image. The stub decompresses the kernel code, on some systems printing dots to the console to indicate progress, and then continues the boot process. Support for LZO,[4]xz,[5]LZ4[6] and zstd[7] compression was added later.
The decompression routine is a negligible factor in boot time, and prior to the development of the bzImage, the size constraints of some architectures, notably i386, were extremely limiting, making compression a necessity.
On the SPARC architecture, the vmlinux file is compressed using simple gzip, because the SILO boot loader transparently decompresses gzipped images.
The filename of the bootable image is not important, but many popular distributions use vmlinuz.
bzImage
Anatomy of a bzImage
As the Linux kernel matured, the size of the kernels generated by users grew beyond the limits imposed by some architectures, where the space available to store the compressed kernel code is limited. The bzImage (big zImage) format was developed to overcome this limitation by splitting the kernel over non-contiguous memory regions.[citation needed]
The bzImage was compressed using gzip until Linux 2.6.30,[3] which introduced more algorithms. Although the bzprefix may suggest that bzip2 compression is used, this is not the case.[1] (The bzip2 package is often distributed with tools prefixed with bz, such as bzless, bzcat, etc.)
The bzImage file is in a specific format. It contains concatenated bootsect.o + setup.o + misc.o + piggy.o.[8]piggy.o contains the gzipped vmlinux file in its data section. The script extract-vmlinux found under scripts/ in the kernel sources decompresses a kernel image. Some distributions (e.g. Red Hat and clones) may come with a kernel-debuginfoRPM that contains the vmlinux file for the matching kernel RPM, and it typically gets installed under /usr/lib/debug/lib/modules/`uname -r`/vmlinux or /usr/lib/debug/lib64/modules/`uname -r`/vmlinux.
↑ "Section 3.5.2 — /boot: Static files of the boot loader". FHS 2.3. 2004-01-29. Retrieved 2014-03-11. The operating system kernel must be located in either / or /boot.
1 2 Linux 2.6.30, released the 9th of June 2009, added support to compress the kernel image with the LZMA and bzip2 algorithms
↑ Linux 2.6.33, released on February 24, 2010, added support to compress the kernel image with LZO
↑ Linux 2.6.38, released on March 14, 2011, added support to compress the kernel image with xz
↑ Linux 3.11, released on September 2, 2013, added support to compress the kernel image with LZ4
↑ Linux 5.9, released on October 11, 2020, added support to compress the kernel image, initrd and initramfs with zstd
Alavoor Vasudevan (2003-08-15). "10. Kernel Files Information". The Linux Kernel HOWTO. Archived from the original on 2017-10-10. Retrieved 2014-03-11.
Daniel Pierre Bovet (2013-01-03). "Special sections in Linux binaries". LWN.net. all special sections appearing in the Linux kernel end up packed in one of the segments defined in the vmlinux ELF header. Each special section fulfills a particular purpose.
gzip is a file format and a software application used for file compression and decompression. The program was created by Jean-loup Gailly and Mark Adler as a free software replacement for the compress program used in early Unix systems, and intended for use by GNU. Version 0.1 was first publicly released on 31 October 1992, and version 1.0 followed in February 1993.
bzip2 is a free and open-source file compression program that uses the Burrows–Wheeler algorithm. It only compresses single files and is not a file archiver. It relies on separate external utilities for tasks such as handling multiple files, encryption, and archive-splitting.
In computing, tar is a computer software utility for collecting many files into one archive file, often referred to as a tarball, for distribution or backup purposes. The name is derived from "tape archive", as it was originally developed to write data to sequential I/O devices with no file system of their own, such as devices that use magnetic tape. The archive data sets created by tar contain various file system parameters, such as name, timestamps, ownership, file-access permissions, and directory organization. POSIX abandoned tar in favor of pax, yet tar sees continued widespread use.
The Lempel–Ziv–Markov chain algorithm (LZMA) is an algorithm used to perform lossless data compression. It has been under development since either 1996 or 1998 by Igor Pavlov and was first used in the 7z format of the 7-Zip archiver. This algorithm uses a dictionary compression scheme somewhat similar to the LZ77 algorithm published by Abraham Lempel and Jacob Ziv in 1977 and features a high compression ratio and a variable compression-dictionary size, while still maintaining decompression speed similar to other commonly used compression algorithms.
rzip is a huge-scale data compression computer program designed around initial LZ77-style string matching on a 900 MB dictionary window, followed by bzip2-based Burrows–Wheeler transform and entropy coding (Huffman) on 900 kB output chunks.
Lempel–Ziv–Oberhumer (LZO) is a lossless data compression algorithm that is focused on decompression speed.
dar is a computer program, a command-line archiving tool intended as a replacement for tar in Unix-like operating systems.
In Linux systems, initrd is a scheme for loading a temporary root file system into memory, to be used as part of the Linux startup process. initrd and initramfs refer to two different methods of achieving this. Both are commonly used to make preparations before the real root file system can be mounted.
The SPARC Improved bootLOader (SILO) is the bootloader used by the SPARC port of the Linux operating system; it can also be used for Solaris as a replacement for the standard Solaris boot loader.
Squashfs is a compressed read-only file system for Linux. Squashfs compresses files, inodes and directories, and supports block sizes from 4 KiB up to 1 MiB for greater compression. Several compression algorithms are supported. Squashfs is also the name of free software, licensed under the GPL, for accessing Squashfs filesystems.
The following tables compare general and technical information for a number of file systems.
Fiwix is an operating system kernel based on the UNIX architecture and fully focused on being POSIX compatible. It is designed and developed mainly as a hobbyist operating system, but it also serves for educational purposes. It runs on the i386 hardware platform and is compatible with a good base of existing GNU applications. It follows the UNIX System V application binary interface and is also mostly Linux 2.0 system call ABI compatible.
The Linux booting process involves multiple stages and is in many ways similar to the BSD and other Unix-style boot processes, from which it derives. Although the Linux booting process depends very much on the computer architecture, those architectures share similar stages and software components, including system startup, bootloader execution, loading and startup of a Linux kernel image, and execution of various startup scripts and daemons. Those are grouped into 4 steps: system startup, bootloader stage, kernel stage, and init process. When a Linux system is powered up or reset, its processor will execute a specific firmware/program for system initialization, such as the power-on self-test, invoking the reset vector to start a program at a known address in flash/ROM, then load the bootloader into RAM for later execution. In IBM PC–compatible personal computers (PCs), this firmware/program is either a BIOS or a UEFI monitor, and is stored in the mainboard. In embedded Linux systems, this firmware/program is called boot ROM. After being loaded into RAM, the bootloader will execute to load the second-stage bootloader. The second-stage bootloader will load the kernel image into memory, decompress and initialize it, and then pass control to this kernel image. The second-stage bootloader also performs several operation on the system such as system hardware check, mounting the root device, loading the necessary kernel modules, etc. Finally, the first user-space process starts, and other high-level system initializations are performed.
Xarchiver is a front-end to various command line archiving tools for Linux and BSD operating systems, designed to be independent of the desktop environment. It is the default archiving application of Xfce and LXDE. Deepin's archive manager is based on Xarchiver.
XZ Utils is a set of free software command-line lossless data compressors, including the programs lzma and xz, for Unix-like operating systems and, from version 5.0 onwards, Microsoft Windows. For compression/decompression the Lempel–Ziv–Markov chain algorithm (LZMA) is used. XZ Utils started as a Unix port of Igor Pavlov's LZMA-SDK that has been adapted to fit seamlessly into Unix environments and their usual structure and behavior.
lzip is a free, command-line tool for the compression of data; it employs the Lempel–Ziv–Markov chain algorithm (LZMA) with a user interface that is familiar to users of usual Unix compression tools, such as gzip and bzip2.
zram, formerly called compcache, is a Linux kernel module for creating a compressed block device in RAM, i.e. a RAM disk with on-the-fly disk compression. The block device created with zram can then be used for swap or as general-purpose RAM disk. The two most common uses for zram are for the storage of temporary files and as a swap device. Initially, zram had only the latter function, hence the original name "compcache". Unlike swap, zram only uses 0.1% of the maximum size of the disk when not in use.
LZ4 is a lossless data compression algorithm that is focused on compression and decompression speed. It belongs to the LZ77 family of byte-oriented compression schemes.
Zstandard is a lossless data compression algorithm developed by Yann Collet at Facebook. Zstd is the corresponding reference implementation in C, released as open-source software on 31 August 2016.
EROFS is a lightweight read-only file system initially developed by Huawei, originally for the Linux kernel and now maintained by an open-source community from all over the world.
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