Readahead is a

system call In computing, a system call (commonly abbreviated to syscall) is the programmatic way in which a computer program requests a service from the operating system on which it is executed. This may include hardware-related services (for example, acc ...

of the

Linux kernel The Linux kernel is a free and open-source, monolithic, modular, multitasking, Unix-like operating system kernel. It was originally authored in 1991 by Linus Torvalds for his i386-based PC, and it was soon adopted as the kernel for the GNU ...

that loads a file's contents into the

page cache In computing, a page cache, sometimes also called disk cache, is a transparent cache for the pages originating from a secondary storage device such as a hard disk drive (HDD) or a solid-state drive (SSD). The operating system keeps a page cache ...

. This prefetches the file so that when it is subsequently accessed, its contents are read from the

main memory Computer data storage is a technology consisting of computer components and recording media that are used to retain digital data. It is a core function and fundamental component of computers. The central processing unit (CPU) of a comput ...

(

RAM Ram, ram, or RAM may refer to: Animals * A male sheep * Ram cichlid, a freshwater tropical fish People * Ram (given name) * Ram (surname) * Ram (director) (Ramsubramaniam), an Indian Tamil film director * RAM (musician) (born 1974), Dutch * ...

) rather than from a

hard disk drive A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device that stores and retrieves digital data using magnetic storage with one or more rigid rapidly rotating platters coated with mag ...

(HDD), resulting in much lower file access latencies. Many

Linux distributions A Linux distribution (often abbreviated as distro) is an operating system made from a software collection that includes the Linux kernel and, often, a package management system. Linux users usually obtain their operating system by downloading one ...

use readahead on a list of commonly used files to speed up

booting In computing, booting is the process of starting a computer as initiated via Computer hardware, hardware such as a button or by a software command. After it is switched on, a computer's central processing unit (CPU) has no software in its ma ...

. In such a setup, if the

kernel Kernel may refer to: Computing * Kernel (operating system), the central component of most operating systems * Kernel (image processing), a matrix used for image convolution * Compute kernel, in GPGPU programming * Kernel method, in machine learn ...

is booted with the boot parameter, it will record all file accesses during bootup and write a new list of files to be read during later boot sequences. This will make additional installed services start faster, because they are not included in the default readahead list. In Linux distributions that use

systemd systemd is a software suite that provides an array of system components for Linux operating systems. Its main aim is to unify service configuration and behavior across Linux distributions; Its primary component is a "system and service manag ...

, readahead binary (as part of the boot sequence) was replaced by systemd-readahead. However, support for readahead was removed from systemd in its version 217, being described as unmaintained and unable to provide expected performance benefits. Certain experimental

page Page most commonly refers to: * Page (paper), one side of a leaf of paper, as in a book Page, PAGE, pages, or paging may also refer to: Roles * Page (assistance occupation), a professional occupation * Page (servant), traditionally a young m ...

-level prefetching systems have been developed to further improve performance.

In filesystem

Bcache bcache (abbreviated from ''block cache'') is a cache in the Linux kernel's block layer, which is used for accessing secondary storage devices. It allows one or more fast storage devices, such as flash-based solid-state drives (SSDs), to act as ...

support readahead of files and metadata. ZFS supports readahead of files and metadata, when using ARC.

References

Preloading and prebinding Linux file system-related software {{Linux-stub