Readahead is a

system call In computing, a system call (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, accessing a hard disk drive ...

of the

Linux kernel The Linux kernel is a Free and open-source software, free and open source Unix-like kernel (operating system), kernel that is used in many computer systems worldwide. The kernel was created by Linus Torvalds in 1991 and was soon adopted as the k ...

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 ca ...

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

main memory Computer data storage or digital 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 processin ...

(

RAM Ram, ram, or RAM most commonly refers to: * A male sheep * Random-access memory, computer memory * Ram Trucks, US, since 2009 ** List of vehicles named Dodge Ram, trucks and vans ** Ram Pickup, produced by Ram Trucks Ram, ram, or RAM may also ref ...

) 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 hard disk drive platter, pla ...

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

Linux distributions A Linux distribution, often abbreviated as distro, is an operating system that includes the Linux kernel for its kernel (operating system), kernel functionality. Although the name does not imply distribution (marketing), product distribution pe ...

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 physical button on the computer or by a software command. After it is switched on, a computer's central processing unit (CPU) h ...

. 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 learnin ...

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. The main aim is to unify service configuration and behavior across Linux distributions. Its primary component is a "system and service manage ...

, 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 mechanism 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), ...

supports readahead of files and metadata. *

ZFS ZFS (previously Zettabyte File System) is a file system with Volume manager, volume management capabilities. It began as part of the Sun Microsystems Solaris (operating system), Solaris operating system in 2001. Large parts of Solaris, includin ...

supports readahead of files and metadata, when using ARC.

References

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

In filesystem

See also

References