Readahead

Readahead is a system call of the Linux kernel that loads a file's contents into the page cache. This prefetches the file so that when it is subsequently accessed, its contents are read from the main memory (RAM) rather than from a hard disk drive (HDD), resulting in much lower file access latencies.^[1][2]

Many Linux distributions use readahead on a list of commonly used files to speed up booting. In such a setup, if the kernel is booted with the profile 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.^[3]

In Linux distributions that use systemd, readahead binary (as part of the boot sequence) was replaced by systemd-readahead.^[4][5] However, support for readahead was removed from systemd in its version 217, being described as unmaintained and unable to provide expected performance benefits.^[6]

Certain experimental page-level prefetching systems have been developed to further improve performance.^[7]

In filesystem

• Bcache supports readahead of files and metadata.^[8]
• ZFS supports readahead of files and metadata, when using ARC.^[9]

See also

• Prefetcher
• preload (software)

References

1. Jonathan Corbet (2005-10-12). "Adaptive file readahead". LWN.net. Retrieved 2014-08-20.
2. "readahead(2) - Linux manual page". man7.org. Retrieved 2014-04-09.
3. Michael Opdenacker (2007-06-15). "Readahead: time-travel techniques for desktop and embedded systems" (PDF). free-electrons.com. pp. 5–6. Retrieved 2014-05-01.
4. "Readahead". fedorahosted.org. Retrieved 2014-04-09.
5. "systemd-readahead-replay.service". freedesktop.org. 2014-03-26. Archived from the original on 2014-04-09. Retrieved 2014-04-09.
6. "systemd/systemd – System and Session Manager: Changes with 217". cgit.freedesktop.org. Retrieved 2018-02-09.
7. Krzysztof Lichota (2008). "Linux solution for prefetching necessary data during application and system startup" (PDF). code.google.com. Archived from the original (PDF) on 2009-12-11. Retrieved 2014-07-28.
8. "bcache.txt\Documentation - linux-bcache.git - Unnamed repository; edit this file 'description' to name the repository". evilpiepirate.org. Retrieved 1 May 2020.
9. "Part 10 - Monitoring and Tuning ZFS Performance| Oracle Community". community.oracle.com. Retrieved 1 May 2020.