The POSIX standard requires operating systems to keep track of the last time each file was accessed by an application or the user, and to store this timestamp as part of the filesystem data. This timestamp, called atime, is used in finding out which files are never used (to clean up the /tmp directory for example) or if a file has been looked at after it was last changed.
A significant downside to atime is that every time a file is accessed, the kernel has to write a new timestamp to the disk, at least after a few seconds of activity. These disk writes keep the disk and the link to the disk busy, which costs both performance and power.
Because some programs use atime, disabling by default is not practical. The Linux kernel has a feature called
relatime, which is an effective compromise between having some of the information that atime provides, without having the disk time updated as regularly. It works by updating the atime field on disk only if the file hasn't been accessed since the last time it was accessed (to provide the new email detection capability) or when the last access was more than 1 day ago (to help programs and users clean up unused files in the
/tmp directory). An improved version of relatime has been
merged upstream by Fedora developers in the 2.6.30 kernel and backported to the Fedora 11 kernel.
