Stat (system Call) - Criticism of Atime

Criticism of Atime

Writing to a file changes its mtime and ctime, while reading a file changes its atime. As a result, on a POSIX-compliant system, reading a file causes a write, which has been criticized. This behaviour can usually be disabled by adding a mount option in /etc/fstab.

However, turning off atime updating breaks POSIX compliance, and some applications, notably the Mutt mail reader (in some configurations), and some file usage watching utilities, notably tmpwatch.

Linux kernel developer Ingo Molnár called atime "perhaps the most stupid Unix design idea of all times," adding: "hink about this a bit: 'For every file that is read from the disk, let's do a ... write to the disk! And, for every file that is already cached and which we read from the cache ... do a write to the disk!'" He further emphasized the performance impact thus:

atime updates are by far the biggest I/O performance deficiency that Linux has today. Getting rid of atime updates would give us more everyday Linux performance than all the pagecache speedups of the past 10 years, _combined_.