Relatime

Relatime is a Linux filesystem mount option that alters how access times (atime) are updated on files and directories. Access times record the last moment a file was read, and updating them on every read can cause substantial disk activity on busy systems. Relatime was introduced to reduce this overhead while keeping atime information useful for typical use.

How it works: under relatime, atime is updated on access only if certain conditions are met. Specifically, atime is updated if the previous atime is older than the file’s modification time (mtime), or if the previous atime is more than about 24 hours old. If neither condition applies, an access does not update atime. If the file has been modified since the last access, atime is updated accordingly. This yields fewer writes than traditional atime behavior while maintaining a reasonable indication of recent activity.

Relation to other options: noatime disables atime updates entirely, improving performance further but removing access-time information. Nodiratime and diratime are related options that control how directory inodes handle atime updates, allowing finer tuning for directory access patterns.

Impact and context: relatime is widely used in modern Linux distributions and filesystems such as ext3, ext4, and others. It reduces disk I/O and wear on storage devices without completely discarding access-time data. Some applications or backup tools that rely on precise atime values may be affected, though strictatime is available for those that require updates on every access. Overall, relatime represents a compromise between correctness of atime and system performance.