3.9. Relatime Drive Access Optimization

The POSIX standard requires that operating systems maintain file system metadata that record when each file was last accessed. This timestamp is called atime, and maintaining it requires a constant series of write operations to storage. These writes keep storage devices and their links busy and powered up. Since few applications make use of the atime data, this storage device activity wastes power. Significantly, the write to storage would occur even if the file was not read from storage, but from cache. For some time, the Linux kernel has supported a noatime option for mount and would not write atime data to file systems mounted with this option. However, simply turning off this feature is problematic because some applications rely on atime data and will fail if it is not available.
The kernel used in Red Hat Enterprise Linux 6 supports another alternative — relatime. relatime maintains atime data, but not for each time that a file is accessed. With this option enabled, atime data is written to the disk only if the file has been modified since the atime data was last updated (mtime), or if the file was last accessed more than a certain amount of time ago (by default, one day).
By default, all file systems are now mounted with relatime enabled. You can suppress it for any particular file system by mounting that file system with the norelatime option.