On Linux, noatime includes nodiratime
By Jon Jensen · February 9, 2010
Note to performance-tweaking Linux sysadmins, pointed out to me by Selena Deckelmann: On Linux, the filesystem attribute noatime includes nodiratime, so there's no need to say both "noatime,nodiratime" as I once did. (See this article on atime for details if you're not familiar with it.)
Apparently the nodiratime attribute was added later as a subset of noatime applying only to directories to still offer a bit of performance boost in situations where noatime on files would cause trouble (as with mutt and a few other applications that care about atimes).
See also the related newer relatime attribute in the mount(8) manpage.