add note on O_NOATIME

Author: vladak

file-api.tex

@@ -797,7 +797,9 @@
 important).  It may also come in handy if your filesystem is on a medium with a
 limited number of erase/write cycles -- a CompactFlash, for example.  If your
 system supports it, it will be a \texttt{noatime} option for the \texttt{mount}
-command.  See the manual page.
+command.  See the manual page. Some systems (Linux based) support a non-portable
+\texttt{open} syscall flag to set this per file opening, based on some
+conditions.
 \item \emph{Metadata} is information about the file -- access mode, access
 times, length, owner, group, etc.  Metadata does \emsl{not} include the file
 data itself, or the file name as the file data can be accessed through