Comment 3 for bug 1378063

Perhaps, docs are http://dev.mysql.com/doc/refman/5.6/en/innodb-parameters.html#sysvar_innodb_flush_method

"For example, on a system with a hardware RAID controller and battery-backed write cache, O_DIRECT can help to avoid double buffering between the InnoDB buffer pool and the operating system's file system cache. On some systems where InnoDB data and log files are located on a SAN, the default value or O_DSYNC might be faster for a read-heavy workload with mostly SELECT statements. "

But the one thing with which I agree completely and which is not being addressed here is:
"Always test this parameter with hardware and workload that reflect your production environment."'