Comment 2 for bug 801989

from open(2)

              The O_DIRECT flag on its own makes at an effort to transfer data
              synchronously, but does not give the guarantees of the O_SYNC
              that data and necessary metadata are transferred. To guarantee
              synchronous I/O the O_SYNC must be used in addition to O_DIRECT.
              See NOTES below for further discussion.

so both XtraDB and the facebook patches are technically incorrect, although they may just happen to work on most kernels/filesystem combinations there is no reason for them to continue to do so.