This bug has been copied from bug #574285 and has been proposed to be backported to 5.5 z-stream (EUS).
in 2.6.18-194.4.1.el5
Technical note added. If any revisions are required, please edit the "Technical Notes" field accordingly. All revisions will be proofread by the Engineering Content Services team. New Contents: When two or more writes which used the O_DIRECT file access interface were run simultaneously on two separate partitions on the same disk, performance of both writes were reduced. This could have caused a write slowdown of approximately 25% when running two simultaneous "dd oflag=direct" commands on two different partitions. This regression has been fixed in this update so that O_DIRECT write performance does not incur a performance penalty.
An advisory has been issued which should help the problem described in this bug report. This report is therefore being closed with a resolution of ERRATA. For more information on therefore solution and/or where to find the updated files, please follow the link below. You may reopen this bug report if the solution does not work for you. http://rhn.redhat.com/errata/RHSA-2010-0504.html