We have a customer who has been experiencing periodic issues with AppServers during the online backup to an NFS mounted drive. After some detailed investigation, we found that OS caching was causing the performance issue. We did a simple test by setting the 'oflag=yes' and performing an 'ls' command. There was no significant drop in performance. When 'oflag' was removed, the performance issue returned during the 'ls' command.
Adding the ability to bypass OS caching would greatly enhance the performance of an online db backup to an NFS mounted drive.