GlusterFS direct I/O mode
Solution 1:
after a long search i've found this article. it's not glusterfs specific, but it may help to understand better: in short, direct i/o mode disable the file buffer implemented by the os for that mount point.
after reading it i've come to this conclusion:
Enabling or disabling may give you better transfer rate depending on which application is using your glusterfs volume: if you are using applications or databases that handle by themselves caches you are going to have some boost enabling it ( because of the missing overhead of a second buffer ). if you are just doing file accesses probably disabling it will increase your performances because of less network access due to local file cache.