Volume set interface needs to be enhanced to support io-threads on the client stack.
CHANGE: http://review.gluster.com/105 (New set option performance.client-io-threads added to enable/disable it.) merged in master by Anand Avati (avati)
CHANGE: http://review.gluster.com/134 (New set option performance.client-io-threads added to enable/disable it.) merged in release-3.1 by Anand Avati (avati)
CHANGE: http://review.gluster.com/135 (New set option performance.client-io-threads added to enable/disable it.) merged in release-3.2 by Anand Avati (avati)
Checked with release-3.2 git head 62adb4d1c204104314e8d1d843dc8ca6c498dc55. gluster volume set <volname> client-io-threads on/off will load/remove the io-threads xlator in the client volfile. Upon enabling io-threads is loaded below io-stats in the client graph.