[ceph-users] where does 100% RBD utilization come from?

oops. I posted this to the "Old" list, but supposedly this is the new list and the better place to ask questions? A google search didnt seem to find the answer on this, so thought I'd ask here: what determines if an rdb is "100% busy"? I have some backend OSDs, and an iSCSI gateway, serving out some RBDs. iostat on the gateway says rbd is 100% utilized iostat on individual OSds only goes as high as about 60% on a per-device basis. CPU is idle. Doesnt seem like network interface is capped either. So.. how do I improve RBD throughput?