block layer: request unplugging threshold: unplug_thresh

Hi, While trying to understand the block layer in Linux, I stumbled upon this request(struct request) unplugging threshold(q->unplug_thresh), which is hardcoded to '4'. So far, my understanding is that there could be explicit request unplugging(e.g. multipath module registering it's unplug_fn and later calling blk_unplug) as well as implicit ones(either due to timeout or read/write request threshold reaching to unplug_thresh. So having, q->unplug_thresh = 4 (ref. blk_queue_make_request), seem like a very small value and even if it's not, then why shouldn't the driver such as RAID framework(dm/md), be allowed to manipulate this threshold value ? -Amit