block layer: request unplugging threshold: unplug_thresh

[Kumar amit mehta]

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