Old known stuff, probably for everybody who is somehow related to networking, but anyway, putting it here too.

IEEE 802.3x – Wikipedia link

If QoS is enabled and you like to prioritize the traffic, flow control needs to be disabled,as it doesn’t care about any higher level prioritization, just when ingress traffic is coming in faster than receiver can accept it, flow control will kick in and send pause frames until the ingress-egress rate will be equalized or ingress rate is lower than egress of that interface.

A bit more info from Dell FTOS 9 documentation about flow control :

 

I would use it only for storage – for example iscsi traffic, in separated network, then it won’t do any harm.. probably 🙂

But of course no way of using it on trunk links, other switch facing links and etc.