Hi Weijia,
What is the SN2700 software version? output of "show version"
Please make sure you either configure PFC according to HowTo Run RoCE over L2 Enabled with PFC
or just for testing purposes configure global flow control on all ports:
interface ethernet 1/1-1/32 flowcontrol receive on force
interface ethernet 1/1-1/32 flowcontrol send on force