We propose PB-FS (Postcard-Based Fast Start), a rate initialization scheme that uses direct feedback from the switches to quickly correct the rates of new datacenter flows that begin at the line rate and cause congestion. PB-FS is designed to easily integrate into any datacenter congestion control protocol. We evaluate PB-FS in two datacenter environments: a lossless network that runs RoCE, and a lossy network that uses RDMA with selective repeat. We show that PB-FS significantly reduces tail latency of short flows while maintaining throughput, in both lossless and lossy datacenters.