As datacenter networks continue to support a wider range of applications and faster link speeds, they face the challenge of managing bursty traffic and transient congestion. End-to-end congestion controls (CCs) find it increasingly difficult to maintain effectiveness due to the inherent feedback delay. To address this issue, per-hop flow control (FC) has gained popularity due to its ability to react promptly to transient congestion. However, existing FC mechanisms either lack fine-grained (i.e., per-flow granularity) control or require an impractical number of queues that exceeds the capabilities of commodity switches. In this paper, we introduce FlowSail, an innovative FC scheme that enables fine-grained control at the per-flow level while requiring a practical number of switch queues, theoretically as few as two. The core of FlowSail is an effective approximation of ideal FC by three key design components: dynamic flow-to-queue mapping, hierarchical congested flow identification, and on-demand isolation. We have implemented a prototype of FlowSail using the programmable P4 switch and conducted extensive testbed experiments and simulations. The results indicate that FlowSail effectively sustains performance with significantly fewer queues compared to existing FC schemes. For instance, FlowSail achieves $4.3\times$ lower tail latency under the same number of queues, matches existing FC schemes with $4\times$ fewer queues, and holds robust performance with a minimum of 2 queues.