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Both carry RDMA at the same line rate. One arrives lossless because the transport was designed that way, the other becomes lossless only if a set of switch settings is correct on every port. What that difference costs in operations, what it saves in money and hiring, and the fleet size where the answer flips.

Thirty-two senders aimed at one port fill a switch buffer in microseconds, and what happens next depends on which of two mechanisms fires first. The buffer arithmetic, why the slow mechanism is the safe one and the fast one is the dangerous one, and the counters that tell you which is happening.

The signature is unusual: many jobs degrade together, throughput collapses, and the drop counters stay at zero. Reading pause counters backward along the paths to find the origin, the three conditions that produce one, and the configuration change that stops it happening again.