write burst

Six and a half terabytes leaving sixteen thousand GPUs at once is a burst no shared filesystem absorbs, and the fix is to stop trying. What the barrier actually has to wait for, the per-node arithmetic that makes it seconds, and the manifest rule that decides whether a checkpoint is usable at all.

One is a write burst of terabytes in a minute followed by half an hour of silence; the other is a steady read that never stops and never spikes. Provisioning for the peak of the first wastes most of its capacity, and provisioning for the average of the second fails the moment a checkpoint lands.