MongoDB primary stalls and throughput collapses when WiredTiger cache reaches high utilization during sustained bulk inserts

WiredTiger relies on an in-memory cache and background eviction to keep dirty pages within limits before checkpoints write to disk. Large, sustained bulk insert bursts can fill the cache faster than eviction/checkpoint can flush to disk, leading to stalls where insertion threads wait for cache pressure to relieve. The observed increase in checkpoint time and disk latency suggests the storage subsystem may be the limiting factor — either because write IOPS are insufficient or because insert batch size produces large working set churn. Short-term mitigations (smaller batches, lower write concern) reduce instantaneous cache pressure; longer-term fixes involve tuning WT cache/checkpoint settings, improving storage write performance, or staging bulk imports differently.