The ultra-large scale and prolonged runtime of Large Language Model (LLM) training—often involving thousands of GPUs and spanning weeks—render reliability a pivotal bottleneck. Hardware failures, stragglers, and runtime issues can waste over 30% of GPU resources, delaying model rollout and driving up costs. This survey focuses on reliability optimization for LLM training systems. The discussion centers on three pillars of reliability: fault detection, fault recovery, and straggler mitigation. For each pillar, we dissect innovative mechanisms, which range from communication-aware fault detection to adaptive load balancing, and we assess their impact on critical reliability metrics such as... More >