ORBIT: Usage-based admission control for file-level HPC multi-tier storage

Hierarchical storage systems in HPC environments combine a high-performance SSD tier with a high-capacity HDD tier, managed at file granularity by policy engines such as Robinhood. A widely used strategy in such systems is to promote on single access (OSA)—migrating any file on its first miss. While OSA maximizes hit ratio, it triggers expensive full file migration and causes SSD/HDD “churn” transfers under skewed access patterns, leading to SSD pollution by weakly reused files, unnecessary data movement, and degraded device endurance. This paper presents ORBIT , a file-level placement policy that couples selective admission with multi-criteria ARC-based eviction ( MC-ARC ). ORBIT admission controller maintains a lightweight, continuously aged usage score that is size-aware and sensitive to the accessed file fraction. When SSD space is needed, ORBIT relies on MC-ARC to evict files by aggregating ARC-derived recency/frequency with predicted remaining lifetime and per-user fairness. Trace-driven simulations on representative HPC workloads show that, compared to the MC-ARC baseline, ORBIT reduces costly data movement across SSD/HDD; the migration volume drops by 95.7%, SSD writes by 97.4% and mean access latency by 78.9%. Despite a lower hit ratio in some cases, avoiding non-profitable promotions yields large end-to-end gains, e.g. 70% average I/O-time reduction across the evaluated workloads and SSD sizes. These results highlight that, at file granularity, disciplined admission and multi-criteria eviction can outperform hit-centric strategies by minimizing migration-dominated overheads in multi-tier HPC storage.