Evaluating phase change memory for enterprise storage systems: a study of caching and tiering approaches

Storage systems based on Phase Change Memory (PCM) devices are beginning to generate considerable attention in both industry and academic communities. But whether the technology in its current state will be a commercially and technically viable alternative to entrenched technologies such as flash-based SSDs remains undecided. To address this it is important to consider PCM SSD devices not just from a device standpoint, but also from a holistic perspective. This paper presents the results of our performance study of a recent all-PCM SSD prototype. The average latency for a 4 KiB random read is 6.7 µs, which is about 16× faster than a comparable eMLC flash SSD. The distribution of I/O response times is also much narrower than flash SSD for both reads and writes. Based on the performance measurements and real-world workload traces, we explore two typical storage use-cases: tiering and caching. For tiering, we model a hypothetical storage system that consists of flash, HDD, and PCM to identify the combinations of device types that offer the best performance within cost constraints. For caching, we study whether PCM can improve performance compared to flash in terms of aggregate I/O time and read latency. We report that the IOPS/$ of a tiered storage system can be improved by 12-66% and the aggregate elapsed time of a server-side caching solution can be improved by up to 35% by adding PCM. Our results show that - even at current price points - PCM storage devices show promising performance as a new component in enterprise storage systems.