UC Santa Cruz

With the dramatic advances in electronic device industry, the availability of high speed non-volatile memory (NVRAM) has introduced a new tier into the storage hierarchy, and holds great promise for reduction in latency, power consumption, and improved performance. Among them, flash-memory has become a popular storage medium to replace hard disk as a permanent storage device and DRAM as a temporary storage device. Yet, despite their fast random I/O performance, the design of a flash-based storage system achieves suboptimal performance or suffers from reduced endurance due to the nature of flash memory, for example, out of place update, asymmetric read-write throughput, and limited write cycles. Lack of application aware design makes flash memory less efficient, and hence cannot meet various performance requirements. To this end, we investigate the roles flash memory plays in different storage applications and their performance and reliability requirements. By examining the behavior of these systems and their consequent data access characteristics, as well as the performance impact, we propose solutions that tradeoff performance, cost, endurance and reliability to achieve high efficiency for flash memory in different storage applications with reduced overhead.

We first explore the use of flash memory as a write-through cache in a tiered storage. We demonstrate the individual and cumulative contributions of cache admission policy, cache eviction policy, flash garbage collection policy, and flash device configuration on a flash caching device. We show that workloads on Solid State Caches (SSCs) have significantly greater write pressures than their storage counterparts. we propose HEC, a High Endurance Cache that aims to improve overall device endurance via reduced media writes and erases while increasing or maintaining cache hit rate performance.

To further characterize the behavior of flash memory used in different storage applications, we focus on flash as a primary backing storage device in the second part of this dissertation. We explore the drawbacks of a typical log-structured file system on top of a log-structured FTL flash device. We characterize the interactions between multiple levels of independent logs, and describe several practical scenarios which arises in real log-on-log systems. We then propose a log-aware coordination to tune the layout of logs, so that when multiple layers of logs exist in the system we can still achieve high performance with minimum interference among each log.

In the third part of this dissertation, we explore the approaches to collapsing logs. While there are several popular ways that utilize the nature of flash memory translation layer to eliminate multiple layers of logs in the entire system, we focus on the benefit we can obtain from a log-less object-based flash aware system. We show from simulation experiments that advanced features could be embedded to improve overall performance for object-based flash system with low overhead through a rich interface.