Mobile devices, such as smartphones, have become a necessity in our daily life. However, users may notice that after being used for a longtime, mobile devices begin to exhibit a sluggish response. Based on an empirical study on a collection of aged smartphones, this work identified that file fragmentation is among the key factors that contribute to the progressive degradation of response time. This study takes a three-step approach: First, this study designed a set of reproducible file-system aging processes based on User-Interface (UI) script replay. Through the aging processes, it confirmed that file fragmentation quickly emerged, and SQLite files were among the most severely fragmented files. Second, based on the workloads of a selection of popular mobile applications, this study observed that file fragmentation did have an impact on user-perceived latencies. Specifically, the launching time of Chrome on an aged file system was 79 percent slower than it was on a pristine file system. Third, this study evaluated existing treatments of file fragmentation, including space preallocation, persistent journal, and file defragmentation to understand their efficacies and limitations. This study also evaluated a state-of-the-art copyless defragmenter, janusd, to show its advantage over the existing methods.

The decreasing lifetime of NAND flash memory, as a side effect of recent advanced semiconductor process scaling, is emerging as one of major barriers to the wide adoption of SSDs in high-performance computing systems. In this paper, we propose Dynamic Erase Voltage and Time Scaling (DeVTS), an integrated approach to extend the lifetime (particularly, endurance) of NAND flash memory. DeVTS is motivated by our key observation that erasing a NAND block with a lower voltage or at a slower speed can significantly improve NAND endurance. However, using a lower erase voltage causes adverse side effects on the write performance and retention capability of NAND flash memory. In order to improve NAND endurance without affecting the other NAND requirements, we take advantage of idle times between write requests and variations of the retention requirement when writing data to a NAND block erased with a lower voltage. We have implemented a DeVTS-aware FTL, called dvsFTL, which exploits the tradeoff relationship between the endurance and erase voltages/times by accurately predicting the write performance and retention requirements. Our experimental results show that dvsFTL can improve NAND endurance by 94 percent, on average, over an existing DeVTS-unaware FTL while all the NAND requirements are preserved.

For NAND flash-based storage systems, managing garbage collection (GC) efficiently is a critical requirement to achieve both high performance and long lifetimes. In this paper, we propose a just-in-time GC technique, called JIT-GC, which invokes background GC operations only when necessary depending on future write demands. JIT-GC was motivated by our measurement study, which strongly suggested that deciding when to invoke background GC operations is a key parameter for efficient GC. By accurately estimating the amount of future SSD writes, JIT-GC can choose the best time to invoke a background GC operation. JIT-GC reserves necessary free space in advance so that high write performance can be achieved while it extends the SSD lifetime by preventing premature block erasures. Our evaluations on real SSDs show that JIT-GC can achieve both high performance and long lifetimes, thus overcoming the shortcomings of existing background GC invocation heuristics.

We propose an efficient software-based out-of-order scheduling technique, called SOS, for high-performance NAND flash-based SSDs. Unlike an existing hardware-based out-of-order technique, our proposed software-based solution, SOS, can make more efficient out-of-order scheduling decisions by exploiting various mapping information and I/O access characteristics obtained from the flash translation layer (FTL) software. Furthermore, SOS can avoid unnecessary hardware-level operations and manage I/O request rearrangements more efficiently, thus maximizing the multiple-chip parallelism of SSDs. Experimental results on a prototype SSD show that SOS is effective in improving the overall SSD performance, lowering the average I/O response time by up to 42% over a hardware-based out-of-order flash controller.