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Abstract-Random accesses are generally harmful to performance in hard disk drives due to more dramatic mechanical movement. This paper presents the design, implementation, and evaluation of Hot Random Off-loading (HRO), a self-optimizing hybrid storage system that uses a fast and small SSD as a bypassable cache to hard disks, with a goal to serve a majority of random I/O accesses from the fast SSD. HRO dynamically estimates the performance benefits based on history access patterns, especially the randomness and the hotness, of individual files, and then uses a 0-1 knapsack model to allocate or migrate files between the hard disks and the SSD. HRO can effectively identify files that are more frequently and randomly accessed and place these files on the SSD. We implement a prototype of HRO in Linux and our implementation is transparent to the rest of the storage stack, including applications and file systems. We evaluate its performance by directly replaying three real-world traces on our prototype. Experiments demonstrate that HRO improves the overall I/O throughput up to 39% and the latency up to 23%.

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Abstract-SPEC CPU2006 benchmark suite has been extensively studied, with efforts focusing on the requirement understanding of memory workloads from the SPEC CPU2006 suite. However, characterizing SPEC CPU2006 workloads from a time dependence perspective has attracted little attention. This paper studies the auto-correlation functions of the arrival intervals of memory accesses in all SPEC CPU2006 traces, and concludes that correlations in memory inter-access times are inconsistent, either with evident correlations or with little and no correlation. Different with the studies focused on the prior suites, we present that self-similarity exists only in a small number of SPEC2006 workloads. In addition, we implement a memory access series generator in which the inputs are the measured properties of the available trace data. Experimental results show that this model can more accurately emulate the complex access arrival behaviors of real memory systems than the conventional self-similar and independent identically distributed methods, particularly the heavy-tail characteristics under both Gaussian and non-Gaussian workloads.