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UNIX Disk Access Patterns

Ruemmler, Chris; Wilkes, John



Abstract: Disk access patterns are becoming ever more important to understand as the gap between processor and disk performance increases. The study presented here is a detailed characterization of every low-level disk access generated by three quite different systems over a two month period. The contributions of this paper are the detailed information we prove about the disk accesses on these systems (many of our results are significantly different from those reported in the literature, which provide summary data only for file-level access on small-memory systems); and the analysis of a set of optimizations that could be applied at the disk level to improve performance. Our traces show that the majority of all operations are writes; disk accesses are rarely sequential; 25-50% of all accesses are asynchronous; only 13-41% of accesses are to user data (the rest result from swapping, metadata, and program execution); and I/O activity is very bursty: mean request queue lengths as large as 89 entries, and maxima of over 1000. Using a simulator to analyze the effect of write caching at the disk level, we found that using a small non-volatile cache at each disk allowed writes to be serviced considerably faster than with a regular disk. In particular, short bursts of writes go much faster - and such bursts are common: writes rarely come singly. Adding even 8 KB of non-volatile memory per disk could reduce disk traffic by 10-18%, and 90% of metadata write traffic can be absorbed with as little as 0.2MB per disk of non-volatile RAM. Even 128 KB of NVRAM cache in each disk can improve write performance by as much as a factor of three. FCFS scheduling for the cached writes gave better performance than a more advanced technique at small cache sizes. Our results provide quantitative input to people investigating improved file system designs (such as log-based ones), as well as to I/O subsystem and disk controller designers.

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