UNIX disk access patterns

TLDR

Disk access patterns are increasingly critical as processor–disk performance gaps widen. This study provides a detailed two‑month characterization of every low‑level disk access from three distinct systems and analyzes disk‑level optimizations that differ markedly from prior literature. The authors used a simulator to evaluate small non‑volatile caches per disk, showing that such caches accelerate write servicing compared to conventional disks. Their traces reveal that most operations are writes, rarely sequential, with 25–50 % asynchronous, only 13–41 % targeting user data, and highly bursty I/O; a 128‑KB NVRAM cache can cut disk traffic by 10–18 % and boost write performance up to three‑fold, with FCFS scheduling outperforming more advanced techniques at small cache sizes, providing quantitative guidance for file‑system and I/O subsystem design.

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 lowlevel disk access generated by three quite different systems over a two month period. The contributions of this paper are the detailed information we provide 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 seen by an incoming request range from 1.7 to 8.9 (1.2‐1.9 for reads, 2.0‐14.8 for writes), while we saw 95th percentile 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 0.2 MB per disk of nonvolatile RAM. Even 128KB 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.