![]() A hard drive’s “geometry” describes how data is arranged into platters, tracks, and sectors. Historically, this was described using three numbers: Cylinders (number of concentric rings from outside to inside), Heads (number of recording surfaces, or the number of tracks per cylinder), and Sectors per track, leading to the well-known acronym CHS. The capacity of a hard drive in sectors is simply C×H×S. Today, C and S are variable and only H is still constant. The number of tracks is not necessarily the same on each recording surface, and the number of sectors per track varies across the disk (more sectors in the longer outer tracks than the inner tracks). This article describes several microbenchmarks that try to extract the physical geometry of hard disk drives, and a few other related measurements. These measurements include rotation period, the physical location (angle and radius) of each sector, track boundaries, skew, seek time, and some observations of defective sectors. I use these microbenchmarks to characterize a variety of hard drives from 45 MB (1989) to 5 TB (2015). There is no attempt to characterize other important performance aspects such as caching. The remainder of this article begins with a background on hard drive geometry. It then describes the collection of microbenchmarks, starting with a basic read access time measurement and building towards increasingly complex algorithms. Data is arranged on recording surfaces (2 sides per platter), tracks, and sectors.īackground: Hard drives consist of spinning disks, and a stack of heads.The second part of the article presents microbenchmark measurements for each of the 17 drives that were tested.
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