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Disk technology (and, indeed, magnetic recording technology in general) is a modern success story. Significant advances have been made in data recording density, access time, and storage costs. The goal of an OEM cost of a dollar per megabyte of disk storage will be achieved in the near future. |
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The problem for the systems designer is that the processor technology has outstripped several aspects of disk technology. Processing speeds have increased much more rapidly than disk access times have decreased. Some historic growth data for processors and storage are: |
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1. Processor speed (MIPS) increases at 2x per year. |
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2. Memory size (MB/die) increases at 2x per 2 years. |
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3. Disk density (bits/unit area) increases at 2x per 3 years. |
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4. Disk transfer (MBps) increases at 2x per 5 years. |
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5. Disk access time (ms) decreases at 2x per 10 years. |
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Despite this, much of the progress has been eclipsed by even more rapid improvements in processor and memory technology. In the areas of both access time and transfer rate, the processor technology has significantly overrun improvements in disk technology. It is this disparity that warrants our careful attention to the effective use of the disk. Illustrating this, Figure 9.9 projects the number of instructions executed in a 20-ms period that corresponds to a typical disk access time. |
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The disk remains the mainstay of the storage system because: |
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1. It represents nonvolatile storage. |
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2. Storage costs are less than one-twentieth of those of DRAM per byte. |
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With the advent of large, relatively low-cost main memory systems (low cost DRAM), the memory system can be used to complement the disk ensemble and maintain relatively high processing rates for the system. |
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Data is stored bit-serial on the surface of a disk platter. Storage tracks are laid out in concentric circles across the surface of the platter. (See Figure 9.10.) The disk device itself consists of a stack of platters. Each platter has two surfaces for data recording. The surface of each platter is divided into tracks, and each track is further divided into sectors. The sector is the smallest addressable data unit. |
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The set of tracks of equal diameter on all disk surfaces is called the cylinder, the cylinder contains the maximum amount of information that can |
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