04_data_access

04_data_access - CPS216: Data-Intensive Computing Systems...

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CPS216: Data-Intensive Computing Systems Data Access from Disks Shivnath Babu
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Outline • Disks • Data access from disks • Software-based optimizations – Prefetching blocks – Choosing the right block size
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Focus on: “Typical Disk” Top View Head assembly Sector Gap Terms: Platter, Head, Cylinder, Track Sector (physical), Block (logical), Gap
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Block Address: • Physical Device • Cylinder # • Surface # • Start sector #
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Disk Access Time (Latency) block X in memory ? I want block X
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Access Time = Seek Time + Rotational Delay + Transfer Time + Other
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Seek Time 3 or 5x x 1 N Cylinders Traveled Time Average value: 10 ms 40 ms
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Rotational Delay Head Here Block I Want
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Average Rotational Delay R = 1/2 revolution Example: R = 8.33 ms (3600 RPM)
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Transfer Rate: t • t: 1 100 MB/second • transfer time: block size t
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• CPU time to issue I/O • Contention for controller • Contention for bus, memory “Typical” Value: 0
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04_data_access - CPS216: Data-Intensive Computing Systems...

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