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Disk Basics

Disk Basics. CS 537 - Introduction to Operating Systems. Hard Drive Technology. track. platter. read arm. . cylinder. read head. sectors. track. Hard Drive Technology. Modern Statistics capacity: 20.5 GB sector size: 512 bytes speed: 7200 RPM platters: 3 heads: 6

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Disk Basics

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  1. Disk Basics CS 537 - Introduction to Operating Systems

  2. Hard Drive Technology track platter read arm  cylinder read head sectors track

  3. Hard Drive Technology • Modern Statistics • capacity: 20.5 GB • sector size: 512 bytes • speed: 7200 RPM • platters: 3 • heads: 6 • cylinders: 17,688 • media transfer rate: 35 MB/sec • interface transfer rate: 14.1 - 23.4 MB/sec • cost: $127.00

  4. Hard Drive Terminology • Seek Latency • time to move the read head to correct cylinder • avg = 9.0 ms • track-to-track = 2.2 ms • full-track = 15 ms • Rotational Latency • time for disk to rotate to needed sector • avg = 4.17 ms • Transfer Latency • time to actually read and send the data • 50 s for 1 KB of data

  5. Disk Access • Cannot access any random byte on a disk • this would be called random access (RAM) • However, don’t have to access them sequentially • have to go through all data to get to desired data (tape drive) • Disks must be accessed per sector • each sector is 512 bytes • this is called direct access

  6. Reading from Disk • To read a single byte from disk • provide address of sector • cylinder, head, sector number • seek to location of data on disk • read entire sector into memory (512 bytes) • index into memory to grab the necessary byte

  7. Writing to Disk • To write a single byte out to disk • read entire sector into memory • modify the necessary byte in memory • seek back to disk location • write the entire sector back to disk • Notice 2 times avg. latency required to write a single block

  8. Disk Performance • Disk is a mechanical device • extremely SLOW!! • Can use DMA and context switching to hide latency • this will only help so much • Want to interact with physical disk as little as possible • Reading and writing a single byte is a bad idea

  9. Prefetching Blocks • If the user wants data in sector 23, odds are they will eventually want data in sector 24 • this assumes intelligent placement of data on disk to start with - more on this later • Amortize the latencies by reading in sector 23 and sector 24 • when user does actually want sector 24, already in memory • Most disks are read in a page at a time • 4 KB of data (8 sectors)

  10. Disk Buffering • If byte 55 of sector 26 is modified, high probability byte 56 of sector 27 will also be modified • Usually wait awhile to write back a sector that is currently in memory • hopefully wait until all data that will be modified is modified • again, allows amortization of latencies over a large number of writes

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