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Disk Average Seek Time

Disk Average Seek Time. Multi-platter Disk. read/write head. read/write arm. Disk. platter. track. cylinder. sector. Multi-platter Disk. Seek Time. measures the amount of time required for the read/write heads to move between tracks over the surface of the platters. 0.0 ms. 0.8 ms.

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Disk Average Seek Time

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  1. Disk Average Seek Time

  2. Multi-platter Disk read/write head read/write arm Disk platter

  3. track cylinder sector Multi-platter Disk

  4. Seek Time measures the amount of time required for the read/write heads to move between tracks over the surface of the platters 0.0 ms 0.8 ms platter Average Seek Time =

  5. Rotational delay After the read/write arm has picked the right track, the time it takes for the requested sector (on that track) to come under the read/write head 0.0 ms 0.2 ms read/write arm platter

  6. Avg. seek time = Other measurements Avg. Rotation latency / rotational delay = latency halfway around the disk Transfer time = the time it takes to transfer a block of bits, typically a sector, under the read/write head - Function of the size, rotational speed, recording density of a track, speed of electronics connecting disk to computer Controller Overhead time = the overhead the controller imposes in performing I/O access

  7. Example taken from: Computer architecture by J. Hennessy and D. Patterson What is avg. time to read or write (access time) a 512-byte sector for a typical disk? Avg. seek time (ST) = 5ms Transfer time (TT) = 40 MB/sec Rotational delay (RD) = 10,000 RPM Controller overhead (CO) = 0.1 ms Assume the disk is idle and so no queuing delay dat = ST + RD + TT + CO = 5ms + (0.5/10,000 rpm) + (0.5KB/40.0 MB/sec) + 0.1 ms = 5.0 + 3.0 +0.013 + 0.1 = 8.11 ms Assuming the measured seek time is 33% of the calculated average. So we obtain: dat = 1.67 ms + 3.0 ms + 0.013 ms + 0.1 ms = 4.783 ms Note that 0.013/4.783 = 0.3% is the disk transferring data in this example

  8. Disk Scheduling

  9. Disk Scheduling Access Time: seek time + rotational delay + transfer time + controller overhead Transfer Time: time to copy the block to and from the disk Disk Bandwidth: total number of bytes transferred, divided by the total time between the first request for service and the completion of the last transfer Seek Time: the for the disk arm to move the heads to the cylinder containing the desired sector. Rotational Delay: time waiting for the disk to rotate the desired sector to the disk head

  10. Disk Scheduling Disk Head Scheduler: After the completion of the current operation, the disk head scheduler examines the current contents of the queue and decides which request to be served next. The Disk Head Scheduler must consider 3 important factors 1. overall performance of the disk 2. fairness in treating processes 3. cost of executing scheduling algorithm Scheduling Algorithms First-In/First-Out (FIFO) Shortest Seek Time First (SSTF) Scan or Elevator Look

  11. 98 98 183 183 37 37 122 122 14 14 124 124 65 65 67 67 Head at 53 45 45 85 + 85 146 + 146 85 + 85 108 + 108 110 + 110 59 + 59 2 + 2 = 640 Disk Scheduling First-In/First-Out FIFO Head at 53 0 14 37 53 65 67 98 122 124 183 199

  12. 98 98 183 183 37 37 122 122 14 14 124 124 65 65 67 67 Head at 53 12 12 2 + 2 30 + 30 23 + 23 84 + 84 24 + 24 2 + 2 59 + 59 = 236 Disk Scheduling Shortest Seek Time First - SSTF Head at 53 0 14 37 53 65 67 98 122 124 183 199

  13. 98 98 183 183 37 37 122 122 14 14 124 124 65 65 67 67 Head at 53 16 16 + 23 23 + 14 14 + 65 65 + 2 2 + 31 31 + 24 24 + 2 2 + 59 59 = 236 Disk Scheduling Scan Head at 53 0 14 37 53 65 67 98 122 124 183 199

  14. 98 98 183 183 37 37 122 122 14 14 124 124 65 65 67 67 Head at 53 16 16 23 + 23 51 + 51 2 + 2 31 + 31 24 + 24 2 + 2 59 + 59 = 208 Disk Scheduling Look Head at 53 0 14 37 53 65 67 98 122 124 183 199

  15. s1 s3 s2 98 98 183 37 122 14 124 65 67 Head at 53 45 45 Disk & Sector Scheduling First-In/First-Out FIFO Head at 53 0 14 37 53 65 67 98 122 124 183 199

  16. s1 s2 s3 98 98 183 37 122 14 124 65 67 Head at 53 45 45 Disk & Sector Scheduling First-In/First-Out FIFO Head at 53 0 14 37 53 65 67 98 122 124 183 199

  17. s2 s3 98 98 183 37 122 14 124 65 67 Head at 53 45 45 Disk & Sector Scheduling First-In/First-Out FIFO Head at 53 0 14 37 53 65 67 98 122 124 183 199

  18. 98 183 37 122 14 124 65 67 Head at 53 45 45 Disk & Sector Scheduling First-In/First-Out FIFO s3 Head at 53 98 0 14 37 53 65 67 98 122 124 183 199

  19. 98 183 37 122 14 124 65 67 Head at 53 45 45 Disk & Sector Scheduling First-In/First-Out FIFO Head at 53 98 0 14 37 53 65 67 98 122 124 183 199

  20. 98 98 183 183 37 37 122 122 14 14 124 124 65 65 67 67 Head at 53 45 45 85 + 85 146 + 146 85 + 85 108 + 108 110 + 110 59 + 59 2 + 2 = 640 Disk & Sector Scheduling First-In/First-Out FIFO Head at 53 0 14 37 53 65 67 98 122 124 183 199

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