Disk Access Time Example
Disk Parameters:
- Transfer size is 8K bytes
- Advertised average seek is 12 ms
- Disk spins at 7200 RPM
- Transfer rate is 4 MB/sec
Controller overhead is 2 ms
Assume that disk is idle so no queuing delay
What is Average Disk Access Time for a Sector?
- Ave seek + ave rot delay + transfer time + controller overhead
- 12 ms + 0.5/(7200 RPM/60) + 8 KB/4 MB/s + 2 ms
- 12 + 4.15 + 2 + 2 = 20 ms
Advertised seek time assumes no locality: typically 1/4 to 1/3 advertised seek time: 20 ms => 12 ms