Accessing a Disk Page
Time to access (read/write -- 存取时间) a disk block:
seek time (寻道时间-moving arms to position disk head on track)
rotational delay (旋转延迟-waiting for block to rotate under head)
transfer time (传输时间-actually moving data to/from disk surface)
Seek time and rotational delay dominate.
Seek time varies from 0 to 10msec . – 平均时间
Rotational delay varies from 0 to 3msec
Transfer rate around 0.2msec per 8K block
Key to lower I/O cost: reduce seek/rotation delays!
Hardware vs. software solutions?