Accessing a Disk Page
Time to access (read/write) a disk block:
- seek time (moving arms to position disk head on track)
- rotational delay (waiting for block to rotate under head)
- often called “rotational latency”
- transfer time (actually moving data to/from disk surface)
Seek time and rotational delay dominate.
- Seek time varies from about 1 to 20msec
- Rotational delay varies from 0 to 10msec
- Transfer rate is about 1msec per 4KB page
Key to lower I/O cost: reduce seek/rotation delays! Hardware vs. software solutions?