Seek
A seek consists of four phases
- Speedup - Accelerate arm to max speed or halfway point
- Coast - At max speed
- Slowdown - stops arm near destination
- Settle - adjusts head to actual desired track
Very short seeks dominated by settle time (~1ms)
Short (200-400 cycles) seeks dominated by speedup
Details
Head switch times comparable to short seek times
- May also require head adjustments
Why do settles take longer for writes than for reads
- If read strays from track, we can catch the error
- If write strays from track, we overwritten other tracks
The disk controller circuit keeps table of pivot motor power
- Maps the seek distance to the power and time needed to move that distance