The techniques presented in Section 2 rely on the driver's ability to accurately predict the disk head location and the cost of disk operations such as track switches, seeks, and rotational placement. The driver also needs information on the layout of the physical sectors on the disk.
Previous proposals that depend on the knowledge of head positions have relied on hardware support [5,27]. Unfortunately, this level of support is not always available on commodity drives. We have developed a software-only head-tracking method. Our scheme requires issuing read accesses to a fixed reference sector at periodic intervals. The head tracking algorithm computes the disk head position based on the time stamp taken immediately after the completion of the most recent read operation of the reference sector. The basic idea is that the time between two read accesses of the reference sector is always an integral multiple of the full rotation time plus an unpredictable OS and SCSI overhead. By gradually increasing the time interval between adjacent read requests to the reference sector, we amortize the overhead of reading the reference sector. Our experiments show that periodic re-calibration at an interval of two minutes yields predictions that have an error of only 1% of a full rotation time with 98% confidence. It is encouraging that we can achieve a high degree of accuracy with a low overhead associated with reading the reference sector every two minutes. To further reduce this overhead, we can exploit the timing information and known disk head location at the end of a request. We have not implemented this optimization.
To obtain an accurate image of the disk, we use methods that are similar to those used by Worthington  for determining the logical to physical sector mapping. We obtain information on disk zones, track skew, bad sectors, and reserved sectors through a sequence of low-level disk operations.
The last piece of information that we measure is the cost of performing track switches and seeks. Small errors in these timing measurements may introduce a penalty that is close to a full rotation. To reduce the number of rotation misses, we introduce a slack of k sectors so that when the mechanism predicts the head to be less than k sectors away from the target, the scheduler conservatively chooses the next rotational replica after the target. This slack can be adjusted by a real time feedback loop to ensure that more than 99% of the requests are on target.