Performance of a two-headed disk system when serving database queries under the scan policy

Abstract

Disk drives with movable two-headed arms are now commercially available. The two heads are separated by a fixed number of cylinders. A major problem for optimizing disk head movement, when answering database requests, is the specification of the optimum number of cylinders separating the two heads. An earlier analytical study assumed a FCFS model and concluded that the optimum separation distance should be equal to 0.44657 of the number of cylinders N of the disk. This paper considers that the SCAN scheduling policy is used in file access, and it applies combinatorial analysis to derive exact formulas for the expected head movement. Furthermore, it is proven that the optimum separation distance is N/2 -1 (TN/2 -11 and LN/2 -1 J) if N is even (odd). In addition, a comparison with a single-headed disk system operating under the same scheduling policy shows that if the two heads are optimally spaced, then the mean seek distance is less than one-half of the value obtained with one head. In fact that the SCAN policy is used for many database applications (for example, batching and secondary key retrieval) demonstrates the potential of two-headed disk systems for improving the performance of database systems.

Why specialized disks for composite operations may be unnecessary

“…Disks with one actuator and two R/W heads have been studied from the viewpoint of minimizing the seek distance by optimizing the R/W head separation [34,3,4,23]. For FCFS scheduling the optimum separation of 0.44657C cuts the expected seek distance for random disk accesses from C/3 [17] to 0.16C [3].…”

Partial match retrieval in two-headed disks

“…These systems exist commercially and it has been proved that they perform better than conventional disks in terms of the distance traveled by the arm mechanism. For example, in [12] it has been shown that the gain in the distance traveled is near 50% if the heads are optimally spaced. This work's contribution is the derivation of the probability distribution for the number of stops of the moving arm.…”

“…In the sequel, we focus on systems having two heads per surface separated by a fixed number of cylinders. For such systems, it has been proved that the optimum separation distance is 0.44657 x C if the FCFS scheduling policy is adopted [1], but if the SCAN scheduling algorithm is applied then this distance should be equal to [C/2] -1 or [C/2] -1 [12]. It is noted that the re-examination of analytical issues in two-headed disk systems is not trivial due to the additional physical constraints.…”

“…The performance of two-headed disks is superior than that of conventional ones because both products of Equation (1) are reduced. The reduction of the distance traveled by the disk heads has been analyzed in the literature [12]. The reduction in terms of the number of arm stops has not yet been evaluated.…”

Αναλυση Επιδοσης Συγχρονων Μαγνητικων Δισκων

“…Στην αναφορά [MaKo89] υπολογίζεται και ο βέλτιστος αναμενόμενος χρόνος αναζήτησης για απόσταση γ -1 (["y -1] και | _^--lj) μεταξύ των κεφαλών. Ο χρόνος αναζήτησης για τον αλγόριθμο SCAN υπολογίζεται από τη σχέση :…”