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Virtual Sectoring on a Disk File

IP.com Disclosure Number: IPCOM000079860D
Original Publication Date: 1973-Sep-01
Included in the Prior Art Database: 2005-Feb-26
Document File: 5 page(s) / 35K

Publishing Venue

IBM

Related People

Grossman, DD: AUTHOR [+2]

Abstract

On a disk file, each track is divided into n sectors, each with its own individual address from 0 to n-1. Among the values of n in common use are 8, 12, 16, and 24. Assume that n=24.

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Virtual Sectoring on a Disk File

On a disk file, each track is divided into n sectors, each with its own individual address from 0 to n-1. Among the values of n in common use are 8, 12, 16, and 24. Assume that n=24.

Consider a sequential data set which is stored on such a disk, occupying a number of sectors which is at least 2 and possibly more than n. Assume that the data set consists of exactly 12 sectors.

Suppose there is a space constraint on computer nain memory, so a program can not allocate internal buffers large enough to read or write more than a few sectors at a time. Assume the main memory buffer is big enough for one sector at a time only. Suppose that the computer program contains the steps shown in the figure. The program contains a loop in which it reads and then writes each sector in sequence. The class of such programs includes editors, garbage collection routines, and sort/ merge routines.

Let T be the time for one rotation of the disk. Assume the time required for step C in the figure plus delay in servicing the end of read interrupt is much Less than T. Assume that the time required for steps E and F plus the delay in servicing the end of write interrupt is more than the duration of the intersector gap, but much much less than T. The speed of program execution is sharply limited by disk rotation delay. The method described here reduces total rotational delay in this class of problem. This method is termed Virtual Sectoring. By Virtual Sectoring, reductions in total rotational delay of up to about 90% occur.

With a 12-sector data set on a 24-sector track, a common method is to arrange data sequentially in sectors 0 through 11. Total rotational delay involved in executing the flow chart is defined below.

On the average, there is a delay of T/2 before Sector 0 is in position to be read. Not until T later is Sector 0 in position to be written. The instant this write is finished, Sector 1 is in position to be read. It was assumed, the time required for flow chart steps E and F plus the delay in servicing the end of write interrupt is more than the duration of the intersector gap, so the disk must make a full rotation before Sector 1 is read. A delay of 25T/24 from the start of the Sector 0 write to the start of the Sector 1 read results. Accesses continue thusly until the start of the Sector 11 write There is then a final delay of T/24 until this write is done.

When all the rotation delays are added up, the total is found to be T/2+12T+11 25T/24+T/24=24T. Out of this 24T, it is seen that T/2 is the initial delay, T is actually used for reads and writes, and the remainder is wasted delay.

In this method, the applications program refers to the 12-sector data set by virtual sector numbers, which run sequentially from 0 to 11. The physical arrangement of these sectors on the disk is not sequential. When an applications program needs to READ virtual sector V, the operating system reads this data from physical sector R. W...