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Sorting Paging I/O Requests for Direct Access Device Using Track Overflow

IP.com Disclosure Number: IPCOM000081283D
Original Publication Date: 1974-May-01
Included in the Prior Art Database: 2005-Feb-27
Document File: 3 page(s) / 42K

Publishing Venue

IBM

Related People

Johnson, EL: AUTHOR

Abstract

A technique for addressing paging I/O records (slots) on a direct-access device is described in the IBM Technical Disclosure Bulletin article entitled "Track Overflow in a Relative Byte Address Environment," published October 1193, Vol. 16, No. 5, pages 1381 to 1385. The present publication deals with a technique that may be used to sort a number of I/O requests in such an environment, so that they may be most efficiently accessed. As in the referenced article, this method is used for records within a single cylinder.

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Sorting Paging I/O Requests for Direct Access Device Using Track Overflow

A technique for addressing paging I/O records (slots) on a direct-access device is described in the IBM Technical Disclosure Bulletin article entitled "Track Overflow in a Relative Byte Address Environment," published October 1193, Vol. 16, No. 5, pages 1381 to 1385. The present publication deals with a technique that may be used to sort a number of I/O requests in such an environment, so that they may be most efficiently accessed. As in the referenced article, this method is used for records within a single cylinder.

Shown is a given allocation unit consisting of 8 tracks and containing 25 (0- 24) slots. Sorting proceeds by first calculating for each requested Slot Number (SN) a Rotational Position Number (RPN). The Rotational Position Number is given by the following equation. RPN = (SN.T) Modulo S where T = the number of tracks in an allocation unit; S = the number of slots in an allocation unit.

After the RPN is calculated for each Slot Number corresponding to an I/O request, the sort method takes as the first request-to-process the Slot Number (SN) with the lowest RPN. Successive requests are selected by the following iterative process. If the next sequential slot (SN+1) is among the Slot Numbers requested, it is selected next. If the next sequential Slot Number is not among those requested, the next request-to-process is chosen by using the RPN of the Slot Number selected last. The rotational position (N) following the slot just selected is calculated by the formula: N = (RPN + T) Modulo S.

The next slot chosen is the Slot Number whose RPN is closest to N and greater than N. The RPN comparison is circular. If there is no RPN greater than N among the requested Slot Numbers, then the smallest RPN is chosen. This process proceeds until all of the Slot Numbers have been sorted into the most optimum I/O sequence.

A simple example cont...