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Defect Skipping Among Fixed Length Records in Direct Access Storage Devices

IP.com Disclosure Number: IPCOM000086544D
Original Publication Date: 1976-Sep-01
Included in the Prior Art Database: 2005-Mar-03
Document File: 3 page(s) / 63K

Publishing Venue

IBM

Related People

Dohermann, JE: AUTHOR

Abstract

This method executes multi-track read and write operations and minimizes the loss of revolution in the accessing of equal fixed length records stored in a direct access storage device (DASD). This is done by embedding forward/backward pointer, which defines the location of the next defect anywhere on a given cylinder surface in the forward direction. In the backward direction, the location of the next defect is measured from the track index of the current track. This look-ahead feature permits comparison between the length of a sequence to be read or written and the likelihood of intercept with the defect location, and permits changing the command list for either skipping the defect or substituting an alternate block location by an appropriate change in the command list driving the DASD.

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Defect Skipping Among Fixed Length Records in Direct Access Storage Devices

This method executes multi-track read and write operations and minimizes the loss of revolution in the accessing of equal fixed length records stored in a direct access storage device (DASD). This is done by embedding forward/backward pointer, which defines the location of the next defect anywhere on a given cylinder surface in the forward direction. In the backward direction, the location of the next defect is measured from the track index of the current track. This look-ahead feature permits comparison between the length of a sequence to be read or written and the likelihood of intercept with the defect location, and permits changing the command list for either skipping the defect or substituting an alternate block location by an appropriate change in the command list driving the DASD. This, in part, avoids the need of expending gaps, as is found, for example, in US Patent 3,771,143, between a control address field and the data. Relatedly, the backward pointer is used where a track switch does not occur at index. It further is desired to obtain information concerning possible defect locations on the current track. In this regard, the "backward pointer" is a "forward pointer" with a reference at index. This pointer defines the initial defective record on the track. In such DASD systems, the indicia of defective record locations are storable anywhere in the appropriately defined cylinder, i.e., the jth track on each of the q disks.

The method comprises the steps of: reading a predetermined portion of the ith-1 record on the jth track containing a backward/forward pointer, the backward pointer value being constant for every record on the track, and referencing a defect by record identification as measured from the track index, the forward pointer identifying the next sequential record i+m, m=1, 2, 3,...,N containing a defect anywhere on the given cylinder surface; determining whether the accessing of a record will intercept the defect location; and changing the command list for either skipping the defective record or substituting an alternate record.

The method is implemented wherein initially all tracks are written with defect information included within each block. This information includes:
D1 - Distance in blocks to next defective block on cylinder.

A1 - An alternate block identifier to be used in place of the defective block indicated by D1.

D2 - Distance in blocks, beginning at the start of the current track, to the first defective block on the current track.

A2 - An alternate block identifier to be used in place of the defective block indicated by D2.

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The D2 and A2 fields are used when an access request is structured not to cause an automa...