Method and means for ensuring single pass small read/write access to variable length records stored on selected DASDs in a DASD array
First Claim
1. In a system comprising an array of N DASDs for storing records in row major order on counterpart tracks of N-1 of said DASDs and for storing a parity image of the counterpart tracks on the Nth DASD of said array, and a control unit responsive to externally supplied read or write requests for accessing records on said array,a method for write updating selected ones among a set of variable length records sorted on DASD tracks, each record including at least a count field and a variable length data field, the fields within a record and between records on each DASD track being spaced apart by a predetermined track extent termed a "gap", any arbitrarily addressable DASD track being denominated as an "ith track", comprising the steps by said control unit of:
- (a) generating a byte level exclusive OR simple parity image of the fields and gaps included within each of the variable length records from each counterpart ith track across N-1 DASDs, and, storing said image on an ith track of the Nth DASD, each gap being coded as an arbitrary constant value (000); and
(b) responsive to each write request to a record stored on a selected DASD,(1) establishing a byte position offset of the record to be modified on the ith track of the selected DASD and the record identify within the parity image on the ith track of the Nth DASD,(2) obtaining and modifying said record from the ith track of the selected DASD and parity from the ith track of the Nth DASD, said parity being modified as the exclusive OR image of the old record, the record modification, and the old parity; and
(3) rewriting said modified record and parity back to the ith track on the selected and Nth DASDs respectively.
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Accused Products
Abstract
Write update of variable length records stored in row major order on an array of N DASDs is facilitated by utilizing the correlation between byte offsets of a variable length record and the byte offset of a byte level parity image of data stored on the same track across N-1 other DASDs.
58 Citations
6 Claims
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1. In a system comprising an array of N DASDs for storing records in row major order on counterpart tracks of N-1 of said DASDs and for storing a parity image of the counterpart tracks on the Nth DASD of said array, and a control unit responsive to externally supplied read or write requests for accessing records on said array,
a method for write updating selected ones among a set of variable length records sorted on DASD tracks, each record including at least a count field and a variable length data field, the fields within a record and between records on each DASD track being spaced apart by a predetermined track extent termed a "gap", any arbitrarily addressable DASD track being denominated as an "ith track", comprising the steps by said control unit of: -
(a) generating a byte level exclusive OR simple parity image of the fields and gaps included within each of the variable length records from each counterpart ith track across N-1 DASDs, and, storing said image on an ith track of the Nth DASD, each gap being coded as an arbitrary constant value (000); and (b) responsive to each write request to a record stored on a selected DASD, (1) establishing a byte position offset of the record to be modified on the ith track of the selected DASD and the record identify within the parity image on the ith track of the Nth DASD, (2) obtaining and modifying said record from the ith track of the selected DASD and parity from the ith track of the Nth DASD, said parity being modified as the exclusive OR image of the old record, the record modification, and the old parity; and (3) rewriting said modified record and parity back to the ith track on the selected and Nth DASDs respectively. - View Dependent Claims (2, 3, 4)
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5. In a system comprising an array of N DASDs for storing records in row major order on counterpart tracks of N-1 of said DASDs and for storing a parity image of the counterpart tracks on the Nth DASD of said array, and a control unit responsive to externally supplied read or write requests for accessing records designated in said requests,
a method for ensuring single pass access to a selected ones of the DASDs especially suited for small track read and write access of variable length records, each record including at least a count field and a variable length data field, the fields within a record and between records on each DASD track being spaced apart by a predetermined track extent termed a "gap", any arbitrarily addressable DASD track being denominated as the "ith track", comprising the steps by said control unit of: -
(a) creating a virtual image as a byte organized result of an exclusive OR operation upon counterpart bytes on the counterpart tracks across N-1 DASDs, and mapping the virtual image into at least one variable length formatted record on the Nth DASD, said counterpart bytes on the counterpart tracks include the fields and gaps within each of the variable length records, each gap being coded as an arbitrary constant value (000); and (b) responsive to a write request on accessing a record on the ith track on a selected on of the N-1 DASDs, (1) ascertaining the byte offset of the record to be updated on the ith track of the selected DASD and the record identify and byte offsets within the parity image on the ith track of the Nth DASD; (1) obtaining and modifying said record from the ith track of the selected DASD and the parity from ith track of the Nth DASD, said parity being modified as the exclusive OR image of the old record, the record modification, and the old parity; and (3) rewriting said modified record and parity back to ith track on the Nth and selected DASD respectively. - View Dependent Claims (6)
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Specification