Browse Prior Art Database

Synchronous Pair Block Recording

IP.com Disclosure Number: IPCOM000050181D
Original Publication Date: 1982-Sep-01
Included in the Prior Art Database: 2005-Feb-10
Document File: 4 page(s) / 80K

Publishing Venue

IBM

Related People

Baird, R: AUTHOR [+2]

Abstract

This invention relates to methods and means for alternately updating a record and its copy in order to minimize DASD (direct access storage device) track latency. In this method, multiple DASD updates to records are made for reduction of the queuing time against record accessing. The method provides a synchronous write to DASD such as is required for a recovery log. Upon completion of a write operation, a designated data block is recorded on the DASD medium. The same data block may be written repeatedly, adding new information each time, such as occurs when records are added to a block. If an error occurs while rewriting a given data block, the immediately preceding version of the block is not affected. That is, the most current version of a given data block is always available for recovery from the physical medium.

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Synchronous Pair Block Recording

This invention relates to methods and means for alternately updating a record and its copy in order to minimize DASD (direct access storage device) track latency. In this method, multiple DASD updates to records are made for reduction of the queuing time against record accessing. The method provides a synchronous write to DASD such as is required for a recovery log. Upon completion of a write operation, a designated data block is recorded on the DASD medium. The same data block may be written repeatedly, adding new information each time, such as occurs when records are added to a block. If an error occurs while rewriting a given data block, the immediately preceding version of the block is not affected. That is, the most current version of a given data block is always available for recovery from the physical medium. An activation can be synchronized in its processing with the successful completion of a write operation and be assured of its reliability. The method and means include a DASD track format, a data recording method, and a synchronization method which in combination enhance the response, reliability of this serial I/O facility.

A DASD track is formatted into pairs of fixed size blocks. The first block of each pair is the primary, and the second block of a pair is the secondary block. Each block must contain a time stamp, version number and/or some identification that distinguishes one block from another. All blocks of a single track represent different versions of the same block A variation of this uses two tracks; one for the primary block and one for the secondary block. The records for primary blocks are denoted by the number 1; the records for the secondary blocks are denoted by the number 2. This arrangement permits orientation on either a primary or secondary block. Either a 1-byte record key or the record ID can be used to designate whether a record is the primary or secondary record of a pair.

Write operations alternate between primary and secondary blocks. This guarantees that data previously in the block is preserved in case of a write error. Since orientation occurs on any arbitrary pair, the latency is reduced from 1/2 disk rotations to n/2 disk rotations, where n is the number of block pairs on a track.

The above paired block recording technique is used in the absence of a hardware function to guarantee that the previously written block is not overwritten. A hardware function that would allow a "WRITE" to be chained to a "SEARCH NOT EQUAL" would not require pairs of blocks.

The feedback from the READ SECTOR command can be used to orient to the exact block in most backout and recovery situations. In some cases, the entire track must be read to obtain the most current version of the block (based upon The time stamp, version number, etc.).

Since the recording technique requires an entire track to support each data block, this mechanism is used in conjunction with a conventional p...