Browse Prior Art Database

Recovery from "Machine Check" in Buffered Tape System

IP.com Disclosure Number: IPCOM000089595D
Original Publication Date: 1977-Nov-01
Included in the Prior Art Database: 2005-Mar-05
Document File: 2 page(s) / 13K

Publishing Venue

IBM

Related People

Cassie, JV: AUTHOR [+4]

Abstract

A buffered tape system is provided with a new command and interlocks to enable an operating system to restart a job if and when a "machine check" condition occurs during read or write operations.

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This is the abbreviated version, containing approximately 52% of the total text.

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Recovery from "Machine Check" in Buffered Tape System

A buffered tape system is provided with a new command and interlocks to enable an operating system to restart a job if and when a "machine check" condition occurs during read or write operations.

Conventionally, during write operations, data is transferred fast from a channel into a buffer control unit, and such transfer will continue as long as a "device end" signal is sent back to the channel after each block is received. Data transfer from the buffer is delayed until the tape drive is up to speed, whereupon transfer continues until the buffer is empty. If a "machine check" occurs at a time when the channel indicates that M blocks have been written into the buffer but in fact less than M blocks have actually been transferred from the buffer to the tape drive, under conventional error recovery procedures the user will not be able to space forward an appropriate number of blocks from a check point. In such case, all blocks from that check point will have to be reprocessed for recovery.

To overcome this, a "flush buffer" command is provided. Any check point or tape mark generation command will be associated with the "flash buffer" command, which causes the control unit to respond with "device end" status only when all data from the buffer is written on tape. This establishes check-point synchronization between the drive and user program. Also, a tape mark is written on the tape before the channel is released. After a block is transferred, a "device end" will be given only if the number of blocks B in the buffer still to be written on the tape is equal to or less than a preselected threshold number C, thus limiting the number of unwritten blocks in the buffer at any time to C (which is adjusted in conjunction with the actual drive characteristics). Now when a "machine check" occurs, the user program will roll the job back to a previously well-defined check...