Magnetic tape packet assembler/disassembler safeguards existing data

Abstract

A tape packet assembler/disassembler (TPAD) for a tape drive accumulates host-transmitted records, which may or may not be blocked by the host. The interblock gaps that would ordinarily separate the host-transmitted records now separate packets, resulting in a considerable increase in the efficiency of tape utilization. When the user attempts to modify an existing collection of application records the entire packet containing that collection must first be read to place its content into the TPAD, the revision created in the buffer, and then a revised version of the entire packet written back to the tape. A failure in this process can result in the loss or corruption of data in that part of the packet preceding the revision. This loss can come as a surprise and be unacceptable to the user. To prevent such corruption or loss the TPAD does not attempt to overwrite the original version of the packet receiving the modification until it has successfully written a demonstration packet (equal in content to the revised packet) a safe distance down the tape beyond the packet to be overwritten. Only then is an attempt made to perform the actual overwrite on the original packet. If it succeeds the demonstration packet is ignored and may eventually be overwritten by subsequent activity. If the overwrite of the original packet fails for any reason, it is simply erased, creating a large (and harmless) interblock gap ahead of the demonstration packet, which now serves as the permanent replacement version.