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Device Disconnect Procedure for Increased SCSI Bus Throughput

IP.com Disclosure Number: IPCOM000034725D
Original Publication Date: 1989-Apr-01
Included in the Prior Art Database: 2005-Jan-27
Document File: 3 page(s) / 41K

Publishing Venue

IBM

Related People

Movall, PE: AUTHOR [+2]

Abstract

A software method is provided by which disconnected status from a small computer systems interface (SCSI) device can efficiently be used by the host system to increase SCSI bus throughput as well as overall system performance. The SCSI standard is local I/O bus developed to provide host systems with a device independent interface within a class of devices. The SCSI standard, ANSI X3.131-1986, provides mechanical, electrical, and functional specifications. It contains provisions for the connection of multiple initiators (SCSI devices capable of initiating an operation) and multiple targets (SCSI devices capable of responding to a request to perform an operation). In a typical system with a local SCSI I/O bus (see Fig. 1), the host (initiator) will issue commands to the various SCSI devices (targets) via the SCSI adapter card.

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Device Disconnect Procedure for Increased SCSI Bus Throughput

A software method is provided by which disconnected status from a small computer systems interface (SCSI) device can efficiently be used by the host system to increase SCSI bus throughput as well as overall system performance. The SCSI standard is local I/O bus developed to provide host systems with a device independent interface within a class of devices. The SCSI standard, ANSI X3.131-1986, provides mechanical, electrical, and functional specifications. It contains provisions for the connection of multiple initiators (SCSI devices capable of initiating an operation) and multiple targets (SCSI devices capable of responding to a request to perform an operation). In a typical system with a local SCSI I/O bus (see Fig.
1), the host (initiator) will issue commands to the various SCSI devices (targets) via the SCSI adapter card. These SCSI devices may include optical disk drives, tapes drives, and hard disk drives.

These commands may include reads, writes, seeks, and various status and selection commands. Within the SCSI standard, there is a provision to allow a target device to disconnect from the bus during a long operation in which it does not use the bus. As an example, the tape drive can be commanded to perform a retention operation to the tape. This operation takes several minutes to complete, during which the bus is not required. The drive may therefore disconnect from the bus, thereby freeing the bus. During this time, the bus is free to be used by another device. If this disconnect status is returned from the adapter to the device driver, the device driver, in turn, can effectively pass this information to the calling application to significantly increase SCSI bus throughput. This method requires only software, providing the SCSI device driver code is notified by the SCSI adapter card of a device disconnect. The key pieces of this software mechanism include: * Return fr...