Browse Prior Art Database

Tape Device Buffer Management Scheme

IP.com Disclosure Number: IPCOM000034525D
Original Publication Date: 1989-Mar-01
Included in the Prior Art Database: 2005-Jan-27
Document File: 4 page(s) / 47K

Publishing Venue

IBM

Related People

Christofferson, AH: AUTHOR [+4]

Abstract

A tape device buffer manager achieves the following two major objectives: 1. Keeps the tape drive device in the streaming mode, which is supplying/receiving data to/from the write/read heads at a fast enough rate to maintain forward tape motion. If data is not supplied/received at a quick enough rate, the tape drive is forced to stop tape motion and resume data transfer when the host has caught up. (Image Omitted) 2. Optimizes performance impact on system DASD operation. The usage of the Small Computer System Interface (SCSI) bus by the tape drive device data transfer can unnecessarily degrade DASD performance.

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Tape Device Buffer Management Scheme

A tape device buffer manager achieves the following two major objectives:

1. Keeps the tape drive device in the streaming mode,

which is supplying/receiving data to/from the

write/read heads at a fast enough rate to maintain

forward tape motion. If data is not supplied/received

at a quick enough rate, the tape drive is forced to

stop tape motion and resume data transfer when the host

has caught up.

(Image Omitted)

2. Optimizes performance impact on system DASD

operation. The usage of the Small Computer System

Interface (SCSI) bus by the tape drive device data

transfer can unnecessarily degrade DASD performance. The tape device referred to in this disclosure is operating in the context of a magnetic media subsystem utilizing a SCSI bus as the DASD and tape device controlling interface. A SCSI tape device has a 256 Kbyte data buffer that is used by the device to buffer data on read and write operations. On a write operation, the buffer is filled from the system across the SCSI bus and emptied to the system across the SCSI bus. In both cases, the device SCSI controller controls the rate that the buffer is filled/emptied across the SCSI bus by controlling
1) the burst length that is transferred at one time across the bus and 2) when these burst transfers will take place. The task is to determine a buffer management scheme that will utilize these two variables such that the tape media is kept moving or "streaming", and that other device transfers on the SCSI bus (most importantly - DASD) are not pre-empted to the point where their performance is adversely affected. The following terminology will be used in this description:

Buffer Manager - The logic (microcode or hardware) that

implements the methods/design described in this

document. This is located in the tape device.

Read Buffer Ratio 1 (RBR1) - A device parameter that

indicates how full the buffer shall be prior to

reconnecting to send data to the system.

Read Buffer Ratio 2 (RBR2) - A device parameter that

indicates to the buffer management logic when the

buffer is beginning to become critically full.

Write Buffer Full Ratio (WBFR) - A selectable device

parameter that indicates how full the buffer shall be

prior to starting tape movement.

Direction of data flow is from SCSI Bus ---> Buffer ---> Tape, (Fig. 1).

1. When the buffer is being filled, tape motion and

transfer to the media will not occur until the Buffer

Full Ratio is reached. During this time, transfer

1

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bursts across the SCSI bus to fill the buffer can be

small, so DASD performance is not affected. It is not

critical to keep the tape moving at this point, since

tape motion has not started.

2. When the Buffer Full Ratio is reached, tape

movement starts, and any transfers across the SCSI bus

take place at the normal burst length. The following

two possibilities can happen when at this point:

a. If the buffer becomes full, the SCSI

controller will disconnect from the SCSI...