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Selecting Small Computer Systems Interface Interrupt Frequencies for Optimal File Performance

IP.com Disclosure Number: IPCOM000105553D
Original Publication Date: 1993-Aug-01
Included in the Prior Art Database: 2005-Mar-20
Document File: 2 page(s) / 51K

Publishing Venue

IBM

Related People

Gramann, PA: AUTHOR

Abstract

A formula for determining the optimal frequency for SCSI DMA interrupts is disclosed that takes SCSI data rate, disk data rate, number of data and non-data bytes per sector, and interrupt processing times into consideration.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 65% of the total text.

Selecting Small Computer Systems Interface Interrupt Frequencies for Optimal File Performance

      A formula for determining the optimal frequency for SCSI DMA
interrupts is disclosed that takes SCSI data rate, disk data rate,
number of data and non-data bytes per sector, and interrupt
processing times into consideration.

      To determine the optimal frequency for the SCSI DMA interrupts,
the time required to completely process both a SCSI DMA interrupt
(Is) and a disk transfer interrupt (Id) must be known.  These times
are constant (i.e., they do not depend on the sector size).  The
other constants required are the number of overhead (non-data) bytes
in each sector (Od), the raw disk transfer rate (Rd), and the raw

SCSI transfer rate (Rs).  The only variable is the sector size (Nd).
When all of the constants are known and a sector size is chosen, the
disk interrupt period (Pd) and the disk leftover time (Ld) can be
derived.

 Pd = (Nd + Od) over (Rd)

 Ld = (Pd) - (Id)

      Since the SCSI DMA processing must be done in the "disk
leftover time", an integer C is calculated that represents the number
of "disk leftovers" required to process the SCSI DMA interrupt.

 C = | (Is) over (Ld) |
p.  In order to ensure that SCSI DMA process does not stall, the
processing for the SCSI DMA interrupt must complete before the next
SCSI DMA interrupt occurs.  The worst case scenario occurs when both
the SCSI DMA interrupt and the disk transfer interrupt happen at the...