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Object-Oriented Windowing Technique to Manage the SCSI Interface

IP.com Disclosure Number: IPCOM000112536D
Original Publication Date: 1994-May-01
Included in the Prior Art Database: 2005-Mar-27
Document File: 4 page(s) / 103K

Publishing Venue

IBM

Related People

Basham, RB: AUTHOR [+2]

Abstract

An object-oriented windowing method to manage SCSI bus error recovery sequences for large data blocks is described.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 52% of the total text.

Object-Oriented Windowing Technique to Manage the SCSI Interface

      An object-oriented windowing method to manage SCSI bus error
recovery sequences for large data blocks is described.

      Object-Oriented SCSI Error Recovery Management

      In an object-oriented approach to the SCSI protocol, the number
of objects needed to represent a SCSI data transfer sequence for a
read or write command can be very large for large data blocks with a
small burst size.  Creating and deleting all these objects generates
too much overhead.

      This solution allows for a clean, object-oriented solution for
the SCSI protocol error recovery management that also gives
acceptable performance during normal operation.  This was done using
an hierarchy of objects and a windowing technique to limit the number
of objects needed for a given SCSI command.

      The SCSI Object Hierarchy

      As a SCSI IO Process executes, it transfers information across
the SCSI bus between the SCSI Initiator (the one that issued the
command) and the SCSI Target (the one that executes the command).
The information is transferred in different phases listed below:

o   Data In

o   Data Out

o   Command

o   Status

o   Message In

o   Message Out

      The phases and order phases are selected giving it structure
and meaning to Initiator and Target communications.  Each coherent
piece of information is called an Interface Logical Element (a term
used in the ANSI SCSI-3 Packetized Protocol draft standard).

      From the perspective of the SCSI bus, an IO Process can be
looked at as a sequence of ILEs.  A write command transferring a
single 32 Kilobyte block with a 4 Kilobyte maximum burst size is
represented by the sequence of ILEs shown in the attached Fig. 1, ILE
Sequence For a 32 Kilobyte Write Command.

As the block size increases or the maximum burst size decreases, the
number of ILEs increase.

      From the standpoint of error recovery, this level of detail is
necessary.  However during a normal data transfer, this level of
detail is not needed.  The hardware manages breaking up the data from
blocks into bursts.  These data blocks are called Target DMA
Elements.  So a 32 Kilobyte block would be represented by one object.

      There is still another layer of data transfer, that being how
much the hardwar...