Glossary; Figure 2. Scsi Client-Server Model - Seagate Ultra160 Product Manual

SCSI Interface Product Manual, Rev. B
Note. In this revision, some new terminology is introduced as taken from the ANSI specifications. In many
instances, the broader scope terms such as "initiator" and "target" are not used, but rather the more
specific terms "Application Client" and "Device Server" appear. In Figure 2, it can be seen that several
"application clients" from a single initiator may have one or more tasks in queue with several "device
servers" in a single target. A drive could be a SCSI target port or it could be one of the device servers
as part of some larger entity. When reading the description, one needs to be able to put the drive of
interest in the proper context in terms of what is shown in Figure 2. For a proper understanding of the
operation of the SCSI protocol, the terms in the SCSI architectural model as described in ANSI specifi-
cation T10/1157D (SAM-2) should be well understood before reading operation descriptions in any
SCSI document. Although a Glossary of terms is provided herein, the definitions may not be adequate
for some. The SAM-2 specification gives a more detailed understanding of some of the new SCSI ter-
minology
Application
Figure 2. SCSI client-server model
1.2.1

Glossary

aborted command—A SCSI command that has been ended by aborting the task created to execute it.
ACA—Auto Contingent Allegiance (see below).
ACA command—A command performed by a task with the ACA attribute. See Section 4.4.2.
application client—An object that is the source of SCSI commands. An object in this sense is not a tangible
piece of hardware, but may be a single numeric parameter, such as a logical unit number, or a complex entity
that performs a set of operations or services on behalf of another object (see ANSI SAM-2, T10/1157D).
asynchronous event notification—A procedure used by targets to notify initiators of events that occur when
a pending task does not exist for that initiator.
asynchronous transfer—An information transfer that uses the REQ/ACK handshake with an offset of zero.
auto contingent allegiance—One of the conditions of a task set following the return of a CHECK CONDITION
status. See Section 4.4.2.
blocked (task state)—The state of a task that is prevented from completing due to an ACA condition.
blocking boundary—A task set boundary denoting a set of conditions that inhibit tasks outside the boundary
from entering the Enabled state.
byte—An 8-bit construct.
call—The act of invoking a procedure.
client-server—A relationship established between a pair of distributed objects where one (the client) requests
the other (the server) to perform some operation or unit of work on the client's behalf (see ANSI standard
SAM-2, T10/1157D).
Device Service Request
Client
Device Service Response
Task Management Request
Task Management Response
Initiator
Logical
Unit
Device
Server
Task
Manager
Target
3