Forward of message on TTD/CIOP Continue Task

Lawrence J. Lamers ljlamers at
Wed May 5 07:56:58 PDT 1999

* From the T10 Reflector (t10 at, posted by:
* "Lawrence J. Lamers" <ljlamers at>
From: 	William F. Renner[SMTP:will at]
To: 	Tom Rauscher
Subject: 	The case for TTD

The TTD and CT messages can be extremely useful in systems where a SCSI
initiator which is active on more than one SCSI bus requires synchronization
of the data phases of targets on the separate busses. In such a system, one
is faced with a deadlock problem if multiple commands are active
simultaneously. For example if disconnecting read commands are issued to
both target 0 and target 1 on both bus A and bus B, it is possible for
target 0 to reselect and go into data phase on bus A and target 1 to
reselect and go into data phase on bus B. Since most targets do not respond
well to an initiator disconnect request following reselection, the initiator
is forced to take the data. In a system where the synchronization of the
multiple channels is a hard requirement, this simply may not be possible.

TTD and CT are the ideal answer to this dilemma: the initiator is informed
when a target is prepared for data phase on any of its busses. It can then
order the data phases of the targets in accordance with its own constraints.
Without those two messages, such an initiator is forced to run in
non-disconnecting mode or to issue only a single command to a single target
on any bus at any time. This leads to a VAST underutilization of resources.

There are few workarounds to this problem without Target Transfer Disable.
The most effective, although it is limited to reads from a block device, is
the SCSI Prefetch command. Unfortunately, it is poorly supported (or not at
all) by most disk drives. As mentioned above, if targets were reliably to
accept a disconnect message from the initiator after reselect, there would
be another solution (although a less efficient one, because the target has
no knowledge of when a reselection might be more appropriate). This,
however, is also rarely implemented in practice. By comparison, Target
Transfer Disable is presently supported by quite a few disk drives.

It may be true that the TTD and CT messages are no longer needed for their
original purpose, however in some systems they serve an integral function,
irreplaceable by any other means available in SCSI as it is implemented in
the field.

Will Renner
Software Engineer
Digidata Corporation

E-mail:  will at
Phone:   (301) 498-0200 ext 228
Fax:     (301) 498-0771

Lawrence J. Lamers                     Adaptec, Inc
email:   ljlamers at            691 South Milpitas Blvd.
Phone: (408) 578-1709                 Milpitas, CA  95035

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