some comments on PLDA tapes using Class 3

[Doug Hagerman, 508-841-2145, Flames to NL: 09-Jul-1996 0025]

* From the SCSI Reflector, posted by:
* "Doug Hagerman, 508-841-2145, Flames to NL:  09-Jul-1996 0025" <hagerman at starch.ENET.dec.com>
*
960708
tape-comments.doc

Here are some additional comments on the tape model as it has been discussed
recently.

Obviously there is a bit of an argument as to whether error recovery should be
done at the command level or at the interconnect transport level, but here I
assume that we're still trying to use SCSI command level recovery. Also note
that I'm not a tape expert ("Doug, you obviously don't know anything about tape
drives!"--Ralph Weber.), and I encourage anyone who IS such an expert to review
and comment on this subject.

I'm assuming that the "large buffer" model is in effect. (That is, every tape
drive guarantees to provide at least 1 MByte of input buffer space, or
alternatively, a mechanism is defined for negotiating the size of said buffer.
Also, every SCSI READ or WRITE command is guaranteed to specify less than 1
MByte (or the negotiated amount) of data.) Using these rules, all the
interconnect-related activity can be completed before any tape media activity
is started. Also it's agreed that after an error is detected in a READ or WRITE
command, a READ POSITION command will be used to synchronize the drive's idea
of the media position with the driver's idea of the media position.

Here are some cases that can occur:

case                    what the          result of         next
                        initiator         READ POSITION     initiator
                        sees              command           action
=====================================================================

1. Command and data     success status    NA                send
get to target, and status                                   next
is returned successfully                                    command

2. Failure in command   failure status    result is OLD     re-send
or data, and status                       value of          command
is returned successfully                  position

3. Command gets lost    command           result is OLD     re-send
on the way to target    times out         value of          command
                                          position

4. Command and data     command           result is NEW     send next
succeed, but status     times out         (i.e. expected)   command
gets lost on the way                      value of
back to initiator                         position

5. Failure in command   command           result is OLD     re-send
or data, and status     times out         value of          command
gets lost on the way                      position
back to initiator

Since the same action is taken by the initiator in every case where the OLD
value of the media position is returned from the READ POSITION command, the
decision process used by the initiator is simple.

It is possible that the READ POSITION command may fail. In this case the
initiator waits until the READ POSITION command times out, then re-issues the
same READ POSITION command. This is acceptable because there is no media
movement associated with this command. This also applies to any other command
that does not move the media. Hopefully all commands can be classified into two
cases. First are those that do not move the media, and it is assumed that all
of these may also be re-issued in case of failure (i.e. there is no harm in
issuing the same command multiple times). Second are those that do move the
media, and they will all be handled using the rules discussed for the READ and
WRITE commands. A list of these two types of commands should be added to the
tape section of PLDA.

Up to this point I do not see a need for Class 2 for tapes using PLDA.


Doug Hagerman
Digital Equipment