Date:          February 2, 1990         X3T9.2/90-28 Revision 0

From:          Mike Racelo

Subject:       Modifying Actual Retry Count/Adding Unrecorded
               Block Count in buffer

The Sense-Key Specific Data of Request Sense has what we believe
to be an overkill:  a 2 byte Actual Retry Count field.  This 2
byte field says some device is capable of doing up to 65,535
retries.  We recommend this field be reduce to 1 byte.  The freed
byte (presently the msb of the Retry Count) is needed to show the
unrecorded block count.  The modified Sense-Key Specific data is
shown below.

               Table 7-37
     Actual Retry and Unrecorded Block Buffer Count
============================================================
 Bit      |  7  |  6  |  5  |  4  |  3  |  2  |  1  |  0  |
Byte      |     |     |     |     |     |     |     |     |
=============================================================
  15      |SKSV |        Reserved                         |
-------------------------------------------------------------
  16      |      Unrecorded Block Count                   |
-------------------------------------------------------------
  17      |         Actual Retry Count                    |
=============================================================

Modifying the Actual Retry Count in the Sense-Key Specific field
will also match the Read-Write Error Recovery Page, Table 9-27,
Retry Counts.

The Unrecorded Block Count field is needed to report unrecorded
blocks sitting in the device buffer as a result of the Write
error.  The Information Field per SCSI-1 contains the blocks or
bytes not transferred to the target.  To simplify describing this
confusing subject see figure 1.   This is a very simple example
of a Fixed Block mode Write error starting from beginning of tape
(BOT).  The Information field will report 3 blocks not
transferred to the target.  The number of unrecorded data blocks
in the target buffer is not known.  This picture gets more
complex and confusing when write errors occur in the middle of
tape and Fixed blocks and Variable blocks were transferred. 
Figure 2 is an example of write errors in mixed Variable and
Fixed block mode.  Note, in this case the Information Field will
be larger than the CDB.  SCSI-2 further requires the Information
Field to include filemarks and setmarks.  

The new section on page 7-47 will add a new paragraph following
the actual retry count field paragraph and shall read as follows:


"The actual retry count field returns implementation-specific
information on the actual number of retries of the recovery
algorithm used in attempting to recover an error or exception.

IMPLEMENTORS NOTE:  It is recommended that this field relate to
the retry count fields within the error recovery page of the MODE
SELECT command.

The unrecorded block count field returns the number of data
blocks, and filemarks and setmarks for sequential devices in the
target buffer that have not been transferred to the recording
medium."


We believe this new field will answer future questions when
Initiators experience Write errors in buffered mode.  This new
field will give the information of where the write error occurred
on tape and how many unrecorded blocks are in the target buffer. 
The unrecorded block count may be used by the Recover Buffered
Data command.      



 Requested     | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | = 10 

 Buffer        | 1 | 2 | 3 | 4 | 5 | 6 | 7 |   |   |    | =  7

 Actual        | 1 | 2 |   |   |   |   |   |   |   |    | =  2

          Residue = Requested - ( Actual + Buffer )
                  =   10      - (  2     +  5   )
          Residue = 3  Blocks not transferred

          Figure 1.  Write error from BOT in Fixed Block Mode





Requested   BOT<--|   1   |2|3|4|5|  6  |  7  | 8 | 9 | -->EOT

Buffer      BOT<--|   1   |2|3|4|5|  6  |  7  | 8 |   | -->EOT

Actual      BOT<--|   1   |2|3| | |     |     |   |   | -->EOT

          Residue = Requested      SCSI-1

          Residue = Requested - ( Actual + Buffer ) SCSI-2
                  
          Figure 2.  Write error not at BOT with mixed Fixed 
                     and Variable Blocks in buffer.