X3T9.2/90-116
                                                                         Rev 1





August 24, 1990

Mr. Brian Earle, Project Engineer
Eventide, Inc.
1 Alsan Way
Little Ferry, NJ 07643

Subject:  Request for Interpretation, #1 on X3.131-199X, SCSI-2

Dear Mr. Earle,

Your Request for Interpretation (#1) on X3.131-1990, Small Computer System 
Interface (SCSI-2), concerning whether Page Size Descriptors should be 
included in the Medium Partition Page(s) for each partition on the Tape has 
been considered by Task Group X3T9.2 and by Technical Committee X3T9.  It is 
the conclusion of these groups that Page Size Descriptors are to be included 
for each partition.

This opinion is based on the contents of the last paragraph of section 
9.3.3.2, which you quoted in your request:

  "Partition size descriptors define the size of the respective partitions in 
  the units specified in the PSUM field.  Up to 64 partitions may be defined 
  using this page.  If additional partitions need to be defined, up to 64 
  partitions can be defined in each of the medium partition pages(2-4) defined 
  in 9.3.3.3."

Furthermore, it is based on the requirement from section 7.3.3 that the target 
reject a MODE SELECT command if the page length is different from the length 
returned in the MODE SENSE command.  This effectively means that the target 
must allocate space in the Medium Partition Pages for the maximum number of 
supported partitions, whether or not they are currently present on the medium.  
It also implies that partition 0, the default partition (see 9.1.3), must be 
included in Medium Partition Page(1) if the Medium Partition Page(1) is 
supported.  Partition 0 could have zero length, but it must exist.

Note that the total number of partitions is defined by the sum of the 
Additional Partitions Defined field in the Medium Partition Page(1) plus one. 
This value may be changed by a MODE SELECT command, but the Page Lengths of 
the Medium Partition Pages do not change.  Repartitioning the medium may cause 
data loss as described in 9.3.3.2.

Partition sizes are approximate and are subject to rounding per section 6.5.4.  
Note that the target must make assumptions to establish partition sizes on the 
medium.  These assumptions include such factors as recording mode, block size, 
and medium defects.  Partition sizes therefore may not represent actual 
capacity obtained for an application.

Since SCSI-2 has not been published yet, X3T9.2 is recommending that the 
description in 9.3.3.2 be revised to read, "...define the size of all the 
respective partitions...".  This is viewed as a non-substantive clarification 
and may help implementors in the future to avoid the variations you have 
encountered in some DAT implementations.

Thank you for bringing this matter to our attention.  

Sincerely,



Del Shoemaker, Chairman X3T9


cc:  Lynn Barra, Coordinator, Nat'l Stds. Processing, X3
     John Lohmeyer, Chairman X3T9.2