Altering block size support for tape devices

tedl at exabyte.com tedl at exabyte.com
Mon Jan 15 07:34:22 PST 1996


* From the SCSI Reflector, posted by:
* <tedl at Exabyte.COM>
          The following X3T10 document proposes a mechanism for
          identifying support of some block sizes instead of all block
          sizes for tape devices.  The proposed change would go into
          the SSC document for SCSI-3.  Some wording may be required
          in the model as well.
          ------------------------------------------------------------

Block size granularity                            X3T10/96-114r1

To:       Membership of X3T10

From:     Edward Lappin
          Exabyte Corporation
          tedl at exabyte.com
          (303) 417-7718

Date:     January 10, 1996
Subject:  Block size granularity for sequential devices

Background

The current model for sequential (tape) devices requires that all
block  sizes be supported in a range.  Support of odd block sizes
is  potentially  costly, particularly with wide SCSI  interfaces.
Additionally, some existing products support more than one  block
size but not sequentially (such as 512 and 1024).

Proposal

I propose that a new field, Granularity, be incorporated in the
Read Block Limits command. The changes do not require any
modification of current SCSI-2 implementations since the new
field returned 0 (reserved) indicating all block sizes within the
range are supported.

The following changes are required in the Read Block Limits
return data in the Sequential-access device section of SSC for
SCSI-3.  Changes are marked with change bars.

                                
                Table 1 - READ BLOCK LIMITS data
                                
  +----+------+------+------+------+------+------+------+------+
  | Bit|  7   |  6   |  5   |   4  |   3  |   2  |  1   |  0   |
  |Byte|      |      |      |      |      |      |      |      |
  +----+------+------+------+------+------+------+------+------+
  | 0  |     Reserved       |           Granularity            |     |
  +----+------+------+------+------+------+------+------+------+
  | 1  |(MSB)              Maximum block                       |
  | 2  |                   length limit                        |
  | 3  |                                                  (LSB)|
  +----+------+------+------+------+------+------+------+------+
  | 4  |(MSB)              Minimum block                       |
  | 5  |                   length limit                   (LSB)|
  +----+------+------+------+------+------+------+------+------+

The Granularity field indicates the supported block size             |
granularity.  The logical unit shall support all block sizes n       |
such that n minus the Minimum block length limit is a multiple of    |
2^granularity  and n is greater than or equal to the Minimum block   |
length limit and less than or equal to the Maximum block length      |
limit.                                                               |





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