From: vail@tegra.COM (Johnathan Vail)
Newsgroups: comp.periphs.scsi
Subject: Re: Logical Block Size for SCSI drives
Date: 17 Aug 92 13:51:51 GMT
Organization: Tegra, Inc., Billerica, MA
In-reply-to: olson@anchor.esd.sgi.com's message of 15 Aug 92 23:28:40 GMT



In article <okb6k8g@zuni.esd.sgi.com> olson@anchor.esd.sgi.com (Dave Olson) writes:

   In <3508@atlas.tegra.COM> vail@tegra.COM (Johnathan Vail) writes:
   | 
   | Also, with 512 byte blocks the most you can transfer in one SCSI
   | command is 64K blocks or 32 Meg.  Larger block sizes increase the
   | amount of data you can move with one SCSI command.

   That is true only if you use the older 6 byte commands.  Many
   systems these days use the 10 byte commands.  A bigger problem
   with the 6 byte commands (for disks) was inability to access
   more than 2^21 sectors (1 Gbyte at 512 bytes).
   --

No, I was talking 10 byte group 2 commands.  Even with READ(10) the
most you get is 2 bytes, 16 bits or 64K blocks to express transfer
length.  That's 32 Meg with 512 byte blocks.  If you know a better way
I would love to hear about it since this is a bottleneck for my
application.

READ(6) is even more limited with only 256 blocks max per SCSI
command.

I agree about the addressing limitations of the READ(6).


Quoting from the book of SCSI, chapter 8, verse 2 paragraph 6:


			       Table 123: READ(10) Command
    +=====-========-========-========-========-========-========-========-========+
    |  Bit|   7    |   6    |   5    |   4    |   3    |   2    |   1    |   0    |
    |Byte |        |        |        |        |        |        |        |        |
    |=====+=======================================================================|
    | 0   |                           Operation Code (28h)                        |
    |-----+-----------------------------------------------------------------------|
    | 1   |   Logical Unit Number    |   DPO  |   FUA  |     Reserved    | RelAdr |
    |-----+-----------------------------------------------------------------------|
    | 2   | (MSB)                                                                 |
    |-----+---                                                                 ---|
    | 3   |                                                                       |
    |-----+---                        Logical Block Address                    ---|
    | 4   |                                                                       |
    |-----+---                                                                 ---|
    | 5   |                                                                 (LSB) |
    |-----+-----------------------------------------------------------------------|
    | 6   |                           Reserved                                    |
    |-----+-----------------------------------------------------------------------|
    | 7   | (MSB)                                                                 |
    |-----+---                        Transfer Length                             |
    | 8   |                                                                 (LSB) |
    |-----+-----------------------------------------------------------------------|
    | 9   |                           Control                                     |
    +=============================================================================+




jv



"There are only two mistakes one can make along the road to truth: not
going all the way and not starting." -- Buddha
 _____
|     | Johnathan Vail     vail@tegra.com     (508) 663-7435
|Tegra| jv@n1dxg.ampr.org    N1DXG@448.625-(WorldNet)
 -----  MEMBER: League for Programming Freedom (league@prep.ai.mit.edu)