Question on Scope: 8-byte vs 2-byte LUNs

George Penokie gop at us.ibm.com
Tue Mar 24 15:00:29 PST 1998


* From the T10 (formerly SCSI) Reflector (t10 at symbios.com), posted by:
* George Penokie <gop at us.ibm.com>
*

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George,
I will try to clear things up, but as you know, this is a complicated subject.

1-This method was defined to allow all logical units to be addressed within the
hierarchy regardless of the level they reside on, without the need for each
level having to know about every other level in the hierarchy. Using the
current method a level only needs to know about the devices that are directly
attached to it. This allows an application client to issue (or pass though) any
legal command to any logical unit even if it is at the lowest level of the
hierarchy without having to put the intermediate devices onto any special pass
though mode. It also provides information on the structure of the hierarchy
(i.e., what is attached to what and the local target/LUN addresses). From this
an application client can build a physical representation of the hierarchy to
use for maintenance or for building configurations.

2-A REPORT LUNs command can only report LUNs attached to the target receiving
the command. So a REPORT LUNs command sent to a device below level 0 can only
report about the LUNs attached to it. It is recommended that REPORT LUNs
commands only be sent to level 0  devices unless the application client has
some special need for that information and can handle it properly.

3-The COPY command is indeed interesting using this structure but will work. I
see four scenarios in which copy could be executed:

A-If the COPY command is between targets at level 0 then it is no different
than today. The target that receives the command becomes the initiator and
moves the data from it's selected logical unit to the selected target and
logical unit. The application client should setup the destination LUN correctly
so the source target(initiator) can address the LUN without manipulation.

B-If the COPY command is between logical units attached to the target then the
target can move the data between the logical units using standard reads and
writes.

C-If the COPY command is between logical units attached to the target and the
target knows both those devices are attached to the same bus and support COPY
it could reform the COPY to use the targets and LUNs those devices know about
and reissue the command to one of those devices.

D-If the application client knows the devices it wants to copy data between are
attached to the same bus in the hierarchy then it can route the COPY command
directly to the device it wants to be the source of the copy by directly
addressing that device. The structure of the hierarchy can be extracted by the
application client by decoding the 8-byte LUN structure.

Hope this helps.
Bye for now,
George Penokie


owner-t10 at Symbios.COM on 03/20/98 11:53:16 PM
Please respond to ericson at worldnet.att.net @ internet
To: George Penokie/Rochester/IBM at ibmus
cc: t10 at Symbios.COM @ internet
Subject: Question on Scope: 8-byte vs 2-byte LUNs


* From the T10 (formerly SCSI) Reflector (t10 at symbios.com), posted by:
* "George M. Ericson" <ericson at worldnet.att.net>
*
This is a multi-part message in MIME format.
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George,

Wonder if you (or others on T10) would clear up rules on how 8-byte and
2-byte LUNs are interpreted.

Some questions that SAM-2, SCC-2, or SPC-2 do not answer.
    1.. SCC-2 uses 2-byte LUNs in data structures.  Implies SCC-2 commands
are restricted to act on Logical Units at same level in the hierarchy as the
target Logical Unit.  So why did SCC-2 need to invent 8-byte 4 level LUNs?
    2.. On Report LUNs, 8-byte LUNs are returned.  How should they be formed
if the target Logical Unit is not at Level 1?  How does the target Logical
Unit know the path used to get to it?
    3.. Similarly, for a Copy command, if the target Logical Unit is not at
Level 1, how are 8-byte LUNs interpreted?  Is level n interpreted relative
to the level 0 initiator, the level n-1 initiator or the target?



----------------------------------------------------------------------------
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One solution is that 8-byte LUNs are interpreted universally by all targets
within the same SCSI Domain.  It would be the responsibilty of each level to
recast any embedded LUN in a form that would preserve that semantic.  In
this scenario, the answers to 2 and 3 above is that the level 0 initiator
passes 8-byte LUNs that have consistent meaning to that initiator.  Lower
levels make appropriate transformations to achieve the semantics intended by
the upper layer.  If this solution is acceptable, then SCC-2 should also be
using 8-byte LUNs.  Since the use of 2-byte LUNs is restrictive in precisely
the place where the more complex 8-byte structure has value.

An alternative solution is that all commands are executed in the scope of
the level of the target Logical Unit.  In this case, the SCC-2 use of 2-byte
LUNs makes sense.  If this is the case, then shouldn't the copy and send
commands also use 2-byte LUNs?

Regards,
George Ericson
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