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

George M. Ericson ericson at worldnet.att.net
Fri Mar 20 20:03:53 PST 1998


* From the T10 (formerly SCSI) Reflector (t10 at symbios.com), posted by:
* "George M. Ericson" <ericson at worldnet.att.net>
*
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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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 George,

Wonder if you (or others on T10) = would clear up=20 rules on how 8-byte and 2-byte LUNs are interpreted.

Some = questions that=20 SAM-2, SCC-2, or SPC-2 do not answer.
 SCC-2 uses 2-byte LUNs in data structures.  = Implies=20 SCC-2 commands are restricted to act on Logical Units at same level = in the=20 hierarchy as the target Logical Unit.  So why did SCC-2 need to = invent=20 8-byte 4 level LUNs? On Report LUNs, 8-byte LUNs are returned.  = How should=20 they be formed if the target Logical Unit is not at Level 1?  = How does=20 the target Logical Unit know the path used to get to it? Similarly, for a Copy = command, if the=20 target Logical Unit is not at Level 1, how are 8-byte LUNs=20 interpreted?  Is level n interpreted relative to the level 0 = initiator,=20 the level n-1 initiator or the target? 

One solution is that 8-byte LUNs are interpreted universally by all = targets=20 within the same SCSI Domain.  It would be the responsibilty of each = level=20 to recast any embedded LUN in a form that would preserve that = semantic.  In=20 this scenario, the answers to 2 and 3 above is that the level 0 = initiator passes=20 8-byte LUNs that have consistent meaning to that initiator.  Lower = levels=20 make appropriate transformations to achieve the semantics intended by = the upper=20 layer.  If this solution is acceptable, then SCC-2 should also be = using=20 8-byte LUNs.  Since the use of 2-byte LUNs is restrictive in = precisely the=20 place where the more complex 8-byte structure has value.
  
 An alternative solution is that all commands are = executed in=20 the scope of the level of the target Logical Unit.  In this case, = the SCC-2=20 use of 2-byte LUNs makes sense.  If this is the case, then = shouldn't the=20 copy and send commands also use 2-byte LUNs?
  
 Regards,
 George Ericson


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