Question On Logical Unit Number field of SPI-4 SPI L_Q information nunit

George Penokie gop at
Fri Jun 14 12:05:31 PDT 2002

* From the T10 Reflector (t10 at, posted by:
* "George Penokie" <gop at>

You ask some interesting questions.

1-According to what is stated today in SAM-2 if the target has either
dependent logical units or uses the single level LUN it would be required
to set the HiSUP bit to one. Because neither SAM-2 nor SPC-2 states
anything about the format of the LUN if the HiSUP bit is zero then anything
goes when it is set to zero. That is not a good thing and there are at
least two letter ballot comments on SAM-2 relating to that issue. So things
may change but I will not know until the next meeting in July.

On the other hand, for devices that only contain a single LUN of zero it
makes no difference what the value of HiSUP is because the value in for the
LUN is all zeros in all cases.

Also, there is no requirement in SPI-4 on the structure of the LUN field in
an L_Q IU.

2-REPORT LUNS is not the only way but any other way requires you to issue
INQUIRY commands to every LUN to see if it valid or not.

3-Right now it is vendor unique except that LUN zero is all zeros. Also,
right now, If the HiSUP bit is zero then anything goes regardless of the
LUN. If the HiSUP bit is one then either the single level LUN structure is
used or the hierarchical structure for dependent logical units is used. If
you look close you will see there is no difference between those two the
way they are defined today. Which, of course makes not sense. This will
also be looked into at the July meeting.

4-Yes the target has to use the same LUN it received in the command it the
L_Q IU responses to that command. If it does not then the initiator has no
idea what logical unit is responding.

Bye for now,
George Penokie

Dept 2C6  114-2 N212
E-Mail:    gop at
Internal:  553-5208
External: 507-253-5208   FAX: 507-253-2880

                      "Spalding, AlanC"                                                                                                
                      <Alan_Spalding at ad        To:       George Penokie/Rochester/IBM at IBMUS                                            
            >               cc:       "Spalding, AlanC" <Alan_Spalding at>                                 
                                               Subject:  Question On Logical Unit Number field of SPI-4 SPI L_Q informatio       n     
                      06/12/02 09:45 PM         unit                                                                                   

Hi George,
I hope you don't mind me emailing you directly (rather than posting to T10
reflector) but you seem to be the person most likely to respond to
I post on the SCSI reflector. Also I believe that you were the author of
Addressing model for SAM proposal.

I have a few questions on the contents of the  Logical Unit Number field of
SPI-4 SPI L_Q information unit:

1) SPI-4 Rev 9 references SAM-2. SAM-2 states that if a device support 256
or fewer LUNs that it SHALL use the Single Level Lun structure format. Goes
on to state that in this format the HiSUP bit of Inquiry data SHALL be set.
Therefore, single Lun SCSI Disk drives supporting packetized transfers must
always have the HiSUP bit set to 1 in Inquiry data, correct?

2) Is using the REPORT LUNS command the only way to determine the number of
luns supported by a target? I.e. Set the Select Report field to 02 (all
luns) and divide the contents of the LUN LIST LENGTH field by 8.

3) What is the format of the SPI L_Q LUN field for devices that return
= 0 in Inquiry data? Is this Vendor unique? I can't find any information.
Does this mean that if a device supported greater than 257 Luns that a
of  00 00 00 00 00 00 00 0x where x is non-zero would be legal?

4) Are targets required to return the LUN field received from the Initiator
in the SPI L_Q(target to init) for a response even for illegal or
unsupported Lun values? I would think so, but I don't see this happening

Thanks for your time (and I hope you don't mind the questions).


Alan C. Spalding
Intra-site work phone # 864-8369
Work phone # 949 455-8369
Work fax # 949 455-8406

* For T10 Reflector information, send a message with
* 'info t10' (no quotes) in the message body to majordomo at

More information about the T10 mailing list