iSCSI: Inquiry, Mode Sense, Read Capacity

[Black_David@emc.com]

These are SCSI questions that are not specific to iSCSI.

> Q1:
> Suppose an iSCSI initiator sends an iSCSI target an iSCSI 
> PDU which encapsulates a SCSI Inquiry command.  In which 
> type of PDU does the iSCSI initiator expect SCSI inquiry 
> data to be received?
> 
> - SHOULD/MUST/MAY target send SCSI inquiry data with 
>   SCSI Response PDU, or
> - SHOULD/MUST/MAY target send SCSI inquiry data with 
>   a PDU other than SCSI Response PDU?
> 
> The same questions are also for SCSI Mode Sense and 
> SCSI Read Capacity commands.

The answers are to be found in careful reading of the relevant
SCSI specs - SAM2, SBC, and SBC2 in this case.  The SCSI specs
make specific distinctions among data, status, and response,
and the T10 document editors are very good about making sure
that such terms are well-defined and used only in the manner
defined.  INQUIRY, MODE SENSE, and READ CAPACITY are all
defined to return data, and hence would use one or more iSCSI
Data-in PDUs for that purpose.  The Response PDU is there
for dealing with command completion and the like.  For example,
if one of these commands starts to transfer data and then fails
at the Target, one would see one or more Data-in PDUs followed
by a Response PDU containing CHECK CONDITION or something
like that, and the data field of the Response PDU would be
needed for the sense data (AutoSense is mandatory for iSCSI)
and the like.
 
> Q2:
> There are 10 target opcodes on page 43 of v.07, and two of 
> them are specified as:
> 
> - 0x21 SCSI Response (contains SCSI status and possibly 
>   sense information or other response information) 
> - 0x25 SCSI Data-in (for READ operations)
> 
> The question is what this "READ" is for?  Is it for reading SCSI 
> data **only**, or can it be also used to read SCSI Inquiry Data
> /Mode Sense Data/Capacity Data, etc.?

See previous answer.  Data-in transfers data for any SCSI command
that has data to transfer.  This includes all three of your examples.
"READ" actually includes any SCSI command that transfers "data" (as
that term is defined by SCSI) from Target to Initiator.  This includes
INQUIRY, MODE SENSE, and READ CAPACITY.

> Q3:
> If both SCSI Response PDU and 0x25 SCSI Data-in PDU can be 
> used to read SCSI Inquiry Data/Mode Sense Data/Capacity Data, 
> how should we specify this in the draft and make it clearer?  Adding
> a sample for SCSI Inquiry/Mode Sense could be helpful for people
> to understand this.

The assumption following the "If" is incorrect, and IMHO, this is
already sufficiently clear from the T10 SCSI specs, which MUST be
read in conjunction with the iSCSI spec to completely understand
how SCSI works over iSCSI.

OTOH, Section 3.7 of the iSCSI draft could use a couple of sentences
pointing out that the Data-in and Data-out PDUs are used for all SCSI
data transfers, not just those associated with the READ and WRITE
commands.

Thanks,
--David

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David L. Black, Senior Technologist
EMC Corporation, 42 South St., Hopkinton, MA  01748
+1 (508) 435-1000 x75140     FAX: +1 (508) 497-8500
black_david@emc.com       Mobile: +1 (978) 394-7754
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