iSCSI RE: Phase collapse

[Black_David@emc.com]

Title: Phase collapse

Yes, most of the time:

- If the INQ succeeds, a single Data-in PDU can carry the

    successful status without a Response PDU.

- If the INQ fails without transferring any data, a single

    Response PDU carries the status and autosense info.

    Recall that autosense is REQUIRED by iSCSI.

But

- If the INQ transfers data and then fails, a Data-In PDU

    is needed to transfer the data and a Response PDU is

    needed to transfer the status and autosense info.

 

--David

 

---------------------------------------------------
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
---------------------------------------------------

 

-----Original Message-----
From: Ken Nicholson [mailto:knicholson@corp.iready.com]
Sent: Wednesday, October 31, 2001 9:45 AM
To: 'ips@ece.cmu.edu'
Subject: Phase collapse

Julian and David:

Could you clarify the information below?

It seems to us that it should be possible for a target to phase collapse the response to an INQ in a single PDU which is both the first and last PDU of that response.

If this true:
>A command and its associated data may be
> shipped together from initiator to target and data and responses
> may be shipped together from targets."

Why is this true:
"...the above text makes
it quite clear that data transferred by the action of a SCSI command
(including Read, Inquiry, Mode Sense, and Read Capacity) cannot be put in
the Response PDU. "

Ken Nicholson
iReady Corp.

----------------------------------------------------------------------------
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To: ips@ece.cmu.edu
Subject: RE: iSCSI: Inquiry, Mode Sense, Read Capacity
From: "Julian Satran" <Julian_Satran@il.ibm.com>
Date: Wed, 3 Oct 2001 22:46:44 +0300
Content-Type: multipart/alternative; boundary="=_alternative
006CB109C2256ADA_="
Sender: owner-ips@ece.cmu.edu

David is right. Response doe not contain data proper.
Phase collapse is for the last DataPDU in which a target can fit a GOOD
status (and thus avoid an additional response) and some residual counts
(when those are not considered errors).

Julo

Black_David@emc.com
Sent by: owner-ips@ece.cmu.edu
03-10-01 20:21
Please respond to Black_David

        To:        lxing@Crossroads.com, ips@ece.cmu.edu
        cc:
        Subject:        RE: iSCSI: Inquiry, Mode Sense, Read Capacity

The governing text is the following from p.62 of -08 on the contents of
the Response PDU:

    3.4.6 Data Segment - Sense and Response Data Segment

       iSCSI targets MUST support and enable autosense.  If Status is CHECK

       CONDITION (0x02), then the Data Segment contains sense data for the
       failed command.

       For some iSCSI responses, the response data segment MAY contain some

       response related information, (e.g., for a target failure it may
       contain a vendor specific detailed description of the failure).

Julian may be able to shed more light on the intended meaning of
"phase collapse" for the response case, but the above text makes
it quite clear that data transferred by the action of a SCSI command
(including Read, Inquiry, Mode Sense, and Read Capacity) cannot be
put in the Response PDU.  A target can send the Response PDU immediately
following the last Data-In PDU for the associated command, which is
similar to the ability of an Initiator to send unsolicited Data-Out
PDUs.  There is no Target analog to the Initiator's ability to send
Immediate Data in the same PDU as the SCSI command.

Thanks,
--David