Re: Bidirectional SCSI commands and iSCSI

["Jim Hafner/Almaden/IBM" <hafner@almaden.ibm.com>]

Julo,

The primary motivation is for object-based storage devices (OSDs).  The
protocol that seems to be needed in that environment is the ability to send
a  CDB, plus some parameter data (DataOut) AND get back read-type data
(DataIn) in the same command sequence.

One (non-OSD) command has been proposed in T10 for an XOR type XDREADWRITE
command.

SAM-2 has now allowed for such commands in the architecture. From this
perspective, it is completely symmetric. Transports that support this
feature will need to have data structures in there PDUs or IUs (or
whatever) to indicate things like DataOut datalength, DataIn datalength,
possible virtual addresses for each buffer, etc.  In other words, there
will potentially be two active buffers (on each end) for use with the same
command.    So, e.g., iSCSI PDU may need to have both "DataIn expected data
transfer length" and  "DataOut expected transfer length" fields.  I'm
assuming that the data direction in the existing iSCSI PDU is derived from
the SCSI opcode, unlike FCP where there are fields to indicate the data
direction.  (I am surprised not to find such things in the iSCSI Command
PDU.)

There are two proposals in T10 to enable this feature in SPI-4 and in FCP-x
(probably FCP-3).

Jim Hafner


julian_satran@il.ibm.com@ece.cmu.edu on 09-28-2000 12:10:53 PM

Sent by:  owner-ips@ece.cmu.edu


To:   ips@ece.cmu.edu
cc:
Subject:  Re: Bidirectional SCSI commands and iSCSI





Jim,

What is the intended use?  Are there already commands that use it?
Is there a major/minor distinction - like output data & input results or
output
parameters & input data - or is it intended to be completely symmetric?

Julo

"Jim Hafner/Almaden/IBM" <hafner@almaden.ibm.com> on 20/09/2000 17:56:27

Please respond to "Jim Hafner/Almaden/IBM" <hafner@almaden.ibm.com>

To:   ips@ece.cmu.edu
cc:    (bcc: Julian Satran/Haifa/IBM)
Subject:  Bidirectional SCSI commands and iSCSI




Folks,

In case you missed this, T10 just approved changes to SAM-2 (rev14) which
remove the restriction that a single SCSI command can have a data flow in
only one direction (DataIn xor DataOut).   Transport specs (that want to
allow this) will need to change to allow for two sets of Data buffer fields
in the headers (length, over/underrun, etc.).

Is anybody looking to modify the iSCSI spec to get these concepts
supported?

Jim Hafner