Re: Bidirectional SCSI commands and iSCSI

[relliott@hobbit.cca.cpqcorp.net (Robert Elliott)]

> 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?

There can be different amounts of write and read data.  The
data-out size and data-in size are set by the initiator.

The target chooses:
* whether to accept write data or provide read data first
* when to switch direction
* how often to switch direction

The initiator is not allowed to change the write data after
sending the CDB - read-modify-write (where the initiator modifies
the data) is not supported.  Write-modify-read (where the
target modifies the data) is supported.

T10/00-315 describes a bidirectional command called XDWRITEREAD
useful for RAID operations.  The object storage device (OSD) 
command set plans to use bidirectional commands.  Other
commands may appear in the future that use this feature.

T10/00-314 describes how this is supported on Parallel SCSI.

T10/00-377 describes how this is supported on Fibre Channel 
(FCP-2).

The 1394 community plans to revise SBP-2 to add bidirectional
command support.

(all T10 documents are on http://www.t10.org)

> 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
> 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
> 


-- 
Rob Elliott      UNIX mailto:relliott@unixmail.compaq.com    
Houston, TX        PC mailto:Robert.Elliott@compaq.com