RE: Data in SCSI Response or SCSI Data

["Douglas Otis" <dotis@sanlight.net>]

Matt,

> -----Original Message-----
> From: owner-ips@ece.cmu.edu [mailto:owner-ips@ece.cmu.edu]On Behalf Of
> Matt Wakeley
> Sent: Friday, September 08, 2000 1:02 PM
> To: ips@ece.cmu.edu
> Subject: Re: Data in SCSI Response or SCSI Data
>
>
> Douglas Otis wrote:
>
> > Bob,
> >
> > Thanks, that helps an understanding of this PDU 0x45.  If iSCSI
> is to copy
> > FCP
>
> I think this needs clarification. iSCSI is *NOT* copying FCP. iSCSI is
> attempting to transport SCSI over TCP/IP.  It just so happens that the end
> result looks similar to what FCP did.

Okay, similar architecture.
>
> > then it should send a response structure as FCP and not one or two and
> > use it according to FCP.  Undefined information of what some
> consider SCSI
> > values cause variants in use.  I have not seen tape drives use
> Good Sense
> > residual but vary in how Check Sense residual is presented.  If
> iSCSI is to
> > copy FC, then why deviate on response symmetry?  At least this
> provides an
> > easier bridge.  Iterative read operations will carry unused values to be
> > examined and opens the door for further variants.  Status
> presented prior to
> > data must then ensure sequence without end confirmation.
>
> Who said anything that status was presented before data?
> iSCSI allows the LAST iSCSI PDU that completes a SCSI command to
> also contain
> the GOOD SCSI status in the header, so that extra overhead to
> send the status
> PDU.

By placing status within the header of perhaps 4G of data payload with no
defined end structure leaves nothing known to follow this transfer.  By
making Response a separate structure that physically follows the data
payload there is an end confirmation.  Should there be an error in sequence,
data will be accepted followed by perhaps some unknown command responses
until everything falls back into sync.  A dubious savings having dummy
status in each read exchange to save one possible structure at the end.
Rather than processing one response structure, you must process two where
one is out of sequence to the function.

Doug
>
> Julian, if it will make this issue go away, maybe the residual
> fields should be
> removed from the data PDU.  If there is residual, it will be sent
> in a status
> PDU.
>
> -Matt
>
> >
> >
> > Doug
> >
> > > -----Original Message-----
> > > From: Robert Snively [mailto:rsnively@Brocade.COM]
> > > Sent: Friday, September 08, 2000 9:20 AM
> > > To: 'Douglas Otis'; Stephen Bailey; ips@ece.cmu.edu
> > > Subject: RE: Data in SCSI Response or SCSI Data
> > >
> > >
> > > In FCP, because FC is a multi-protocol environment that may have
> > > drivers independent of the SCSI drivers, an actual buffer allocation
> > > was transmitted independently of the SCSI command.  Both overrun
> > > and underrun indicators and a residual are returned.  They are
> > > protocol related, not SCSI related.
> > >
> > > There are cases where incorrect length indications are provided
> > > in the SCSI model for both under-run and over-run cases, but they
> > > are rare and typically associated with legacy tape drive programs.
> > >
> > > Bob
> > >
> > > >  -----Original Message-----
> > > >  From: Douglas Otis [mailto:dotis@sanlight.net]
> > > >  Sent: Wednesday, September 06, 2000 6:26 PM
> > > >  To: Stephen Bailey; ips@ece.cmu.edu
> > > >  Subject: RE: Data in SCSI Response or SCSI Data
> > > >
> > > >
> > > >  Steph,
> > > >
> > > >  Although a logical use of a residual, I can not see where it
> > > >  is defined
> > > >  should the allocation length be greater than the returned
> > > >  length.  Should
> > > >  the allocation be less than the response, this is a Check
> > > >  Condition and the
> > > >  residual is defined.  It would seem a residual in a case
> > > >  with adequate
> > > >  allocation is not interesting or defined.  Do you know where
> > > >  this mechanism
> > > >  you describe is defined?  In normal use, without a Check
> > > >  Condition such
> > > >  information is not returned to the application.
> > > >
> > > >  Doug
> > > >
> > > >  > -----Original Message-----
> > > >  > From: owner-ips@ece.cmu.edu
> > > >  [mailto:owner-ips@ece.cmu.edu]On Behalf Of
> > > >  > Stephen Bailey
> > > >  > Sent: Wednesday, September 06, 2000 3:45 PM
> > > >  > To: ips@ece.cmu.edu
> > > >  > Subject: Re: Data in SCSI Response or SCSI Data
> > > >  >
> > > >  >
> > > >  > > When do you get GOOD status and residual counts on a read?
> > > >  > What is causing
> > > >  > > the target to get the length wrong?
> > > >  >
> > > >  > One example is an INQUIRY command.  The inquiry data length is
> > > >  > target-specific.  Typically the CDB allocation length (and
> > > >  DL) are set
> > > >  > to some arbitrary large value (0xff), and the target sends back
> > > >  > everything it has.  The transfer ends with success status.
> > > >  >
> > > >  > There can certainly be transfer residual and no SCSI
> error status.
> > > >  >
> > > >  > Steph
> > > >  >
> > > >
> > > >
> > >
>