RE: Data in SCSI Response or SCSI Data

[julian_satran@il.ibm.com]

Overrun/Underrun situations are a "bridging patch" from the block count
model
used by the "classical SCSI" (perhaps matched by some buffering model in
some transports) and the byte count model specified by SAM.

As a command has a byte count associated to it and the CDB has a block
count
the resulting inconsistency (if any) is reported in overrun/underrun byte
count delivered by
the target.

As for a non-zero residual being an error, as far as I can read it - it is
today a target decision
and except for some legacy devices most do not report it as an error ( a
far cry from
the 360 channel days where it was an error unless explicitely requesting it
not to be by
a bit in the command).

Julo

"Douglas Otis" <dotis@sanlight.net> on 08/09/2000 21:50:16

Please respond to "Douglas Otis" <dotis@sanlight.net>

To:   "Robert Snively" <rsnively@Brocade.COM>, "Stephen Bailey"
      <steph@cs.uchicago.edu>, ips@ece.cmu.edu
cc:    (bcc: Julian Satran/Haifa/IBM)
Subject:  RE: Data in SCSI Response or SCSI Data




Bob,

Thanks, that helps an understanding of this PDU 0x45.  If iSCSI is to copy
FCP 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.

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
> >  >
> >
> >
>