Re: iSCSI : Initiators expected to fake CHECK CONDITIONS.

[Stephen Bailey <steph@cs.uchicago.edu>]

Julian,

This seems like the zillionth time aired this same disagreement.

I think we should try to reach a WG consensus on whether iSCSI should
use SCSI status as a means for reporting protocol-related errors, and
kill it once and for all.

I'm strongly against it.

> And an error in the iSCSI layer gets reported by the next layer - that is
> the regular layering technique (and BTW I am getting a bit uneasy about all
> this preaching on layering when it is not obvious that you understand all
> the implications of the point).

Santosh seems to understand 100%.  I agree with Santosh.

SAM defines three pieces of status returned by Execute Command()
(which is, in turn implemented by each SCSI protocol, e.g. iSCSI):
  1) Service Response: task complete, linked command complete, service
     delivery or target failure.
  2) SCSI status byte
  3) SCSI sense data

SAM clearly suggest that a protocol's means for signalling
protocol-detected errors is the service response status when it says:

  The actual protocol events corresponding to a response of TASK
  COMPLETE, LINKED COMMAND COMPLETE or SERVICE DELIVERY OR TARGET
  FAILURE shall be specified in each protocol standard.

Note that defining protocol error events in terms of TC, LCC, SDF and
TF, remains an abstraction.  An actual implementation can chose to
signal these events by whatever means.  All SCSI implementations I've
seen do have a status return component which exactly corresponds to
SAM's service response status, but has more than just these four
alternatives.  Typically, that includes things like success, command
timeout, addressing failure (selection timeout in ||SCSI, bad AL_PA in
FCAL, etc.), command aborted, bus parity error, etc..  What iSCSI does
need to do is clearly define its error events AS protocol events,
which is what describing them in terms of the SAM specified set does.

||SCSI, FCP and the other SCSI protocol standards do this.

Operationally, this means that in iSCSI:

  o protocol-specific errors for a task detected by the target without
    a CLEARLY corresponding SCSI error return should be signalled
    using the iSCSI response mechanism.

    The initiator will handle these errors by recording them in some
    appropriate way, and selecting an appropriate service response
    status value.

  o errors for a task detected directly by the initiator are handled
    by recording them in some appropriate way, and selecting an
    appropriate service response status value.

I don't know that there's a single sentence or section in SAM which
says this, but it clearly implies that the components of SCSI status
(status byte and sense) are data which are CARRIED (not created) by
SCSI protocols, for the use of the protocol-independent components.
For example, a disk peripheral driver reacts to SCSI status and sense
generated by disks for SCSI operations that it starts.

SCSI status should be equivalent to the SCSI status returned by the
logical unit.

SCSI protocols are not citizens of the SCSI status space, which means
that the real citizens (the command standards, and SAM), may define
semantics of SCSI error codes which conflict with iSCSI's selections.
The `hardware error' sense key might be defined to mean something very
specific within a particular SCSI peripheral command set, and your
choice of synthesizing SCSI status within the protocol could cause
this behavior to misfire.

> [js} the error was generated by a faulty controller and I did not find
> any other SCSI sense fit for it[/js]

That's because there IS no SCSI sense fit for it.  It is a
protocol-detected, protocol-unique error.  Therefore it should be
signalled using the service response status.

Steph