Re: iSCSI:flow control, acknowledgement, and a deterministic recovery

[Santosh Rao <santoshr@cup.hp.com>]

Doug [& All],

Some comments on this thread :

1) The immediate command feature can be exploited by initiators which
are not required to provide strict command ordering for their SCSI
sub-system. (The majority of today's scsi stacks). IOW, all commands can
be sent with CmdSN=0 to indicate no ordering required.

Any scheme to restrict the number of "immediate commands" that can be
sent prohibits such a feature and is un-desirable.

2) The (ExpCmdSN, MaxCmdSN) based acknowledgement and flow control is a
freebie that targets can use to implement additional flow control.
There's nothing in the spec that mandates that a target MUST use this
feature to throttle its window and apply flow control.

Hence, any dependence and assumptions on the existence of CmdSN based
flow control may be incorrect, since the target may be using QUEUE FULL
(TASK SET FULL) to apply flow control.

> I also suspect it is a mistake to allow commands
> to remain trapped within the sequencer during emergency or abnormal events
> signified by the use of these immediate commands.

3) If the initiator suspects that the CmdSN queue at the target is
stuck, it can always use a task mgmt command or even an iSCSI login with
the X (restart) bit to perform a cleanup of stuck commands at the
target. There's no need to build in implicit assumptions that a CmdSN
with the immediate flag result in a clean-up of previously pending
commands at the CmdSN queue of the target.

- Santosh


Douglas Otis wrote:
> 
> David,
> 
> > > To not support rejects, you are then relying on the target to
> > > handle these now out of sequence commands.
> >
> > Immediate commands are "out of sequence" by definition - immediate
> > means deliver immediately without regard to CmdSN order.  Only the
> > target can do this, so I don't see any problem with relying on the Target
> > to do so.
> 
> The commands that are NOW out of sequence are those commands bypassed by the
> immediate command.  If this immediate command was Rewind, then a long list
> of write commands become invalid as a new tape is now needed for those
> operations.  You are expecting that the target to handle a long list of
> invalid commands made possible by the iSCSI sequencer.  Commands invalidated
> by immediate commands become out of sequence commands.
> 
> I'll reserve further comment until Julian has made some progress in

> 
> Forgive my advocacy, but if one does not attempt to support a position, then
> the subject is not explored.
> 
> Doug
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