Re: iSCSI : digest error handling violates EMDP/InDataOrder

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

julian_satran@il.ibm.com wrote:
> 
> David,
> 
> I read Bob's mail and my interpretation is similar to his. However I think
> that SPC explicitly states that different transports are free to interpret
> and make use of this page as they find appropriate.
> 
> I have a hard time understanding Santosh's objection as it does not refer
> to the reason the EMDP is there but to the way it is written in FCP (not
> iSCSI).

Julian,

As has been stated earlier, EMDP allows control over the order in which
the target requests outbound data or sends inbound data. EMDP can be
used by initiators to control this order and turn off out-of-order R2T
requests [as well as turn off out of order read data pdus].

This is a useful control option and is already provided by other SCSI
transports. What good reason exists to deny this provision in iSCSI ?

Also, I have some concerns about the ambiguous definition of DataOrder.

Per the spec :
"DataOrder=<yes|no> 
    
The default is yes but targets MAY support no. No is used by iSCSI to
indicate that the data PDUs can be in any order (EMDP = 1). Yes is used
to indicate that incoming data PDUs have to be at continuously
increasing addresses (EMDP = 0)."

Based on the above definition wording :

a) How is DataOrder interpreted for WRITE I/Os ?
b) Is the ordering across the entire SCSI command or a subset of the I/O
? If so, what constitutes this subset ?

Different implementors can arrive at different interpretations reading
the above definition !

- Santosh
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