Re: iSCSI : digest error handling violates EMDP/InDataOrder

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

In previous exchanges in this thread, it was stated that EMDP only
governs data order within a sequence. This, to me [and others from FC
world, as was noted by Bob] reads as the definition of Random relative
Offset [as defined by bob].

However, Section 2.7.5 on Buffer Offset states that :
"Output data within a burst MUST be delivered in increasing buffer
offset order".

This contradicts the above by indicating that iSCSI forbids Random
Relative Offset.

So, does EMDP apply to WRITEs or does it not (?). 

Also, does iSCSI allow data overlay or is this prohibited ? I'd suggest
the following be clearly described in the draft :

a) iSCSI defines EMDP to govern the order of data PDUs within a SCSI
command for both READ and WRITE operations. An EMDP setting of 0 implies
data overlay is forbidden.

b) The out-of-order transmission of a given sequence of WRITE data PDUs
in response to an R2T (the random relative offset property) be
explicitly dis-allowed by iSCSI [which is currently stated in Section
2.7.5].

Regards,
Santosh


julian_satran@il.ibm.com wrote:
> 
> Santosh,
> 
> Let's take a systematic approach to it.
> 
> Restriction on data ordering are required if the source or the destination
> of the data is unable to deliver or take data data in any order other that
> sequential.
> 
> Semiconductor or other direct access memories don't  have this restriction.
> 
> Tapes and other sequential media do have this type of restriction and so
> some streaming devices.
> 
> If the restricted device is a target of a SCSI operation with an
> unrestricted initiator then:
> 
> a. on reads the target can always ship its data in sequential order
> b. on writes the target can  always request the data in sequential order
> 
> However if the restricted device is an initiator then:
> 
> a. on reads the initiator will request the target to send the data in order
> b. on writes the restricted initiator will have to get the R2Ts in order
> from the target and will be able to support data recovery through an R2T
> only if it has enough buffered data.
> 
> A restricted device will act as an initiator only if it becomes a third
> part copy manager (CM) in a third party operation an does copy from one of
> its devices to another device.
> 
> Introducing a new mode bit (as Robert Snively seems to suggest) will not
> change the fact that the restriction can't be upholded
> and do recovery unless the restricted initiator has enough memory.
> 
> The spec should only specify a way to terminate a command in those
> conditions and leave it at that.
> 
> I will change the wording of the DataOrder to make it clearer but I
> consider the whole issue entirely academic and overblown.
> 
> Recall also that a CM implemented which such severe buffering restrictions
> violates the basic SCSI assumption that a target is the data master.
> 
> Regards,
> Julo
> 
> Santosh Rao <santoshr@cup.hp.com> on 28/04/2001 00:03:26
> 
> Please respond to Santosh Rao <santoshr@cup.hp.com>
> 
> To:   Julian Satran/Haifa/IBM@IBMIL
> cc:   Black_David@emc.com, ips@ece.cmu.edu
> Subject:  Re: iSCSI : digest error handling violates EMDP/InDataOrder
> 
> 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
>  - santoshr.vcf
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