Re: iSCSI : digest error handling violates EMDP/InDataOrder

[julian_satran@il.ibm.com]

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