Subject: RE: iSCSI : digest error handling violates EMDP/InDataOrder

From: Robert Snively <[email-masked]>
Date: Mon, 23 Apr 2001 07:46:51 -0700

Seems to me that there are some unclarities in this area as well.

There are really two pieces being discussed as one:

	EMDP (a SCSI functionality)

	Random relative offset (a transport functionality)

EMDP is used to allow a target to request or deliver its data
out of order.  This is used for things like passing a stripe
segment from a RAID data extent as soon as it has been accumulated,
rather than waiting until all previous parts of the RAID data
extent have also been accumulated and delivered.  It is also used
for things like "start anywhere" reading of a disk track.

It says nothing about the ordering of data within a PDU or sequence
which must be ordered according to the rules of the protocol.  Fibre
Channel allows the data within a sequence to be transmitted in order
or out of order by using the login parameter "random relative offset".
Almost all devices choose to login and require "continuously increasing
relative offset".

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From: Santosh Rao <[email-masked]>
Date: Tue, 24 Apr 2001 11:30:09 -0700

What is the final resolution on this EMDP issue. IMHO, iSCSI must retain
the EMDP semantics as defined in FCP, SRP. i.e. It controls the order of
the data across the entire SCSI command. (which includes sending R2T
requests in order, if EMDP was set to 1).

Some additional thoughts on this topic are :

1) Is it worth a finer granularity of control wherein the initiator be
allowed to negotiate with the target that R2T requests be sent in-order
, while not imposing any constraints on the Read Data PDU order.

2) Should control be provided over a "Random Relative Offset" feature,
as Bob describes it below, or is it to be assumed that iSCSI Data PDUs
will always be in-order within a sequence ?

3) Speaking of sequence, this terminology has been often used in this
thread. Where is the notion of a sequence defined in iSCSI ? What is the
definition of an iSCSI sequence.

- Santosh