ISCSI: More on Urgent pointer

[Black_David@emc.com]

The discussion on where the Urgent pointer points has
been oversimplified.  Returning to Silvano's question:

>  if Command A and Command B are transmitted in the same
> segment, do you care to specify if the urgent pointer points to the
> first or second command?

The Urgent pointer will definitely not point to the first
command because both commands are in the same segment and
the Urgent pointer is coalesced by the sending TCP.  It
might point to the second command, or it might point to
a command in a future segment that has not arrived (if the
future command was queued before the segment containing
A and B was sent).  iSCSI cannot be any more specific
than this, because to do so (e.g., always require the
Urgent pointer to point within the segment or to the
first command within the segment) would violate RFC 793.

I think Matt and Doug are talking past each other in part
by assigning different meanings to the term "record mark".
Doug appears to be using the term "record mark" to refer
to something that marks every record boundary between
SCSI commands, allowing any command to be recovered from
any point in the stream based solely on knowing where its
"record mark" is.  The Urgent pointer cannot do this (e.g.,
the above paragraphs describe a counter-example with two
record boundaries and only one Urgent pointer).  Matt seems
to have used the term "record mark" to refer to something
that marks a record boundary somewhere (in the hope that
his implementation will find the boundary to be useful).

I will again note that concrete indications from implementers
of how much of a difference this makes in specific scenarios
and how often those scenarios occur/can be expected to occur
would be useful.

Thanks,
--David

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David L. Black, Senior Technologist
EMC Corporation, 42 South St., Hopkinton, MA  01748
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