RE: iSCSI - change proposal LUN field definition on every PDU

[Robert Snively <rsnively@Brocade.COM>]

Folks,

I have followed this thread up to the current time, and I have
to agree with the people who think that this is probably
not necessary and has undesirable consequences.

One reason that it is not necessary is that as we write, the
SCSI MIB people are defining the properties and statistic
gathering capabilities of SCSI logical units.  These capabilities
are clearly going to require logical units to capture additional
performance information using SCSI logging commands (which
already capture a huge amount of error information).

A second reason that it is not necessary is that the command
and data counts can be extracted from the command PDU's
CDB field, which is labeled with a logical unit.  Given that
the command completes correctly, the counting is already done
and the count is in the command PDU.

A third reason that it is not necessary is that any decent
instrumentation on the line has to already be capable of sorting
through the TCP/IP mapping, the security mappings, and the
basic PDU identification.  All the monitors that we have today
in the SCSI environment apply a little simple postprocessing to
the data they have captured and presto, they not only identify
PDU's related to particular logical units, but to particular
commands, and they have interpreted the commands and verified
that the responses are appropriate to the commands.

If the instrumentation is inline with the receiving code,
it is by definition aware of the states and identifications
already available to it and does not need this additional information.

As for undesirable consequences:

Paul Koning's objection to the additional functional steps
required to provide the LUN is a valid concern.

Mallikarjun Chadalapaka's concern about the requirement for
additional consistency checking (and a whole new class of
non-SCSI protocol error checking) is also a valid issue.

Let's talk to your colleague and explain to him/her how there
are better ways to achieve his/her goals.

Bob Snively                        e-mail:    rsnively@brocade.com
Brocade Communications Systems     phone:  408 487 8135
1745 Technology Drive
San Jose, CA 95110



> -----Original Message-----
> From: Julian Satran [mailto:Julian_Satran@il.ibm.com]
> Sent: Monday, November 12, 2001 4:31 AM
> To: ips@ece.cmu.edu
> Subject: iSCSI - change proposal LUN field definition on every PDU
> Importance: High
> 
> 
> Dear colleagues,
> 
> A colleague interested in instrumentation approached me with 
> a question 
> about stateless logging of specific LU activity.
> With the current iSCSI PDU formats this is not possible.
> We have consistently avoided having fields that are redundant 
> and will 
> need consistency checking.
> However I think we should consider including the LU field in 
> all PDUs that 
> are referencing a specific LU to simplify this type of 
> instrumentation (as 
> we did with the direction bit in the op-code).
> 
> As I am already in "count-down" mode for version 09 I would 
> appreciate 
> your comments ASAP.
> 
> Julo
>