RE: rationale to send PDUs in increasing CmdSn on single co nnection

David - I admit I did not use the right word - I should have said not "visible" but "good" motivation. And the immediate vs. non-immediate shipping can be solved within any of the layers above. I do not even recall a heated debate about it.

Julo

-----David Sheehy <[email protected]> wrote: -----

To: Julian Satran/Haifa/IBM@IBMIL, Parav Pandit <[email protected]>
From: David Sheehy <[email protected]>
Date: 08/31/2007 09:34PM
cc: [email protected]
Subject: RE: [Ips] rationale to send PDUs in increasing CmdSn on single co nnection

>
and
there was no visible motivation for out of order commands on a single
connection

On the contrary, there was plenty of motivation. It was
overridden by the desire for simplified implementations over single TCP
connections as the Parav conjectured. There are DMA related efficiencies that
can be realized in situations with a mix of solicited and immediate/unsolicited
I/O traffic. The folks in favor of this optimization were outvoted by those who
wanted to simplify their single connection based
implementations.

Dave

----------

From:
Julian Satran
[ mailto:[email protected] ]
Sent:
Friday, August 31, 2007 8:54
AM
To:
Parav Pandit
Cc:
[email protected]
Subject:
Re:
[Ips] rationale to send PDUs in increasing CmdSn on single
connection

Parav Pandit <[email protected]>
wrote on 31/08/2007 12:52:04:

> Hi,
>
> RFC 3720,
section 3.2.2.1 says
>
> "On any connection, the iSCSI initiator
MUST send the
> commands in increasing order of CmdSN, except for
>
commands that are retransmitted due to digest error
> recovery and
connection recovery. "
>
> (Assuming Single TCP connection ISCSI
session)
>
> 1. I interpret above 3.2.2.1 statement as
>
SCSI layer gives SCSI commands to the ISCSI stack in
> the order of Cmd-1
and Cmd-2.
> Cmd-1 will have CmdSn = 10.
> Cmd-2 will have CmdSn =
11.
> ISCSI stack CAN send PDUs to the TCP layer in
> following
order ONLY.
> PDU-1 with Cmd-1.
> PDU-2 with Cmd-2.
>
>
Is this correct interpretation?
> Or
>

Yes
> 2. On a SINGLE connection can ISCSI stack send the
>
PDU-1 with Cmd-2 followed by
> PDU-2 with Cmd-1?
>

NO
> Assuming the answer of the question
#2 is No,
>
> 3. If there are multiple connections in a session
then
> command MAY any way reach out of order. And targets
> need to
wait for the previous expected commands.
>
> So targets will
receive out of order ISCSI PDUs from
> the TCP layer and ISCSI stack
handles them.
>
> So then why initiators have restriction of
sending
> command in the increasing order of CmdSn on SINGLE TCP
>
connection?
>

To simplify recovery and
to...
> Is it to simplify the implementation of targets
> supporting
only single TCP connection?
>

>

and there was no visible
motivation for out of order commands on a single connection

> Regards,
> Parav Pandit
>
>
>
>
>
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