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RE: 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:Julian_Satran@xxxxxxxxxx]
Sent: Friday, August 31, 2007 8:54 AM
To: Parav Pandit
Cc: ips@xxxxxxxx
Subject: Re: rationale to send PDUs in increasing CmdSn on single connection



Parav Pandit <paravpandit@xxxxxxxxx> 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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