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

[Julian Satran <Julian_Satran@xxxxxxxxxx>]

As assigning CMD sequence number is
the prerogative of the initiator I do not  understand the point of
send out of order commands. If you want to optimize within an independent
RDMA layer at the initiator the assign the numbers there.  Initiators
sending out-of-order packets will probably be flagged as bad by any of
the certification packages.

Julo

Parav Pandit <paravpandit@xxxxxxxxx>

01/09/07 09:18

To	David Sheehy <David_Sheehy@xxxxxxxxxxxxxx>, Julian Satran/Haifa/IBM@IBMIL
cc	ips@xxxxxxxx
Subject	RE: rationale to send PDUs in increasing CmdSn on single co        nnection

Exactly thats the reason which lead me to asko this
question.

DMA optimization is possible if we can send out of
order commands to the TCP layer on single connection.

If I take pragmatic approach, 
Any idea, currently how current single connection
target implementation is?

I mean, if current targets are smart enough (accept
out of order PDUs on single connection- liberal on
point 3.2.2.1), then initiators can use their DMA
optimization.
What do you say?

Parav


--- David Sheehy <David_Sheehy@xxxxxxxxxxxxxx> wrote:

> > 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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