Re: iSCSI: changing MaxPDUDataLength

["Mallikarjun C." <cbm@rose.hp.com>]

Eddy,

I am not clear on why you think the target code becomes messy on 
a PDU size change.  The last para in section 4.4 states the following -

"Parameters negotiated by a text exchange negotiation sequence become
effective only after the negotiation sequence is completed."

Since the target gets to complete a text negotiation with a Text Response
PDU, it can time the applicability of a changed MaxRecvDataSegmentLength.

Requiring that an initiator must idle the connection before changing this
parameter, IMHO, is counter-productive when there's a dynamic PMTU
degradation in the presence of long-running commands (writes in particular).
Once the initiator starts the renegotiation, target would ideally want to quickly 
renegotiate its receive size as well to receive ULPDU-contained segments.

In short, seems to me that the draft is saying what you would like.
--
Mallikarjun

Mallikarjun Chadalapaka
Networked Storage Architecture
Network Storage Solutions
Hewlett-Packard MS 5668 
Roseville CA 95747
cbm@rose.hp.com


----- Original Message ----- 
From: "Eddy Quicksall" <eddy_quicksall@ivivity.com>
To: "Julian Satran" <Julian_Satran@il.ibm.com>
Cc: <ips@ece.cmu.edu>
Sent: Tuesday, June 11, 2002 7:56 AM
Subject: RE: iSCSI: changing MaxPDUDataLength


> I think it should be a requirement because if it is not, all target code
> will have to have the messy code. Or, we could say that if it is not idle
> commands, then the target may reject it.
>  
> Eddy
> 
> -----Original Message-----
> From: Julian Satran [mailto:Julian_Satran@il.ibm.com]
> Sent: Tuesday, June 11, 2002 9:11 AM
> To: Eddy Quicksall
> Cc: ips@ece.cmu.edu; pat_thaler@agilent.com
> Subject: RE: iSCSI: changing MaxPDUDataLength
> 
> 
> 
> That is a fair request - we may slip in a recommendation to that effect (in
> chapter 11?) 
> 
> Julo 
> 
> 
> 
> Eddy Quicksall <eddy_quicksall@ivivity.com> 
> 
> 
> 06/11/2002 04:28 AM 
> Please respond to Eddy Quicksall 
> 
> 
>         
>         To:        Julian Satran/Haifa/IBM@IBMIL 
>         cc:        ips@ece.cmu.edu, pat_thaler@agilent.com 
>         Subject:        RE: iSCSI: changing MaxPDUDataLength 
> 
>        
> 
> 
> How about if we say that an initiator must not change the MaxPDUDataSize
> unless it first idles the commands (at least the ones with the R bit) or if
> ErrorRecoveryLevel==0? 
>   
> That would simplify target code greatly and would meet the design goals; and
> since it should be rare to change it, it would be of little impact to idle
> the commands for a split second. 
>   
>   
> Eddy 
> -----Original Message-----
> From: Julian Satran [mailto:Julian_Satran@il.ibm.com]
> Sent: Monday, June 10, 2002 8:06 PM
> To: Eddy Quicksall
> Cc: ips@ece.cmu.edu; pat_thaler@agilent.com
> Subject: RE: iSCSI: changing MaxPDUDataLength
> 
> 
> I said only that when you change length you can ask for all PDUs after the
> ack! (no need to keep a tall - the old offsets are good up to the hole). 
> 
> Julo 
> 
> 
> Eddy Quicksall <eddy_quicksall@ivivity.com> 
> 
> 
> 06/11/2002 12:32 AM 
> Please respond to Eddy Quicksall 
> 
>         
>        To:        Julian Satran/Haifa/IBM@IBMIL 
>        cc:        ips@ece.cmu.edu, pat_thaler@agilent.com 
>        Subject:        RE: iSCSI: changing MaxPDUDataLength 
> 
>       
> 
> 
> 
> Julian, 
>  
> Another problem here is that the target has to calculate the offset from the
> DataSN #. And as BegRun can be any value. E.g., BegRun=4 and RunLngth=0
> means starting DataSN=4, send all the data for that sequence. 
>  
> I think it would be a performance hit and waist of memory to keep a tally of
> all PDU sizes just for an occasional SNACK. 
>  
> It's not that it can't be done ... it is more that it is messy and I think
> there is a solution that will satisfy the design requirements but keep the
> software simpler. 
>  
> Eddy 
> -----Original Message-----
> From: Julian Satran [mailto:Julian_Satran@il.ibm.com]
> Sent: Saturday, June 08, 2002 10:16 PM
> To: Eddy Quicksall
> Cc: ips@ece.cmu.edu; owner-ips@ece.cmu.edu; pat_thaler@agilent.com
> Subject: RE: iSCSI: changing MaxPDUDataLength
> 
> 
> That is not completely accurate. 
> The only problem is when PDU size decreases and then SNACK must be for all
> data. 
> Target can also keep a mapping of numbers/to offsets (the list should be
> small and if it gets long ask for ack (A-bit). 
> 
> Julo 
> 
> Eddy Quicksall <eddy_quicksall@ivivity.com> 
> Sent by: owner-ips@ece.cmu.edu 
> 
> 
> 06/08/2002 10:54 PM 
> Please respond to Eddy Quicksall 
> 
>         
>       To:        pat_thaler@agilent.com 
>       cc:        ips@ece.cmu.edu 
>       Subject:        RE: iSCSI: changing MaxPDUDataLength 
> 
>      
> 
> 
> 
> 
> Thanks,
> 
> As a target, I won't be able to let it change until all of the outstanding
> commands are finished (running with ErrorRecoveryLevel>=1). This is because
> I must use the PDU size to compute the offset from a SNACK's
> BegRun/RunLength.
> 
> So, I plan to not give the text response for a MaxPDURecvDataLength in FFP
> until I get back an ExpStatSN == next StatSN.
> 
> This will cause a delay of unknown time before the PDU size can actually
> change ... do you see any problem with this?
> 
> Eddy
> 
> -----Original Message-----
> From: pat_thaler@agilent.com [mailto:pat_thaler@agilent.com]
> Sent: Friday, June 07, 2002 1:13 PM
> To: eddy_quicksall@ivivity.com; ips@ece.cmu.edu
> Subject: RE: iSCSI: changing MaxPDUDataLength
> 
> 
> Eddy,
> 
> If one uses a message sync and steering that relys on the transport segments
> carrying a full PDU, e.g. TCP ULP Framing Protocol (TUF), then if the path
> MTU changes one would want to change the PDU data length to fit the new path
> MTU.
> 
> Pat
> 
> -----Original Message-----
> From: Eddy Quicksall [mailto:eddy_quicksall@ivivity.com]
> Sent: Wednesday, June 05, 2002 12:24 PM
> To: ips@ece.cmu.edu
> Subject: iSCSI: changing MaxPDUDataLength
> 
> 
> Does anybody know a case where it is necessary to support a new PDU data
> length during full feature phase?
> 
> Eddy
> mailto: Eddy_Quicksall@iVivity.com
> 
> 
> 
> 
> 
> 
> 
>