Re: iSCSI: SNACK R2T/Data

["Julian Satran" <Julian_Satran@il.ibm.com>]

Santosh,

None of your arguments makes much sense.
The ACK (a third form of SNACK) can be done at explicit boundaries and only
by initiators
supporting the within command class.  For most of the initiator it will
require 0 additional resources as they either
don't do they recovery, or the input data is short (the ack is not needed
at the end as the status ack acks the data too).
For the ones that have long exchanges (tape and 3rd party) it is a big
help.

But you did not fail (again) to make your point.

Julo


                                                                                        
                    Santosh Rao                                                         
                    <santoshr@cup.       To:     ips@ece.cmu.edu                        
                    hp.com>              cc:                                            
                    Sent by:             Subject:     Re: iSCSI: SNACK R2T/Data         
                    owner-ips@ece.                                                      
                    cmu.edu                                                             
                                                                                        
                                                                                        
                    20-09-01 18:54                                                      
                    Please respond                                                      
                    to Santosh Rao                                                      
                                                                                        
                                                                                        




Matthew,

We have gone through this thread of discussion regarding DataSNa long time
back on
ips and the consensus has been that I/O transfer sizes  are not large
enough to
justify OUT_OF_BAND acknowledgement of data. [achieved by having the
initiator
generate NOP-OUTs in response to received data pdu's to send a DataSN ack.]

The primary dis-advantage with that scheme was that the data ack's were
being
generated out-of-band, and therefore, implied extra cost in terms of
initiator
resources for each I/O, as well as increased wire traffic per I/O,
performance
penalty, etc.

I am opposed to the draft requiring initiators to send out-of-band ack's to
data
pdu's through the use of initiator generated NOP-OUT pdus. This is a
performance
penalty, a resource overhead, and not really justified in most I/O traffic
due to
the avg. data xfer sizes.

Regards,
Santosh


Julian Satran wrote:

> Matthew,
>
> I am also in favor of such a mechanism.  It is easy to implement (another
> form of SNACK) and we have already built-in a mechanism by which an
inbound
> stream can be marked for acking (the inbound sequence separator F bit).
> It can also be specified as mandated only if the within-command recovery
> class is present.
>
> However I am reluctant to open again this issue except if there are more
> supporters. Data ACKs where hastily dropped at the interim meeting in
> Orlando.  I recall Somesh Gupta, Pierre Labat and Santosh Rao as being
very
> vocal against them (arguing complexity) and carrying the room with them.
>
> Julo
>
>
>                     "BURBRIDGE,MATTH
>                     EW                     To:     Julian
Satran/Haifa/IBM@IBMIL,
>                     (HP-UnitedKingdo        ips@ece.cmu.edu
>                     m,ex2)"                cc:
>                     <matthew_burbrid       Subject:     RE: iSCSI: SNACK
R2T/Data
>                     ge@hp.com>
>
>                     19-09-01 17:25
>                     Please respond
>                     to
>                     "BURBRIDGE,MATTH
>                     EW
>                     (HP-UnitedKingdo
>                     m,ex2)"
>
>
>
> I am very much in favour of having a positive ACK mechanism to control
> buffer resources at the target.  If there is a very large transfer (e.g.
1
> Mb) then the sender can release buffer space once it knows that the
> receiver
> has received the data.  It is worth pointing out that this mechanism is
for
> buffer control and is not for flow control which, as we all know, is
> handled
> by TCP.
>
> Cheers
>
> Matthew Burbridge
> Senior Development Engineer
> NIS-Bristol
> Hewlett Packard
> Telnet: 312 7010
> E-mail: matthewb@bri.hp.com
>
> -----Original Message-----
> From: Julian Satran [mailto:Julian_Satran@il.ibm.com]
> Sent: Wednesday, September 19, 2001 6:28 AM
> To: ips@ece.cmu.edu
> Subject: Re: iSCSI: SNACK R2T/Data
>
> There is no ACK mechanism. The group wisdom was that there is no need for
> one.
> Incoming data and R2Ts are not acked (a mechanism that did that existed
and
> was based on NOP-Out).
>
> Julo
>
> Michael Schoberg <michael_schoberg@cnt.com> on 18-09-2001 19:09:51
>
> Please respond to Michael Schoberg <michael_schoberg@cnt.com>
>
> To:   "'ips@ece.cmu.edu'" <ips@ece.cmu.edu>, Julian
Satran/Haifa/IBM@IBMIL
> cc:
> Subject:  iSCSI: SNACK R2T/Data
>
> Old subject, but I couldn't find any discussion on this:
>
> When does the target know it no longer needs to hold R2T & Data PDUs?
> StatSN responses are acknowledged through the ExpStatSN field received in
> future I->T requests.  What's the acknowledgement method for R2T & Data
> PDUs?  Is it tied to the original request and acknowledged through the
> ExpStatSN acknowledgment of the request's response?
>
> Thanks.

 - santoshr.vcf