RE: iSCSI: freeing resources during data-in and r2t

[Black_David@emc.com]

> As I said to David, it is probably too late for a change here but
> it seems like we could have used StatSN (actually with a different name)
> as a counter of everything going from the target to the initiator.
> That way, the target does not have to associate the TCP ACK in order
> to tell if the initiator got a PDU.

A word of caution - the TCP ACK indicates that TCP delivered the bytes
- if CRC digests are in use, a digest error could cause those bytes not
to form a usable PDU.

I view the A bit as an 80% solution that addresses the major portion
of the need for targets to free resources prior to command completion,
and I think Eddy's in reluctant semi-violent agreement that no change
should be made here.

Thanks,
--David

-----Original Message-----
From: Eddy Quicksall [mailto:eddy_quicksall@ivivity.com]
Sent: Monday, July 22, 2002 3:12 PM
To: Julian Satran
Cc: Black_David@emc.com; ips@ece.cmu.edu; owner-ips@ece.cmu.edu
Subject: RE: iSCSI: freeing resources during data-in and r2t


There are other resources besides the buffers that are used to transmit the
data and it would be nice if those could be freed as soon as I know the
initiator has them.
 
As I said to David, it is probably too late for a change here but it seems
like we could have used StatSN (actually with a different name) as a counter
of everything going from the target to the initiator. That way, the target
does not have to associate the TCP ACK in order to tell if the initiator got
a PDU.
 
Eddy
-----Original Message-----
From: Julian Satran [mailto:Julian_Satran@il.ibm.com]
Sent: Friday, July 19, 2002 8:44 PM
To: Eddy Quicksall
Cc: Black_David@emc.com; ips@ece.cmu.edu; owner-ips@ece.cmu.edu
Subject: RE: iSCSI: freeing resources during data-in and r2t



        Eddy, 

What resources?  R2Ts for which the initiator has finished sending data can
be discarded. 
Targets build R2Ts as needed. 
So what resources do you have in mind?  Data Buffers at initiator? According
to SAM2 you have to keep them up to the bitter end unless 
you request delivery strictly in order in which as you are better off
issuing in sequence short writes if you want to allow recovery. 
If that is what you have in mind there is a basic asymmetry build in SCSI
(and others) that the command issuers has 
the resource to keep the command going while the target (that can't plan the
commands hitting it) has to be taken care of. 

iSCSI is not worsening this (neither improving on it). 

Regards, 
Julo 


Eddy Quicksall <eddy_quicksall@ivivity.com> 
Sent by: owner-ips@ece.cmu.edu 
07/19/2002 03:43 PM 
        
        To:        Black_David@emc.com 
        cc:        ips@ece.cmu.edu 
        Subject:        RE: iSCSI: freeing resources during data-in and r2t 

       


Yes, I agree that it is probably too late to think about this. But just to
put you in perspective, I was referring more to the data-in, not the R2T's
and the A bit causes extra traffic. 
  
I mostly wanted to point this out, I am not really asking for a change
unless the group thinks it is relevant. If no one has an issue (and I really
don't), then we can drop the thread. 
  
Eddy 
-----Original Message-----
From: Black_David@emc.com [mailto:Black_David@emc.com]
Sent: Wednesday, July 17, 2002 9:57 PM
To: eddy_quicksall@ivivity.com
Cc: ips@ece.cmu.edu
Subject: RE: iSCSI: freeing resources during data-in and r2t

Eddy, 
  
The A bit in Data-In plus Data ACK (SNACK type 2 in -14, Julian's 
renumbering of the SNACK types in the working version of -15 is/will 
be/has been undone) already provides the intermediate resource freeing 
for Data-In.  This is only available for ErrorRecoveryLevel >= 1 
We've never thought that R2Ts would consume enough resources to 
be worth putting in support for intermediate resource 
freeing of them.  Adding another sequence number at this late date 
to deal with a "slight inefficiency" when the A bit is used properly 
(SNACK-Data ACK PDUs vs. a piggybacked count) does not seem like a 
good idea. 
  
Thanks, 
--David 
-----Original Message-----
From: Eddy Quicksall [mailto:eddy_quicksall@ivivity.com]
Sent: Wednesday, July 17, 2002 12:15 PM
To: Mallikarjun C. (E-mail)
Cc: ips@ece. cmu. edu (E-mail)
Subject: RE: iSCSI: freeing resources during data-in and r2t

I think see a slight inefficiency in how resources are freed when multiple
data-in and r2t's PDU's are used when ErrorRecoverLevel > 0 or TCP ACK is
not available to the iSCSI layer. 
  
Data-in and R2T's use DataSN and R2TSN. To free those resources, ExpStatSN
is used. 
  
But if several R2T's and/or Data-in PDU's were used, resources used for
those PDU's are tied up for the entire command. 
  
Since other commands could be received during this time, it would be nice if
those commands could contain information that would tell the target that the
R2T and/or Data-in PDU's have been received. 
  
I know a radical change at this point is probably a bad idea but I just
wanted to throw out the idea ... if the StatSN/ExpStatSN were changed to
something like RespSN/ExpRespSN and if everything going from the target to
the initiator carried a new RespSN, then the resources could be freed up
sooner. 
  
I would hate to use the A bit for this because it causes more traffic. 
  
Eddy 
mailto:Eddy_Quicksall@iVivity.com