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

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

        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