Re: iSCSI: Flow Control

["John Hufferd/San Jose/IBM" <hufferd@us.ibm.com>]

Matt,
I am not sure that you answered David's Question.  Even if the SCSI layer
uses RTT, what if ISCSI did not?  What would be the result?

.
.
.
John L. Hufferd



"Matt Wakeley" <matt_wakeley@agilent.com>@ece.cmu.edu on 09/21/2000
12:35:55 AM

Please respond to Matt Wakeley <matt_wakeley@agilent.com>

Sent by:  owner-ips@ece.cmu.edu


To:   "Reflector, IPS" <ips@ece.cmu.edu>
cc:
Subject:  Re: iSCSI: Flow Control



Well, "modern" disk arrays imployed on SCSI and FC use XFER_RDYs (RTT).
They don't
seem to question it.

-Matt

David Robinson wrote:

> Matt Wakeley wrote:
>
> > Ok, so now the SCSI processes the first command, and sends an RTT
(XFER_RDY in
> > FC terms) to the initiator.  Now, the initiator sends the data down the
same
> > TCP connection, and it gets stuck behind all those 998 commands in the
TCP
> > receive buffers.  The command can't complete because it can't get the
data,
> > and the data can't be delivered because there's no room for the
commands in
> > front of it.  Deadlock.  Do you see the issue now?  (this is a good
example of
> > why the single TCP connection model, be it synchronous or asynchronous,
is
> > bad).
>
> As I said in other e-mail I question the use of RTT in a modern
> environment with large buffering. Data immediately following the
> command makes much better sense to me, but if we must support
> this environment then you are right that deadlock can occur.
>
>         -David