Re: iSCSI: Flow Control

[rbg@ipperformance.com]

In "modern" disk arrays, what problem does XFER_RDYs solve (or is it a
feature) ? Is there a concept of "command+immediate data" for these 
disk arrays ?  

Just as a general question.. do we need to comprehend and accomadate 
all legacy issues ? 

Matt, Are you suggesting that iSCSI should be limited to disk arrays ? 


> On Thu, 21 Sep 2000 00:35:55 -0700 you said:
> 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
> 
    _____________________________________________________________
    Robert Gordon                      rbg@ipperf.com
    IP Performance, Inc  
    Austin, Texas.                     <http://www.ipperf.com>