Re: iSCSI: Flow Control

["Matt Wakeley" <matt_wakeley@agilent.com>]

rbg@ipperformance.com wrote:

> 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 ?

Charles Binford answers this question very well in a separate message.


> 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 ?

No, but disk array storage attach (and tapes) is the major market.

-Matt

>
>
> > 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>