Re: Status summary on multiple connections

[David Robinson <David.Robinson@EBay.Sun.COM>]

There are two types of out of order processing we need to
consider:
	1) Commands that have been received and ACK'd by the transport
	2) Commands that have been received and not ACK'd

In the first case this is purely a SCSI issue on if the commands
are ordered and how the target processes them. As far as the
transport is concerned the packets are "delivered".  In the second
case the transport is still required to track the possible
retransmissions and process the transport ACK for all of
the data when the missing segment arrives.  It will overly
complicate the iSCSI layer to successfully track which parts 
of the sequence space it has processed, defend against 
retransmissions, and handle packets that have more than one 
command, and retransmissons that may be only part of a command.

This is all doable in an implementation (a bit messy at times)
but to support this as an explicit feature we need to start
adding in an iSCSI layer sequence space.

I propose that we leave the processing of transport
level un-ACK'd commands and data as an implementation detail
and not make it an explicit iSCSI feature. Dropped or reordered
TCP segments are rare enough in high performance environments
that this is really a non-issue.

	-David

julian_satran@il.ibm.com wrote:
> 
> David,
> 
> I think that RDMA out-of-order processing is important only in order not to
> have data
> piling up in adapters. It does not mean that data gets really "committed".
> The commands can be executed in or out-of-order - this is a pure SCSI
> story.
> But if they have to be executed in order the ordering provided by SCSI
> will enable it.
> And as you and others have pointed out the same mechanism is good for both
> ordering and flow control.
> As far as I understand it windowing - although more expensive - is a better
> technique over a wide variety of latencies than credits.
> 
> And BTW we even considered credits for a "prefetching mechanism" for
> chained
> commands but where told that those are very much "out of fashion" (i.e. not
> worth speeding up on Elefants).
> 
> Julo
> 
> David Robinson <David.Robinson@EBay.Sun.COM> on 29/09/2000 21:25:52
> 
> Please respond to David Robinson <David.Robinson@EBay.Sun.COM>
> 
> To:   ips@ece.cmu.edu
> cc:    (bcc: Julian Satran/Haifa/IBM)
> Subject:  RE: Status summary on multiple connections
> 
> > I am left with the following impression as to what was indicated here:
> > - In general, command ordering is not relevant
> > - If the initiator filesystem detects an ordering dependency, it will
> wait
> > until outstanding commands are complete before issuing the dependant
> > command.
> >
> > This may be a reasonable means of operation for the disk world. It is
> > woefully inadequate for the tape world, as follows:
> > - In general, command ordering is crucial - out of order command
> processing
> > will lead to data corruption.
> > - This would require the initiator backup application to block on
> completion
> > of every single write command of a backup operation before issuing the
> next
> > command.
> >
> > If this blocking were performed, both the throughput and capacity of a
> tape
> > device/media would be negatively impacted by an order of magnitude or
> more.
> > This would occur even assuming an instantaneous transport.
> 
> I am hearing different stories on the issue of ordering.  One side
> is pushing hard for techniques that will allow out of order
> execution using various RDMA techniques. This clearly states for
> a certain class of devices (e.g. tapes) ordering is crucial.
> I thought this problem was already solved at the SCSI layer
> through the use of ordered commands which in general are not used
> for disks but always used for tapes?  Since FC will reorder this
> has to be a solved problem. Would not an initiator talking to
> a tape target simply set the ordering flag?
> 
> Lastly, for a TCP based connection ordering can easily be made a
> non-issue, simply don't try to process segments out of order.  I
> will defer to a transport expert, but I believe processing
> TCP segments by an application out of order might cause problems.
> In particular since the out of order segment is not ACKed until
> after the missing segments arrive, they can be retransmitted
> multiple times.  SACK helps this but does not guarentee that
> segments will not be retransmitted. So to process out of order
> segments the application must maintain a list of which segments
> have been processed as well, yuck!
> 
>      -David