RE: Addition of CmdSN in Data-out PDU

["BURBRIDGE,MATTHEW (HP-UnitedKingdom,ex2)" <matthew_burbridge@hp.com>]

Since I started this thread I feel I must at least contribute!

The reason why I proposed putting CmdSN (actually it should be RefCmdSN) in
the Data-out PDUs was to enable the target to have a faster search to
associate unsolicited data out PDUs with its SCSI Command PDU.  Solicited
Data-out PDUs do not require this as they have a Target Task Tag.

If all Command PDUs were queued then I believe this would work just fine.
However, as Santosh correctly pointed out they are not and without repeating
what he said this mechanism would not work for immediate command PDUs.

I am sure that particular implementations could make this work but the
underlying argument is that it needs to work and be useful to all
implementations.  The only benefit I now see of having CmdSN in the data PDU
is as a check as implementations must (and can only) use the initiator task
tag to associated the Data-out PDU with the command PDU.  Therefore, IMO it
is not a good enough reason for having CmdSN in the Data-out PDUs simply for
a consistency check.

The benefit of having the data sent unsolicited to minimise if not eradicate
round trip times far out weighs the overhead in having to perform a search
on receipt of unsolicited data. If we could have developed a well defined
mechanism to overcome this overhead then all well and good and that is what
I attempted.  Still, if someone can do this and the solution is simple and
straight forward then I am sure that it will have my backing but until then
...

Cheers

Matthew Burbridge
Senior Development Engineer
NIS-Bristol
Hewlett Packard
Telnet: 312 7010
E-mail: matthewb@bri.hp.com
 


-----Original Message-----
From: Paul Koning [mailto:pkoning@jlc.net]
Sent: Wednesday, October 10, 2001 5:07 PM
To: Julian_Satran@il.ibm.com
Cc: ips@ece.cmu.edu
Subject: Re: Addition of CmdSN in Data-out PDU


Excerpt of message (sent 10 October 2001) by Julian Satran:
> Inconsistency can be legitimate. CmdSN is ephemeral. It can be reused, it 
> may have large holes and using it in an implementation is as bad as a 
> hashed index.

Not true.

CmdSN values are sequential, by definition.  Yes, clearly there will
be small holes because commands complete out of order.  But "large"
holes are unlikely.

In any case, the target has control over that.  I can use an array
whose size is given by the number of pending commands times a
correction factor to account for the likely density of holes.  Then
MaxCmdSN would be updated based on two considerations: the ability to
handle more pending commands, and the need to keep the distance
between oldest (lowest) still active CmdSN and MaxCmdSN bounded by the
size of the lookup array.

So having CmdSN in the DataOut PDU allows this approach, thereby
replacing a hash lookup on a rapidly changing ID space by a simple
array indexing operation.  Without CmdSN, you're forced to use a
mechanism that has a lot more overhead (in the insert/remove or in the
lookup, depending on the mechanism chosen).

	paul