RE: Addition of CmdSN in Data-out PDU

["Somesh Gupta" <somesh_gupta@silverbacksystems.com>]

Matthew,

Since the unsolicited data does not follow the
command, you need the link list. But since the
unsolicited data must be sent in the same order
as the commands, a link list is enough.

Let us say that you have 8 commands. The ones
for which we expect unsolicted data are marked
as Cnn(ud). And I have marked the unsolicted
data PDUs as UD(nn). The (nn) with data implies
that it is implicit and not actually carried with
the PDU itself.

C01(ud) C02(ud) C03(ud) C04 C05 C06(ud) UD(01) --->
--> C07(ud) UD(02) UD(03) D04 D04 D05 D05 UD(06) --->
--> UD(07) C08(ud) UD(08) --->

After the target receives the command C01, C02, and C03
for which it expects unsolicited data, it puts them in
a link list. It also receives C04 and C05 for which
unsolicited data is not expected and they don't go
on the list. It then receives C06 for which unsolicited
data is expected, and it is added to then end of the list.
Then an unsolicited data PDU is received. It must go
with the command at the head of the list which is C01.
Use the ITT to make sure and you can then take C01 off
the list and so on.

Somesh


> -----Original Message-----
> From: owner-ips@ece.cmu.edu [mailto:owner-ips@ece.cmu.edu]On Behalf Of
> BURBRIDGE,MATTHEW (HP-UnitedKingdom,ex2)
> Sent: Friday, October 12, 2001 7:53 AM
> To: 'Binford, Charles'; ips@ece.cmu.edu
> Subject: RE: Addition of CmdSN in Data-out PDU
>
>
> Charles,
>
> As you have described the spec states that "that unsolicited data MUST be
> sent in the same order as the commands".  This is not the same as
> unsolicited data must follow the command associated with it:  For example:
>
> (Cx = SCSI Command PDU, Dx = The unsolicited data PDUs. The x in all the
> example can be the ITT. It is not the CmdSN.
>
> This is allowed:
>
>   C1 C2 C3 D1 D2 C4 D3 D4
>
> and the target will have to use the ITT to associate the data with the
> command.
>
> Matthew
>
>
> -----Original Message-----
> From: Binford, Charles [mailto:CBinford@pirus.com]
> Sent: Friday, October 12, 2001 2:50 PM
> To: ips@ece.cmu.edu
> Subject: RE: Addition of CmdSN in Data-out PDU
>
>
> I have not verify version 8 is still the same, but version 07-97
> had a rule
> that unsolicited data MUST be sent in the same order as the commands.
>
> Thus, there is no need for a search on the ITT.  The target just needs to
> keep of linked list of I/Os waiting on unsolicited data.  New commands are
> added to the tail, any unsolicited data *should* be associated
> with the I/O
> at the head of the list.  The ITT is used as a sanity check and
> you're done.
>
> What am I missing?
>
> Charles Binford
> Pirus Networks
> 316.315.0382 x222
>
>
> -----Original Message-----
> From: BURBRIDGE,MATTHEW (HP-UnitedKingdom,ex2)
> [mailto:matthew_burbridge@hp.com]
> Sent: Thursday, October 11, 2001 3:21 AM
> To: 'Paul Koning'
> Cc: ips@ece.cmu.edu
> Subject: RE: Addition of CmdSN in Data-out PDU
>
>
> 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
>