iSCSI : Command Ordering Proposal.

["David Peterson" <dap@cisco.com>]

Howdy Charles,
The CmdRN has been renamed to CmdSN (Command Sequence Number).
The previously reserved field (word 0, byte 2) is now the placeholder for
SAM's CRN.
Dave

> -----Original Message-----
> From: owner-ips@ece.cmu.edu [mailto:owner-ips@ece.cmu.edu]On Behalf Of
> Charles Binford
> Sent: Friday, January 26, 2001 9:10 AM
> To: Ips (E-mail)
> Subject: RE: iSCSI : Command Ordering Proposal.
>
>
> I'm confused on a couple of points:
>
> - Are we talking about 'CmdSN' (a field that doesn't exist unless I missed
> an update since iscsi-03) or 'CmdRN'?
>
> - Yes, CRN was added to SAM, but I see nothing in iSCSI that utilizes it.
> Is this thread suggesting a new field be added?  I certainly don't see how
> the current CmdRN field could fill the roll of SAM's CRN and its current
> roll concurrently (different rules on initial value, when to reset, etc.)
>
> Seems to me like adding a new CRN field for command ordering (if the
> application so chooses) and re-defining the CmdRN field to only
> provide the
> flow control satisfies the requirements without overloading the
> functionality of a single field.
>
>
> Charles Binford
> Blue Spruce Networks
> office/cell: (316) 210-6404
> e-fax: (509) 756-4425
>
> -----Original Message-----
> From: owner-ips@ece.cmu.edu [mailto:owner-ips@ece.cmu.edu]On Behalf Of
> csapuntz@cisco.com
> Sent: Friday, January 26, 2001 3:28 AM
> To: Santosh Rao
> Cc: csapuntz@cisco.com
> Subject: Re: iSCSI : Command Ordering Proposal.
>
> Santosh Rao <santoshr@cup.hp.com> writes:
>
> > [1  <text/plain; us-ascii (7bit)>]
> > Julian & All,
> >
> > Proposal :
> > =======
> > CmdSN should be used to enforce ordering only when the Ordered Task Tag
> > attribute is set in the SCSI Command PDU.
>
>
> Santosh,
>
> I believe you may be missing a subtlety to the SCSI task queueing model.
> I may also be missing a subtlety of your argument and, if so,
> please excuse
> this e-mail.
>
> Consider three commands sent down a FIFO SCSI transport:
>
> 	1) Ordered Command
> 	2) Simple Command
> 	3) Simple command
>
> The ordered command is received first and then the two simple commands.
> The ordered command is guaranteed to be executed before either SIMPLE
> command.
>
> Relevant text from SAM-2 rev 14, sec 7.5.1
>
> "The [simple] task shall not enter the Enabled state until all
> older Head of
> Queue and older Ordered tasks have ended."
>
> Your proposed change means that CmdSN no longer is able express that
> the Ordered task is "older" that the "simple" task. Thus, an initiator
> might need to stop-and-wait before issuing #2 and #3.
>
> I say "might" because iSCSI now transports SCSI Command Reference
> Numbers (CRNs) in the command PDU. The CRN is a SAM-2 rev. 15 (and
> FCP) construct that tells us which commands are older than others. So,
> in some cases, the CmdSN may no longer be needed at all to tell which
> commands are "older" than others.
>
> Given that we have CRNs, why do we need CmdSNs? Well, CmdSNs provide
> a couple extra features:
> 	- ordering on task management (SAM doesn't defined
>           that CRNs are used with task management)
>         - ordering when application doesn't support CRNs
>        (though couldn't the iSCSI layer fabricate CRNs?)
> 	- flow control of the iSCSI target queue
>
> -Costa
>
>