RE: StatSN and overlapped commands

[pat_thaler@agilent.com]

Julian,

For recovery, the target has to remember the status, but that doesn't mean it has to keep the task information around. It would have the status message linked to StatSN but it would be typical to clear away any task context when that message is generated. Remember that when things are very busy it might get a single acknowledgement that acknowledges multiple status messages from multiple tasks. It isn't efficient to have to go back to do the task context clean up when the status ack comes. 

It is the initiator's job to not shoot itself in the foot by issuing a new command with the same task tag as one that it hasn't gotten status on yet. For the target, once the status is generated, the task is gone. The status message may still be there available for a resend, but the task is gone. I don't see any requirement in iSCSI or SCSI to do anything other than that.

Regards, 
Pat

-----Original Message-----
From: Julian Satran [mailto:satran@haifasphere.co.il]
Sent: Tuesday, August 05, 2003 8:48 PM
To: THALER,PAT (A-Roseville,ex1)
Cc: julian@cs.haifa.ac.il; Black_David@emc.com; dcuddihy@attotech.com;
ips@ece.cmu.edu
Subject: Re: StatSN and overlapped commands


pat_thaler@agilent.com wrote:

> Julian,
> 
> I agree that the initiator is misbehaving, but I don't agree that the target should detect that misbehavior. The target SCSI layer thinks the command was done when it generated the status. As David said, the target keeps the status around so that it can resend it if requested by Status SNACK. It doesn't need to keep track of the tag anymore at that point.
> 
> If the target had to generate an error when a command came with for a tag before the status for a prior command with the same tag was acknowledged, then it would have to clear the memory of tags when status acks came in which is less efficient than doing it when posting the status.
> 
> Pat
> 
> -----Original Message-----
> From: Julian Satran [mailto:julian@cs.haifa.ac.il]
> Sent: Tuesday, August 05, 2003 10:56 AM
> <snip>
> 
>>
>>No, iSCSI at the target is retaining the SCSI status of the completed
>>command for retransmission.  SCSI believes the command to be completed,
>>and any retransmission request (e.g., Status SNACK) is not visible to
>>SCSI at the target.  In this case "command recovery" does not execute
>>any commands at the target; it just causes retransmission of the saved
>>status.
>>
>>Thanks,
>>--David
>>----------------------------------------------------
>>David L. Black, Senior Technologist
>>EMC Corporation, 176 South St., Hopkinton, MA  01748
>>+1 (508) 293-7953             FAX: +1 (508) 293-7786
>>black_david@emc.com        Mobile: +1 (978) 394-7754
>>----------------------------------------------------
>>
> 
> My only caveat to this would be that an initiator that reuses the 
> Initiator Task Tag but does not acknowledge the reception of the status 
> by an ExpSataSN is definitely misbehaving.
> 
> The target should not consider it as an implicit ack (as intermetiate 
> status PDUs may have been lost - it should reject the command that 
> reuses the Initiator Task Tag.
> 
> That is not necesarily related to the way an initiator maps SCSI tags to 
> iSCSI tags - it is specific to iSCSI expectations about tag reuse.
> 
> You correctly stated that an iSCSI tag should not be reused before it's 
> status is acknowledged but violating this rule is an iSCSI protocol 
> error and not a SCSI error (overlapped command).
> 
> Julo
> 
Pat,

The target has to remember the status the status for recovery - if both 
I and T have agreed on recovery. If it drops it some recovery scenarios 
won't work.

You are right that when no recovery is agreed status can be dropped but 
not so if recovery is agreed. And while within connection recovery is 
not based on ITT a recovered status containing a wrong ITT is not a good 
idea!

Julo