RE: Response Fence Flag

SAM-3 and SAM-4 require all transport protocols to tag the TMF responses
with the requests:

"Each SCSI transport protocol shall allow a Received Task Management
Function Executed confirming completion of the requested task to be
associated with the corresponding Send Task Management Request."

Although iSCSI was based on SAM-2, it complies with that rule. The Fence
concept is asking for more than that - it wants the target to be able to
ensure that all previous commands/TMFs are complete before delivering a
particular TMF response (e.g., for a LOGICAL UNIT RESET).  Since iSCSI
doesn't have ACKs, the target must wait for the next PDU from the initiator
with an updated ExpStatSN.

06-341r0 discussed an alternative - add a Delivery Result output to the Send
Command Complete and Task Management Function Executed confirmations (as
previously proposed in 04-072r0 for a different purpose). This would let the
device server/task manager wait for all the previous commands/TMF responses
to complete (be ACKed) before proceeding to make additional calls (e.g.,
Task Management Function Executed for a LOGICAL UNIT RESET).

However, iSCSI allows the target port to send a NOP-IN to force delivery of
a NOP-OUT with ExpStatSN, rather than passively wait for a PDU to show up.
The device server/task manager must instruct the target port when to do
this, and the Request Fence argument serves that purpose.  Target ports
using protocols without such a force mechanism are still OK - they will just
wait for protocol-specific delivery confirmations (e.g. ACKs).

-- 
Rob Elliott, [email protected] 
Hewlett-Packard Industry Standard Server Storage Advanced Technology 
https://ecardfile.com/id/RobElliott 


 

  _____  

From: Robert Snively [mailto:[email protected]] 
Sent: Wednesday, January 03, 2007 4:46 PM
To: Elliott, Robert (Server Storage); [email protected]
Subject: RE: [Ips] Response Fence Flag


I need a little tutorial here.  Note that this is an architectural hack that
is very "un-SCSI" and is not required if the response is qualified with
identification
information associated with the original request.  I know that such
qualification
exists for commands, but does it exist for task management requests in
iSCSI?  
 
I know that it does exist for parallel SCSI (which is strictly interlocked)
and for SAS and Fibre Channel SCSI, making such "fences" unrequired by
those technologies.
 
Note that Rob's previous revision of 06-341 is available on www.t10.org.
 
I would hate to see such a hack creep into the SCSI architecture.
 
Bob
 

  _____  

From: Elliott, Robert (Server Storage) [mailto:[email protected]] 
Sent: Wednesday, January 03, 2007 1:56 PM
To: [email protected]
Subject: RE: [Ips] Response Fence Flag


If T10 agrees, T10 proposal 06-341 will add it to SAM-4.
 
-- 
Rob Elliott, [email protected] 
Hewlett-Packard Industry Standard Server Storage Advanced Technology 
https://ecardfile.com/id/RobElliott 



  _____  

From: Eddy Quicksall [mailto:[email protected]] 
Sent: Wednesday, January 03, 2007 2:00 PM
To: [email protected]
Subject: [Ips] Response Fence Flag


Section 3.3.1 talks about a Response Fence Flag:
 
SCSI protocol layer instructs the SCSI transport layer of a 
"Response Fence" associated with the response in question when 
the "Send Command Complete" protocol data service (SAM-2, clause 
5.4.2) ...
 
But I don't see any reference to that in SAM-2.
 
Is this strictly an iSCSI flag? Where is the flag specified?
 
Eddy

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