Immediate Commands in iSCSI

[<Jacob_Cherian@xxxxxxxx>]

Title: Immediate Commands in iSCSI

Ken/Julo,

  We may have been talking past each other here J  I agree that defining a status code for the Response field of the TMF Response PDU is the simplest option:

0x07 - Immediate Command Reject - Too many immediate commands

0x06 is already defined as “Function authorization failed.” Or were you recommending changing the description for the 0x06 response code?

Thanks,

Jacob

RE: Immediate Commands in iSCSI

by Ken Sandars Dec 04, 2007; 07:08pm ::

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Hi Jacob,

A Reject PDU which indicates its reason as "Immediate Command Reject - too many immediate commands" seems pretty granular to me. :-) That's what the reject code 0x06 means (as Julo had recommended). The initiator may resubmit the PDU whenever it deems appropriate. Does that make sense?

IMHO, a target implementation constrains the number of non-TMF commands and might throttle repeated TMF commands when they are in progress (and the clearing effects are taking a long time to resolve). Even with a large number of LU's it would be unusual to constrain the number of concurrent LU Reset operations. Have you got a trace showing this behaviour?

HTH,
Ken

Subject: RE: [Ips] Immediate Commands in iSCSI
Date: Mon, 3 Dec 2007 14:15:19 -0600
From: Jacob_Cherian@...
To: Julian_Satran@...
CC: ips@...; Nam_Nguyen@...

Julian,

    No offense taken. The scenario that you brought up was considered since most configurations with iSCSI are with multiple hosts. However, the allocation of immediate commands to connection/sessions/hosts depends on the actual implementation. RFC 3720 already requires support for at least one per connection. Even we cannot agree on a key, having a more granular status for a rejected TMF that says the target is currently busy processing a TMF and cannot process another at this time (but will be able to sometime in the future), helps initiators to wait and retry, instead of assuming a failure and escalating the reset.

 

 Jacob

 

From: Julian Satran [mailto:Julian_Satran@...]
Sent: Monday, December 03, 2007 8:42 AM
To: Cherian, Jacob
Cc: ips@...; Nguyen, Nam
Subject: RE: [Ips] Immediate Commands in iSCSI

 

Jacob,

I do not want to appear offensive - but did you consider the "larger picture"? Assuming that the negotiation key or status is there and assuming that the target talks to 7 (seven) different initiators what would it tell each of them (it has 7 different sessions)? Should he reserve a quota for each of them - so that it can meet what it declared? What should a target do with commands that exceed the number?

In iSCSI as a rule we avoided setting or negotiating parameters that we don't need to set/negotiate and I think the number of accepted immediate commands is one of them.

Julo

        RE: Immediate Commands in iSCSI

 

Jacob_Cherian

to:

Julian Satran

12/03/07 04:06 PM

 

Cc:

ips, Nam_Nguyen

       
 

The goal of the new status or key is to help the initiator to know the correct number of immediate commands that the target supports. The issue with overloading a status that does clearly define the error condition is that it does help in achieving interoperability between the myriad of initiators and targets that are out there. I was looking to see if there was another way that I had not considered.
 
Jacob
 
 
From: Julian Satran [mailto:Julian_Satran@...]
Sent: Monday, December 03, 2007 1:23 AM
To: Cherian, Jacob
Cc: ips@...; Nguyen, Nam
Subject: Re: [Ips] Immediate Commands in iSCSI
 

It looks to  me that the Reject response with reason 0x06 should be enough for the initiator to handle the situation you describe.
If the TM are the cause then each of those has a response indicating termination. After receiving a reject an initiator should reissue new TMs only after getting back one or more responses.  The initiator may even accurately guess the number of commands the targets is capable of handling per connection and per LU. The only case in which this handling may be awkward is over a WAN but if you are operating over a WAN and TM performance is your only worry you are in good shape :-)

Julo

         

Immediate Commands in iSCSI

 

 

Jacob_Cherian

to:

ips

12/03/07 05:23 AM

 
 

Cc:

Nam_Nguyen

       

 

 

RFC 3720 states:

The number of commands used for immediate delivery is not limited and their delivery for execution is not acknowledged through the numbering scheme. Immediate commands MAY be rejected by the iSCSI target layer due to a lack of resources. An iSCSI target MUST be able to handle at least one immediate task management command and one immediate non-task-management iSCSI command per connection at any time.

 

With storage systems that have very large number of logical units, we run into an issue when we use LU Reset or Abort Task for reset recovery. I couldn’t find a way to tell an initiator how many immediate commands a target can accept. The issue with this is that if we have to issue multiple task management commands and it is more than what the target can accept, it will reject the task management request. Since we don’t know why it was rejected, we will have to escalate the reset. This can be very disruptive with nodes that have very large number of logical units. There may be two ways to resolve this:

 

1)      Define a new Text negotiation key for number of outstanding immediate commands supported. Unlike non-immediate (data) commands where we can send TASK SET FULL, there is no way for a target to report in the Task Management Function Response PDU that is unable to process the current request before it is out of resources.  The default vaule for this new key can be 1, so if this key is not negotiated initiators can assume one per connection.

2)      Define a new value for the Response field in the Task Management Function Response PDU for reporting that the target cannot process the request currently.

 

Any suggestions on how to resolve this?

 

Thanks,

Jacob

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