Wow there is a name I have not seen for awhile

Lou Dickens dickens at
Tue May 2 05:43:45 PDT 2006

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We what happens when you post to the reflector, you never know how will

harass you.. How have you been ?  Are they keeping you busy ?

 Lou Dickens						 

 Software Engineer					 

 9000 South Rita Road					 

 Tucson, Az, 85744					 

 Tel: 520-799-4139 (T/L: 321-4139)			 

 Fax: 520-799-5607:					 

 Email address: dickens at 		 

	     Gary.Franco at Emule						   


	     Sent by:							To 

	     owner-t10 at	       <David.Peterson at>,	   

				       <t10 at>, <t11_3 at>	   


	     05/01/2006 12:56	       <Bob.Nixon at Emulex.Com>		   

	     PM 						   Subject 

				       RE: FCP-4: Items for discussion	   

From: owner-t10 at [mailto:owner-t10 at] On Behalf Of David


Sent: Monday, May 01, 2006 2:23 PM

To: t10 at; t11_3 at

Subject: FCP-4: Items for discussion


Below is an email thread from Claudio that was discussed a bit at the last

FCP-4 working group meeting:

I please ask you to discuss in the FCP-4 WG the possibility to mandate

continuous increasing SEQ_CNT in FCP-4. Relying on it greatly simplifies

the detection of a missing Sequence and consequently simplifies error

recovery, but today is optional in FCP-x (while is mandated by IP over FC

and is going to be mandated by FC-SATA).

Additional items for FCP-4 discussion follows (all due to doubts submitted

to me...):

- Bidirectional Commands: I think we need to count bytes for both data-in

and data-out. Which of these two counters should be put in the FC4VALUE

field of the REC ELS?

Wouldn’t that be dependant on the type of command? FCP read or write types.
The device knows the current command type.

- Data Overlay: the FCP-3 definition of data overlay says "see SAM-3", but

SAM-3 says nothing on data overlay.

I would say that EMDP should be disabled cause with the end to end data

protection I do not think you can even enable this option because the DIFs

would get corrupted would they not?

- Data Overlay: In which way could it be possible detecting a missing

Sequence when data overlay is used and continuously increasing SEQ_CNT is

not used? (it seems to us that there is no way, but others may have a

different opinion...).

I agree, I think there is no possible way, at least real-time as the data

is being streamed in. At FCP response time the overrun/underrun and

residual length would cause a re-execution of the command.

- Data Overlay: how can the FC4VALUE counters can be accurate when data

overlap (i.e., how to avoid to count twice the overlapping data)?

I would think that the ability to resend data blocks already sent was

discussed in the specification as a bad thing. If not then I believe the

only way to catch a misbehaving target would be to track the data blocks

already received and keep track of the holes left behind. The device would

also have to track the resent data and not include the resent data as data

received to satisfy a read command.

The members of the working group came to no real concensus/resolution per

the questions and would like to open up the discussion to the T10/T11

commitees, specifically making Continuously Increasing SEQ_CNT a

requirement for FCP-4.

Making this a requirement would be a much need improvement, and one that

has been needed for a while.

Regarding Continuously Increasing SEQ_CNT:

We have discussed requiring Continuously Increasing SEQ_CNT during each

previous FCP-x standard development efforts and folk opted to not specify

it as a requirement since some vendors did not yet fully support it. We may

have now moved past that issue.

Regarding Bidirectional Commands:

FCP-3 states: "Sequence level error recovery as described in 12.4 shall not

be used for bidirectional SCSI commands." So the question regarding the

FC4VALUE field is moot until if/when we want to support FCP-x error

detection and recovery for bidirectional commands.

Regarding Data Overlay:

The reference to SAM-3 is not intended to refer the reader to SAM-3 for

data overlay, but since it is outside the sentence this is what it means.

The reference will be removed since it provides value in this context

(i.e., the intent was to refer the reader to SAM-3 for application client

buffer offset but that is already covered).

The other two questions are vendor implementation specifc in my mind,

others may share if they wish...


(no disclaimer)

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