[T11.3] Re: FCP-4: Items for discussion

Gary.Franco at Emulex.Com Gary.Franco at Emulex.Com
Mon May 1 12:56:21 PDT 2006

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From: owner-t10 at t10.org [mailto:owner-t10 at t10.org] On Behalf Of David
Sent: Monday, May 01, 2006 2:23 PM
To: t10 at t10.org; t11_3 at t11.org
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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