Re: iSCSI: DataPDUInOrder=No Clairification

["Julian Satran" <Julian_Satran@il.ibm.com>]

Dear Nicholas,


"Nicholas A. Bellinger" <nick@pyxtechnologies.com>
wrote on 21/05/2003 12:48:15:

> [This is a resend due to mailer problems, please excuse any duplicates.]
> 
> Greeting all,
> 
>    I am inquiring about the author's intent and proper usage
of
> DataPDUInOrder=No.  Of course the definition of the aforementioned
> parameter is defined in section 12.18:
> 
> "No is used by iSCSI to indicate that the data PDUs within sequences
>  can be in any order."
> 
> I had originally used the interpretation that it was mainly designed
to
> aid within-command recovery in the following DataOUT (not excluding
> DataIN) example:
> 
> When a DataOut PDU fails a Data Checksum, DataPDUInOrder=No allows
the
> target to continue to receive the rest of the DataOut sequence without
> having to discard PDUs received after the initial failure.  Hence,
> the Recovery R2T only has to contain Failed Offset + Failed PDU Length.
> 
> This compared to the same example with DataPDUInOrder=Yes:
> 
> When a DataOut PDU fails a Data Checksum, DataPDUInOrder=Yes requires
> the target to NOT acknowledge the any futher PDUs in the DataOut
> sequence until the initial failure is recovered.  Hence, the
Recovery
> R2T must contain Failed Offset -> Length to end of Sequence.
> 
> This type of example seems like the most logical application of
> DataPDUInOrder=No, that is one assumes the pdus in a given sequence
will
> be still sent in increasing order.  If an error occurs one is
allowed to
> continue to receieve pdus out of order and request only the missing
> ones.
> 
> Now let us consider another more unorthadox (and perhaps a bit
> illogical) example:
> 
> Consider a DataOUT Sequence of 32k, interpreting the definition in
> section 12.18 can one assume the following is legal?
> 
> Initiator -> Target
> 
> DataSN: 0, Offset: 24576, Length: 8192
> DataSN: 1, Offset: 0, Length: 8192
> DataSN: 2, Offset: 16384, Length: 8192
> DataSN: 3, Offset: 8192, Length: 8192
> 
This is indeed legal

> This type of example assumes the pdus in a given sequence may be sent
in
> any order, and the target handles the metadata for each pdu.  If
this is
> indeed legal, then its safe to assume the following 128k DataIN example
> for DataSequenceInOrder=No is legal as well:
> 
> Target -> Initiator
> 
> Sequence 0: Offsets: 65536->98304
> Sequence 1: Offsets: 32768->65536
> Sequence 2: Offsets: 98304->131072
> Sequence 3: Offsets: 0->32768
> 
> My guess is these both are indeed legal.
> 

Yes they are legal.

The stricter sequencing requirement is not there to
avoid recovery but rather to enable devices to optimize
transmission in face of "mechanical" delay
and lack of resources.

> Thanks,
> -- 
> Nicholas A. Bellinger <nick@pyxtechnologies.com>
>