Re: Read data chunk size and segmentation negotiation

["Elliott, Robert" <Robert.Elliott@compaq.com>]

Stephen Bailey wrote:
> Mark Burton,
> 
> > I understand from your response that it is an (implicit) requirement
> > that all initiators must always accept a data response of any length
> > up to the expected data transfer length.
> 
> This behavior parallels FCP.  The target can control the flow of write
> data to it, but the initiator can not do the same for read data.
> Furthermore, an FCP target can send read data in as many `bursts' as
> it choses, but an FCP initiator can only send a single burst in
> response to an XFR_RDY.
> 
> > I just wonder if there is (or will be) any situation that would
> > require the size of the data responses to be limited by the initiator
> > whilst still allowing for a larger data transfer length. If no such
> > situation can ever occur then the negotiation is not required.

Fibre Channel FCP targets must implement the Disconnect-Reconnect 
Mode Page 02h, which includes two fields that might be 
applicable to iSCSI and relate to the discussion above:

* Enable Modify Data Pointers - a 1 lets the target deliver data
in any order, while a 0 requires it deliver data in continuously 
increasing order.  This lets the application client force the 
target to deliver data in order if the initiator cannot handle 
it out of order.  This bit is optional in FCP targets; they
must deliver data in-order if they don't support the bit.

* Maximum Burst Length - limits the maximum burst that a target
may use.  This lets the initiator limit data bursts to meet its 
buffering requirements.  This field is required in all FCP targets.


SPC-2 says "The parameters appropriate to each protocol and their 
interpretation for that protocol may be specified in the 
individual protocol documents."  

Julian, you may want to add a section documenting the 
Disconnect-reconnect mode page, Port control mode page, and 
Logical Unit control mode page for iSCSI.  FCP-2 defines them
in section 10 (fcp2r05).  T10/00-426r1 describes adding these 
pages to the SCSI over RDMA protocol (SRP). 

There is already reference to one field in the Disconnect-reconnect
mode page in iSCSI revision 02b:
   Outgoing SCSI data (initiator to target - user data or command 
   parameters) will be sent as either solicited data or unsolicited 
   data.  Solicited data are sent in response to Ready To Transfer (R2T) 
   PDUs. Unsolicited data can be part of an iSCSI command PDU 
   ("immediate data") or an iSCSI data PDU.  An initiator may send 
   unsolicited data (immediate or in a separate PDU) up to the SCSI 
   limit (initial burst size - mode page 02h). All subsequent data have 
   to be solicited.   

(A few editorial comments on this reference:
* Change "initial burst size" to "first burst size," to match the 
name used in SPC and SPC-2.  
* Consider referring to the Disconnect-reconnect mode page by name 
rather than number.)

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