Re: iSCSI: burst size, response and sense data

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

Mallikarjun Chadalapaka wrote: 
> Julian,
> 
> 1. The new iSCSI draft states that: 
>   "An initiator may send unsolicited data (immediate or in a
>    separate PDU) up to the SCSI limit (initial burst size - mode 
>    page 02h)." (section 1.2.5, para 4).
> 
> SPC-2 has the following on "FIRST BURST SIZE" in mode page 02h -
>   "The FIRST BURST SIZE indicates the maximum amount of data that
>    a target may transfer for a command during the same interconnect 
>   tenancy in which it receives the command." (Clause 8.3.7, last para)
> 
> Note that the mode page 02h is specifying the burst size in the 
> *opposite* direction - from target to initiator.  I would suggest 
> that a new "iSCSI mode page" be created to specify parameters of 
> this nature.  Or, rely only on what is negotiated in the iSCSI 
> login dialogue, on a per-session basis.

Interesting.  FCP-2 (since revision 2) defines it the opposite way, 
similar to iSCSI:

"When write transfer ready is disabled, the FIRST BURST SIZE field 
 indicates the maximum amount of data that shall be transmitted in 
 the first FCP_DATA IU sent from the initiator to the target. If 
 all data is transmitted in the first IU, no subsequent FCP_XFER_RDY 
 IUs shall be transmitted by the target. If the maximum amount of data
 has been transmitted, but more data remains to be transferred, 
 the target shall request that data with subsequent FCP_XFER_RDY IUs."

I think FCP-2 and iSCSI are using it as intended.  Remember that 
in parallel SCSI the target controls data transfers in 
both directions.  If you interpret:
    "that a target may transfer" 
more like:
    "that a target may request to transfer" 
you can accept the FCP-2 interpretation.

It appears that FCP-2 is the only SCSI protocol using that field 
so far, so SPC-2 could be reworded without breaking anything.  It's
in T10 letter ballot resolution now so is still (barely) open for
changes.

By the way, SCSI provides for a Protocol-specific Port Control
mode page 19h which could serve as an "iSCSI mode page" if 
a need arises.
---
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