Re: iSCSI : Holes in StatSN

[Santosh Rao <santoshr@cup.hp.com>]

julian_satran@il.ibm.com wrote:

> With a data sequence we may want to use a similar mechanism to ask for a
> missed data block as soon as we see one of its successors or the status.

Julian,

The missing data PDU is missing due to either a header format error, header
digest error or data digest error. [in all other cases, TCP ensures reliable
delivery].

In 2 of the above 3 cases, [header format error & header digest error] the
initiator CANNOT do a safe interpretation of the PDU header. Without
interpreting the PDU header, the initiator does NOT get the Initiator Task
Tag. Any request to re-send a particular data PDU MUST be qualified by :
I.T.T + missing_DataSN [+ T.T.T + CmdSN, optionally].

Since I.T.T. cannot be reliably determined in 2 of the 3 cases, such a
re-send request cannot be reliably achieved.

The alternate proposal that was made should be considered in its place, which
was to :
- dis-allow overlapped data xfer's
- initiators do a count check
- a command level retry is performed at the iSCSI layer on detecting an
underrun [due to a missing PDU].

On several ocassions, requests from different people have been made on this
list to dis-allow overlapped data xfers. Can a WG consensus be sought on this
issue to see if the benefits of allowing overlapped data xfer's offset its
complexities and justify its support ?

Regards,
Santosh
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