Subject: [Ips] Re: iscsi + REPORT LUNS + overflow question

From: Julian Satran <[email-masked]>
Date: Thu, 28 Apr 2005 11:25:23 +0300

Dear All,

A question was raised recently regarding
(again) the use of the Overrun or Underrun bits in iSCSI.

An iSCSI target is supposed to report
an Overrun or Underrun only if there is a mismatch between the Expected-Data-Length
and the data length derived by the SCSI target from the data presented
in the CDB and the actual data.  In the case of a Report LUNs the
Allocated Length is part of the CDB and as long as the Expected-Data-Length
matches the Allocated-Data-Length and the target transferred the number
of bytes indicated neither Overrun nor Underrun should be set. If target
would have transferred less data than the Expected-Data-Length (either
because EDL is larger that ADL or because the actual data sent is less
than EDL) the Underrun bit must be set (and the value computed). If the
target attempts to send more data than EDL (either because EDL is lower
than ADL or the target SCSI is bad and attempts to send more than ADL)
the Overrun bit must be set (and the overrun value computed).

An ADL lower or higher than the actual
list length is not considered an exception according to SCSI (SPC2 and
3).

Note that if EDL is larger than ADL
you can have an underrun and an incomplete list and in the case of EDL
lower than ADL you can have an overrun and an incomplete list.

A good implementation should set EDL
= ADL.

Julo

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