SAM-3 byte alignment change in 03-002r3

Elliott, Robert (Server Storage) Elliott at
Thu Jan 30 08:57:08 PST 2003

* From the T10 Reflector (t10 at, posted by:
* "Elliott, Robert (Server Storage)" <Elliott at>
I think this change proposed by 03-002r3 went too far:

"Change 20 [relax byte-alignment in SAM-3]:
In (Data transfer SCSI transport protocol services
modify the fourth paragraph from the end of the subclause as follows:
All SCSI transport protocol standards (shall changed to) should define
support for a resolution of one byte for the above arguments. A SCSI
initiator device (shall changed to) should support a resolution of
one byte. A SCSI target device may support any resolution."

Although SCSI transport protocols are free to require alignment for most
phases during a command (e.g. the DATA frames that are not the last DATA
might have to be 4-byte aligned), they need to provide byte resolution
for the
command as a whole (e.g. there needs to be a way to indicate an odd
number of
bytes in the last DATA frame).

This is why parallel SCSI has an IGNORE WIDE RESIDUE message and why
like SAS and FCP have "number of fill bytes" fields in their frame
It's fair to require the "number of fill bytes" field be zero until the
DATA frame of the command; but I think it's important for SCSI to
mandate that
they offer it for the last DATA frame.  Otherwise, the transport
protocol can
only support commands that perform multiple-of-n byte transfers (where n
protocol-specific, since there's no requirement in SAM-3 any more).

Some wording changes from the original may be appropriate to communicate
intermediate vs. last distinction, but I think just changing those
to "should"s goes too far.

>From the CAP minutes 03-047r0:

4.8 E-mail SAM-3 topic [Penokie]
George Penokie presented an issue with SAM-3 requiring byte alignment in
transfer operations. He noted that most if not all of the serial
violate the SAM-3 statement. The group agreed to discuss the change as
part of item 8.2.

8.2 Remove SPI from SAM-3 and SPC-3 (03-002) [Weber]
Ralph Weber presented a proposal to remove the SCSI parallel bus
|from SAM-3. He noted that the proposal removes contingent allegiance
completely. The group requested minor changes including text to resolve
issue described in item 4.8 and Ralph agreed to prepare a new revision.
Ralph Weber moved that 03-002r3 (r2 as revised) be recommended for
in SAM-3 and SPC-3. Mark Evans seconded the motion. In the absence of
objections, the motion passed unanimously.

Rob Elliott, elliott at
Hewlett-Packard Industry Standard Server Storage Advanced Technology

Rob Elliott, elliott at
Hewlett-Packard Industry Standard Server Storage Advanced Technology

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