RE: sector alignment for DataOut PDUs?

["Rod Harrison" <rod.harrison@windriver.com>]

	I have no objection in principle to this but I have a concern
over non-media access commands. Things like mode select payloads,
i.e. defect lists etc are not necessarily 512 byte multiples. The
wording would need to allow "odd" sizes when the SCSI transfer
length is not a 512 byte multiple.

	- Rod

-----Original Message-----
From: owner-ips@ece.cmu.edu [mailto:owner-ips@ece.cmu.edu]On
Behalf Of
Paul Koning
Sent: Tuesday, February 26, 2002 9:51 PM
To: ips@ece.cmu.edu
Subject: sector alignment for DataOut PDUs?


In the past, a number of login parameters were specified as
multiples
of 512 bytes, which makes a lot of sense for disk targets (but
not so
much for tape targets).  With the spec as it stands now, you can
negotiate pretty much arbitrary burst sizes etc.  And in any
case, the
sender can pick "strange" PDU sizes even if the negotiated sizes
are
multiples of 512, because after all those are limits, not
required
sizes.

It would be very attractive for disk targets to be able to
specify
that they require DataOut PDUs to be multiples of 512 bytes in
length.
That way, any PDU would correspond exactly to one or more
sectors,
rather than potentially having several PDUs straddling sector
boundaries as is currently permitted.  This could be done by a
Boolean
key (512 byte alignment, yes or no).  (An integer key would allow
other values than 512, but I'm not sure that such flexibility is
a
whole lot more useful.)

Basically, the effect of this feature would be to tell the
initiator
that it must send DataOut PDUs and immediate data whose length is
always a multiple of 512.  Obviously, targets can't ask for that
unless the devices on that target already come with such a
limitation.

       paul