sector alignment for DataOut PDUs?

[Paul Koning <ni1d@arrl.net>]

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