RE: sector alignment for DataOut PDUs?

["Martin, Nick" <Nick.Martin@compaq.com>]

Paul,

For all data carrying PDUs except the last in a sequence, the sender is
expected to send maximum sized PDU allowed.  When the negotiated maximum
is a multiple of 512, this effect is what you request.

I thought this was a requirement, but I did not find it as such in draft
10.  I did find this:

: 8.5 Unsolicited Data and Performance
: Unsolicited data on write are meant to reduce the effect of latency on
: throughput (no R2T is needed to start sending data). In addition,
: immediate data are meant to reduce the protocol overhead (both
bandwidth
: and execution time).
: However, negotiating an amount of unsolicited data for writes and
: sending less than the negotiated amount when the total data amount to
: be sent by a command is larger than the negotiated amount may
negatively
: impact performance and may not be supported by all the targets.

This is a warning that an initiator sending less than the negotiated
maximum when the expected data transfer is greater than the maximum a)
may reduce performance and b) may not be supported by all targets.

IMHO, it makes more sense to include stronger wording encouraging
maximum negotiated length transfers rather than to add another parameter
requiring PDU breaks on different boundaries.

If such initiator behavior may not be supported by all targets, then the
initiators SHOULD NOT do it.

A disk target which can not handle such behavior is forgiven in advance
;).

Thanks,
Nick


> -----Original Message-----
> From: Paul Koning [mailto:ni1d@arrl.net]
> Sent: Tuesday, February 26, 2002 3:51 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
> 
>