RE: sector alignment for DataOut PDUs?

[Paul Koning <ni1d@arrl.net>]

Excerpt of message (sent 1 March 2002) by Robert Snively:
> 
> There has always been a feeling that it was nice to do things
> on convenient boundaries, including memory page boundaries and
> device physical block boundaries, but SCSI has long since 
> elected to perform any operation on almost any boundary.  SCSI drives
> and related operating systems have commonly used block sizes of 512
> bytes and 520 bytes and less commonly of other values.  
> 
> The only requirement we were able to enforce in previous
> SCSI protocols is that all but the last PDU of a transfer
> were required to be on 4 byte boundaries.  We were also
> able to enforce the maximum values and a requirement that
> the transferred data count exactly match the requested byte
> count.

I don't see that last point.  Certainly not for unsolicited data --
and for R2T I can find no stated requirement to send exactly the
requested count either.  Or did you mean the total for the entire
operation as opposed to the individual bursts?
 
> I would strongly suggest that you not bother to enforce any
> other boundary restrictions.
> 
> I believe Eddy was right when he asked:
> 
> 	Given that the TCP issues of reassembly 
> 	are so much more complicated than
> 	the concatenation issues necessary for 
> 	target data alignment, is there
> 	really a problem here?
> 
> My answer to him would be, no, there is not a problem.

I answered it before when Eddy said that, but I'll summarize it again:
I'm not talking about a problem, I'm talking about an opportunity for
simplifying the target and making it more efficient.

Given alignment, each iSCSI PDU that carries data can be sent to disk
by itself, because it corresponds exactly to one or more disk blocks.
Without alignment, doing writes when the data arrives is still
possible, but it clearly adds complexity because PDU boundaries don't
match disk block boundaries.

I made the proposal because it clearly helps the target and appears to
add no significant burden on the initiator.  The feedback to date
indicates that this is indeed the case.

Are you saying "I don't care one way or the other" or are you saying
"I feel this is a bad idea because it creates problems you haven't
thought of"?  I'm reading your comment as the former; if you meant the
latter, could you elaborate?

	paul