RE: iSCSI: padding on intermediate sequences

[Robert Snively <rsnively@Brocade.COM>]

Since there is no requirement by the SCSI ULP to say anything about data until

the SCSI Status has been presented, there is no need to concern ourselves with

device dependent burst boundaries.  We can make the boundaries convenient

for the technology.  In FCP, all bursts were required to begin on a

4-byte boundary, and all except the last to end on a 4-byte boundary.  This

simplified DMA and segmentation/reassembly mechanisms

in all hardware paths.  Looking at the TCP/IP formats, I see a similar

bias toward 4-byte structures, which implies that the same rule would be

a constructive addition to the document.

 

This simply prohibited the target from asking for a number of intermediate

bytes that was not a multiple of 4.

 

The folks using odd byte block counts had to manage the buffers in

a manner appropriate to such behaviors.  Practically, I do not believe that

there were many such devices, nor was this a burden to any devices that did

support such behavior.

 

Bob

-----Original Message-----
From: Julian Satran [mailto:Julian_Satran@il.ibm.com]
Sent: Wednesday, March 27, 2002 12:03 AM
To: Eddy Quicksall
Cc: ips@ece. cmu. edu (E-mail); owner-ips@ece.cmu.edu
Subject: RE: iSCSI: padding on intermediate sequences

Eddy,

Now that we understand better you issue here are some reasons why we may not want to do what you propose:

• the target may ask for a number of bytes that is not a multiple of 4 (should we outlaw this? I don't think we want)
• there might be devices that require/generate in "bursts" that are not multiples of 4
  
  
  I think that we avoided abuse by what we have and we should leave further optimization to the implementer for specific devices.
  
  Julo
  
  
  

  Eddy Quicksall <Eddy_Quicksall@ivivity.com> Sent by: owner-ips@ece.cmu.edu 27-03-02 02:41 Please respond to Eddy Quicksall

  To:        "ips@ece. cmu. edu (E-mail)" <ips@ece.cmu.edu>         cc:                 Subject:        RE: iSCSI: padding on intermediate sequences

  
  
  It is the sequences I am speaking of, not the segments. The current wording
  of the spec would allow you to send sequences with padding in each sequence
  (the segments within the sequence are ok because they can't have padding).
  But if a transfer is made up of several sequences then it is more work for
  the hardware if intermediate sequences were allowed to have arbitrary pad
  bytes.
  
  And there is no good reason (that I am aware of) that intermediate sequences
  could not just be pure data.
  
  Now, if there is a reason, then we should come up with a scheme that allows
  one party to "just say no". One possibility is that if DataInOrder=yes, then
  padding would not be allowed except in the sequence that represents the last
  portion of the transfer.
  
  If that is objectionable, then we should have a new key
  (PaddingOnlyInLastSequenceOfTotalTransfer=yes). :-)
  
  Eddy
  
  -----Original Message-----
  From: Robert D. Russell [mailto:rdr@io.iol.unh.edu]
  Sent: Tuesday, March 26, 2002 2:48 PM
  To: Eddy Quicksall
  Cc: ips@ece. cmu. edu (E-mail)
  Subject: Re: iSCSI: padding on intermediate sequences
  
  
  Eddy:
  
  I may not be understanding you correctly, but the
  words in section 9.7.7 I believe mean that these data
  segments should contain an integer number of 4 byte words
  of data, not arbitrary "pad" bytes.  If the data segment
  length is a multiple of 4 then there will be NO pad bytes.
  
  There seems to be confusion over your use of "pad"
  and the standard's use of "pad".  Would you please clarify?
  
  Thanks,
  Bob
  
  
  On Tue, 26 Mar 2002, Eddy Quicksall wrote:
  
  > Section 9.7.7 says:
  >
  > The Data Segments of Data-in and Data-out PDUs SHOULD be filled to the
  > integer number of 4 byte words (real payload) unless the F bit is set
  > to 1.
  >
  > This allows one to pad intermediate sequences of a transfer. But, there
  are
  > reasons why padding on intermediate sequences within a transfer can make
  > life difficult.
  >
  > Can we forbid padding on all but the last PDU for the transfer? If that is
  > objectionable, could we forbid padding on all but the last PDU of a
  transfer
  > if DataInOrder=yes.
  >
  > Eddy
  >