RE: iSCSI: Markers

["Williams, Jim" <Jim.Williams@emulex.com>]

> -----Original Message-----
> From: John Hufferd [mailto:hufferd@us.ibm.com]
> Sent: Sunday, January 06, 2002 4:58 PM
> To: ips@ece.cmu.edu
> Subject: iSCSI: Markers
> 
> 
> [...]
> 16. If the HW Pipelining iSCSI HBA is to
> send COWS Markers to another HW
> Pipelining HBA, there may be a problem.
> The outgoing pipeline would prefer
> to have the pointers pointing backwards,
> but the receiving pipeline would
> prefer to have the pointers going forwards.
> Therefore with COWS, either
> the sending or the receiving HW Pipelining
> HBA will be unhappy and pipeline
> design will get very much more complicated.

With the disclaimer that I am skeptical of the
value of markers and definitely opposed to 
mandating them, I would add the following
comment regarding COWS.

I believe its more important to make the
transmit side "happy" than the receive side.
There are two reasons here.

1.  It is desired to have much wider deployment
    of transmit side COWS, since the receive
    side is an option that many (most) vendors
    will choose not to support.  Transmit side
    may want (need) to be supported anyway
    for the benefit of other implementations
    that require it.

2.  NIC designers can defer
    fetching transmit data until late in the
    pipeline.  All protocol processing, header
    formatting, and bookkeeping can be done 
    prior to fetching the data.  Therefore
    the transmit pipeline will have a small
    amount of time to fully implement the COWS
    algorithm.

    On the receive side pipeline, the COWS algorithm
    can be done in parallel with the protocol
    processing since that is done when all the data
    is present in the receive buffer.  The 
    receive COWS algorithm can be done in place in the
    receive buffer, so backward pointers are
    not a problem.

    An efficient transmit COWS algorithm requires that
    hardware check each word in the outgoing
    PDU, whereas an efficient receive algorithm requires
    checking only the pointer (other than exceptional 
    cases).

> John L. Hufferd
> Senior Technical Staff Member (STSM)
> IBM/SSG San Jose Ca
> Main Office (408) 256-0403, Tie: 276-0403,  eFax: (408) 904-4688
> Home Office (408) 997-6136, Cell: (408) 499-9702
> Internet address: hufferd@us.ibm.com
>