RE: Why FCP doesn't need RDMA? It has a better way.

["GUPTA,SOMESH (HP-Cupertino,ex1)" <somesh_gupta@am.exch.hp.com>]

Even iSCSI does not really need RDMA. The underlying note in
the whole debate is really the preservation of message boundaries.

If we could use one of the reserved bits in the TCP header to say,
do the repacketize or adjust segment boundaries, and each TCP
segment starts with an iSCSI header, there should be no need for
another protocol (or TCP option header).

The iSCSI header contains enough information in it to enable
the recepient to determine where to put the data.

Somesh

-----Original Message-----
From: Robert Snively [mailto:rsnively@Brocade.COM]
Sent: Thursday, September 28, 2000 8:29 AM
To: 'Matt Wakeley'; IPS Reflector
Subject: RE:Why FCP doesn't need RDMA? It has a better way.


>  I object to mandating iSCSI use an RDMA option because:
>  
>  - (main reason) there isn't any standardized mechanism now, and I
>    don't want to hold up iSCSI while one crawls through the 
>  standards process.
>  
>  - I don't think RDMA is needed.  FCP doesn't use it, and it 
>  works great with
>  the
>    FC protocol chips that "accelerate" FCP.

Actually, RDMA is not needed in FCP because all protocol chips
implemented perform a real peer-to-peer DMA straight to the 
data areas specified by the user's interaction with the operating
systems allocation algorithms.  The combination of the FCP/SCSI
pointer structure, task tagging, and the FC relative offset perform the
function you would otherwise have to use RDMA to accomplish.

Bob Snively
Brocade Communications           Phone  408 487 8135
1745 Technology Drive
San Jose, CA 95110               Email   rsnively@brocade.com