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    RE: iSCSI: No Framing

    Right now, I'd say disk-disk backup/restore and mirroring.  The individual
    disk devices of today can perform at ~70 MB/s.  One need only gang 14 of
    these together, streaming, to reach nearly 1000 MB/s, or 1 GB/s, or 8 Gb/s.
    The disk devices of tomorrow are going to perform at rates approaching 100
    MB/s per spindle.  Only ten devices are then necessary.
    This would be a single TCP stream (one initiator, one target, many LUs).
    If one has to mirror (/move) 10 TB in a reasonable amount of time, say, then
    a stream of 1 GB/s is not only necessary, but paramount.  Even at that rate,
    it would still take nearly three hours to move just 10 TB.  In a
    business-critical recovery situation, three hours is an eon.
    Don't think application (CPU/memory in a server) to storage.  Think storage
    to storage.
    Rob Peglar
    Corporate Architect
    XIOtech Corporation, a Seagate Company
    (314) 308-6983
    >-----Original Message-----
    >From: Prasenjit Sarkar []
    >Sent: Sunday, February 03, 2002 9:23 PM
    >To: THALER,PAT (A-Roseville,ex1)
    >Cc:; WENDT,JIM (HP-Roseville,ex1); 
    >Subject: RE: iSCSI: No Framing
    >Hi Pat,
    >I had a few questions about your 10 Gbps example. If we assume 
    >that this is
    >a single TCP stream, I would like to know which application in 
    >the near and
    >forseeable future can sustain a 10 Gbps data rate for a 
    >reasonable amount
    >of time? Or if they are multiple TCP streams, are you assuming 
    >packet drops
    >in all streams simultaneously?
    >   Prasenjit Sarkar
    >   Research Staff Member
    >   IBM Almaden Research
    >   San Jose
    >                      "THALER,PAT                              
    >                      (A-Roseville,ex1)        To:       
    >"WENDT,JIM (HP-Roseville,ex1)"                    
    >                      "                         
    >                      <pat_thaler@agile        cc:             
    >            >                  Subject:  RE: 
    >iSCSI: No Framing                             
    >                      Sent by:                                 
    >                      owner-ips@ece.cmu                        
    >                      .edu                                     
    >                      02/03/2002 03:01                         
    >                      PM                                       
    >In answer to your questions:
    >Agilent is planning to implement framing. We view both FIM and
    >COWS as having about the same utility so we would implement
    >whichever one went into the standard.
    >A smoothing buffer on the chip is feasible wrt your point 2.
    >There are some configurations that would use external memory.
    >Also, one concern is that the very situation where one would
    >need large window size for efficiency - high bandwidth long
    >distance communication - is where out of order receipt becomes
    >increasingly likely so the amount of memory for this situation
    >could push up cost to an excessive extent.
    >Reducing the amount of traffic that has to be shunted to an
    >external memory affects the bandwidth that needs to be provided
    >for that memory. If we can handle most PDUs internally then the
    >external memory doesn't have to be as wide and as fast. For
    >instance, if a drop means that the iSCSI PDU with the drop
    >is put into external memory then that takes less memory bandwidth
    >than if a drop means the whole data stream for that session will
    >be going into the buffer until the hole is filled.
    >This decision also effects latency after a drop and required
    >backplane bandwidth.
    >Let's say one is on a 10 Gbit/s extreme long distance WAN
    >connection and that there is a drop. If the round trip delay
    >for getting the whole filled is 100 ms, then without framing,
    >one could have about 1 Gbit stored in the local buffer memory
    >by the time the hole is filled. One will have to process all
    >of this through iSCSI and across the backplane before one can
    >deal with the new traffic coming in. If the backplane data rate
    >is closely sized to the external data rate, an extra 100 ms latency
    >has just been inserted into the path until traffic slacks off.
    >(Congestion control might mitigate this to the extent that one
    >is talking to just one source, but with multiple sessions, one
    >will still have to keep up.) To get the buffer emptied to make
    >space for further drops and to get the latency back down, one
    >will have to size the backplane bandwidth wider and have the
    >iSCSI engine process at a higher data rate.
    >With FIM or COWS, one can end the incident with much less data
    >in the buffer as one processes PDUs after the hole while waiting
    >for it to be filled. Then, when the hole is filled one just has
    >to process a small amount of data to catch up.
    >Pat Thaler
    >-----Original Message-----
    >From: WENDT,JIM (HP-Roseville,ex1) []
    >Sent: Tuesday, January 29, 2002 10:47 PM
    >Subject: iSCSI: No Framing
    >So, it would be good to hear from several iSCSI
    >NIC/chip implementors who:
    >- have or plan to implement FIM or COWS (or some
    >other framing mechanism) and take advantage of it to
    >minimize demands on on-NIC buffer memory
    >- believe that all-buffers-on-chip solutions are
    >feasible and valid (wrt the points above, including
    >- can quantify the memory/pin/power/space cost
    >savings for all-buffers-on-chip solutions


Last updated: Mon Feb 04 13:18:05 2002
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