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    RE: iSCSI: Flow Control



    I agree with Peter.
    
    We should allow immediate data, but only up to a limit previously set by the
    target (so we have a chance that it is received).  But the target has to be
    given the ability to dynamically change (in Peter's terms, advertise) the
    resources available, and be able to do it selectively (i.e. to one initiator
    and not another).  This inability to change the resources deployed for this
    purpose over time is why current schemes are not implemented (no target
    wants to get "caught" and so drastically underadvertizes the actual
    resources it has available).
    
    Jim
    
    
    
    
    -----Original Message-----
    From: Peter Johansson [mailto:PJohansson@ACM.org]
    Sent: Friday, September 29, 2000 2:57 PM
    To: IP Storage
    Subject: RE: iSCSI: Flow Control
    
    
    At 04:39 PM 9/29/00, Black_David@emc.com wrote:
    
    >The open question to the list is whether there's value in allowing some 
    >amount of immediate data (e.g., for targets that need fast startup on
    >long latency connections, and are prepared to deploy the buffering 
    >required to make it work reliably), or whether we ought to follow FCP-2 
    >and forbid immediate data, which will impose a round trip delay (command 
    >out, R2T back) before data starts to flow.  I think I've seen a couple of 
    >comments in favor of this, but more discussion is in order.
    
    I suggest that you split the question:
    
    1) Is immediate data desirable when the initiator has certain knowledge 
    that the LU is idle (with respect to the initiator)?
    
    2) Is immediate data desirable when the LU may be busy.
    
    I think the answer to 1) is "Yes", make provision for the initiator to send 
    immediate data---but also make provision for the initiator to obtain some a 
    priori knowledge as to the quantity of data the LU will accept in the idle 
    state. There need not be a negotiation between initiator and LU; it could 
    be sufficient for the LU to advertise its capabilities.
    
    In the case of 2), the answer might vary dependent upon whether or not 
    those LU resources used to obtain data from the initiator are fully 
    occupied when the LU is other than idle.
    
    If most of the optimization is realized in case 1), it might obviate the 
    need for complicated schemes needed to make immediate data in case 2)
    useful.
    
    
    
    
    Regards,
    
    Peter Johansson
    
    Congruent Software, Inc.
    98 Colorado Avenue
    Berkeley, CA  94707
    
    (510) 527-3926
    (510) 527-3856 FAX
    
    PJohansson@ACM.org
    


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Last updated: Tue Sep 04 01:06:52 2001
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