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    RE: iSCSI: Use of the A bit



    Why is it not of paramount importance? 
    
    How would the target throttle its Data-In's if DataACK is optional when
    ErrorRecoveryLevel != 0?
    
    I don't know a lot about FC but parallel SCSI has this capability.
    
    Eddy
    
    -----Original Message-----
    From: Julian Satran [mailto:Julian_Satran@il.ibm.com]
    Sent: Wednesday, March 13, 2002 8:17 PM
    To: Eddy Quicksall
    Cc: ips@ece. cmu. edu (E-mail); BURBRIDGE,MATTHEW
    (HP-UnitedKingdom,ex2); owner-ips@ece.cmu.edu
    Subject: RE: iSCSI: Use of the A bit
    
    
    
    Eddy,
    
    You need the target to support SNACK so that you need some recovery
    capabilities.
    And yes the consensus of the group was that the scenario you describe is
    not of paramount importance.
    So you may ask for ACK and keep the data for a while hoping to get it :-)
    
    Julo
    
    
     
    
                          Eddy Quicksall
    
                          <Eddy_Quicksall@i        To:       "BURBRIDGE,MATTHEW
    
                          vivity.com>               (HP-UnitedKingdom,ex2)"
    
                          Sent by:                  <matthew_burbridge@hp.com>,
    "ips@ece. cmu.   
                          owner-ips@ece.cmu         edu (E-mail)"
    <ips@ece.cmu.edu>              
                          .edu                     cc:
    
                                                   Subject:  RE: iSCSI: Use of
    the A bit         
     
    
                          13-03-02 22:02
    
                          Please respond to
    
                          Eddy Quicksall
    
     
    
     
    
    
    
    
    Then my only concern is that the initiator may ignore the A bit if it deems
    that the bit is being set aggressively.
    
    If it ignored it, then the target would be stalled waiting for he ACK.
    
    Eddy
          -----Original Message-----
          From: BURBRIDGE,MATTHEW (HP-UnitedKingdom,ex2)
          [mailto:matthew_burbridge@hp.com]
          Sent: Wednesday, March 13, 2002 1:39 PM
          To: 'Eddy Quicksall'; ips@ece. cmu. edu (E-mail)
          Subject: RE: iSCSI: Use of the A bit
          Importance: High
    
          Eddy,
    
          The target is quite within its rights to use the A bit when at
          recovery level 0.  If the session is re-established due to recovery
          7.11.4 then the relevant command is aborted anyway and so there is no
          reason to keep hold of the data any way: With recovery level 0 there
          is no recovery mechanism that requires the target to keep the data.
          Therefore the A bit is redundant when the recovery level is 0.
    
          The spec says that the initiator MUST issue a SNACK if the A bit is
          set.  However, the MaxBurstSize restriction is there to prevent the
          initiator from having to send a SNACK on every PDU in the case where
          a target inadvertently sets the A bit in (for example) every data in
          PDU. The target may set the A bit more often than the MaxBurstSize
          but it should not expect a SNACK more often than this.
    
          Matthew Burbridge
                -----Original Message-----
                From: Eddy Quicksall [mailto:Eddy_Quicksall@ivivity.com]
                Sent: Wednesday, March 13, 2002 3:12 PM
                To: ips@ece. cmu. edu (E-mail)
                Subject: iSCSI: Use of the A bit
    
                Here is a case that I want to go over and if there is not
                already a solution, I think a refinement to the A bit could
                solve it.
    
                The problem is that a target (perhaps an iSCSI disk drive) does
                not have enough memory to transfer the full READ request so it
                must read from the medium as much as it can, transmit that,
                when that transmission is known to be good, read the next
                bunch, transmit that and so on.
    
                The problem we have is that the target must keep the buffer
                around until the transfer has been "ack'd" via ExpStatSN. But
                that status can't be sent because all of the requested data has
                not been sent. So the target would have to refuse to do the
                command.
    
                I was going to use the A bit for this thinking it would force
                the initiator to give an "ack" but our current wording does not
                make this a sure fire thing:
    
                1) The initiator may not want to run at ErrorRecoveryLevel 1.
                2) The initiator may ignore the A bit if it deems that the bit
                is being set aggressively.
                3) The target may set the A bit no more frequently than
                MaxBurstSize.
    
                Comments?
    
                mailto:Eddy_Quicksall@iVivity.com
    
    
    
    


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Last updated: Thu Mar 14 09:21:37 2002
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