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    RE: iSCSI question



    Pat,
    
    Thanks for your excellent description. I now fully understand the
    issue.
    
    However, I disagree that the text is clear on the difference between
    error recover classes and error recovery levels. As a proof, look at figure 2, in section 5.15:
    
      +-------------------+--------------------------------------------+
      |ErrorRecoveryLevel |  Associated Error recovery capabilities    |
      +-------------------+--------------------------------------------+
      |        0          |  Session recovery class                    |
      |                   |  (Section 5.14.4 Session Recovery)         |
      +-------------------+--------------------------------------------+
      |        1          |  Digest failure recovery (See Note below.) |
      +-------------------+--------------------------------------------+
      |        2          |  Connection recovery class                 |
      |                   |  (Section 5.14.3 Connection Recovery)      | 
      +-------------------+--------------------------------------------+
    
    In which, each recovery level is associated with only one recovery class.
    
    
    Yours,
    -Shahram
    
    > -----Original Message-----
    > From: pat_thaler@agilent.com [mailto:pat_thaler@agilent.com]
    > Sent: Thursday, August 08, 2002 5:58 PM
    > To: Shahram Davari; pkoning@equallogic.com
    > Cc: ips@ece.cmu.edu
    > Subject: RE: iSCSI question
    > 
    > 
    > Shahram,
    > 
    > There are two recovery heirarchies. You seem to be confusing 
    > error recovery level with error recovery class.
    > 
    > One is the heirarchy for implementation requirements. With 3 
    > forms of recovery, there are 8 possible combinations but to 
    > simplify interoperability and to specify minimum acceptable 
    > operation, we don't want to allow all eight. Session 
    > qualifies two ways to be level 0 of this heirarchy - it is 
    > simplest to implement and it should be able to recover from 
    > any recoverable error. 
    > 
    > Note that these are the error recovery levels that are 
    > described in 5.15. A sessions recovery level indicates the 
    > set of recovery classes it is capable of using. The recovery 
    > levels are defined such that a lower recovery level includes 
    > a subset of the recovery classes available at a higher recovery level.
    > 
    > The other heirarchy is the order in which the available 
    > classes are applied once an error occurs. This is the subject 
    > of 5.14. The classes are disjoint. 
    > 
    > Once an error occurs, the device will choose an error 
    > recovery class from the set of recovery classes in its 
    > recovery level. If that recovery class fails, it may try the 
    > next higher class.
    > 
    > Error recovery level 0 includes the Session recovery class.
    > Error recovery level 1 includes the two classes useful for  
    > Digest failure recovery (Within-Command and 
    > Within-Connection) plus the Session recovery class from level 0.
    > Error recovery level 2 includes  Connection recovery class 
    > plus the classes included in level 1. 
    > 
    > I think the text already makes this clear and the pyramid 
    > correctly represents this, but perhaps the chart below the 
    > period could add "plus the capablilities of 
    > ErrorRecoveryLevel n" to the boxes for Associated Error 
    > recovery capablities for levels 1 and 2 ("n" = 0 and 1, respectively).
    > 
    > I hope this helps remove some of the confusion.
    > 
    > Pat
    > 
    > -----Original Message-----
    > From: Shahram Davari [mailto:Shahram_Davari@pmc-sierra.com]
    > Sent: Thursday, August 08, 2002 2:22 PM
    > To: 'Paul Koning'
    > Cc: ips@ece.cmu.edu
    > Subject: RE: iSCSI question
    > 
    > 
    > Paul,
    > 
    > 
    > > But that's not what "hierarchy" refers to here.
    > > 
    > > The hierarchy is one of increased capability, not increased
    > > desperation.  Session recovery is the minimum required; the 
    > additional
    > > levels are optional capabilities in addition to the minimum.  Each
    > > higher level in the hierarchy is a superset of the one below.
    > 
    > It all depends on the definition of these recovery classes.
    > 
    > 1) If they are defined in a superset/subset fashion, then I agree
    > that level of complexity increases as: Session->PDU->connection.
    > Then I suggest changing texts in other parts of the draft such as
    > section 5.14 to indicate that if you have the capabilities of
    > class X, then you don't need to escalate to lower classes, because
    > class X already has those capabilities itself. Also I suggest
    > changing the hierarchy figure as following:
    > 
    > 
    >                              +
    >                             / \
    >                            / 2 \      
    >                           +-----+
    >                          /  1,2  \     
    >                         +---------+
    >                        /   0,1,2   \   
    >                       +-------------+
    > 
    > 
    > 2) If they are defined as disjoint classes, then the hierarchy for
    > complexity makes no sense. Rather you need a hierarchy for escalation
    > or transition.
    > 
    > 
    > Based on the emails that I have received so far it seems that 
    > the intent is the former definition.
    > 
    > Yours,
    > -Shahram 
    > 
    


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Last updated: Fri Aug 09 13:18:58 2002
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