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    iSCSI: establish a nexus with everything you see?



    I've changed the subject line cause we're going off on a tangent that isn't
    really associated with conservative reuse.
    
    > > If that's indeed so, the assertion that it's only the external factors
    > > that decide
    > > nexii is not always true.  Consider the case of an FC initiator 
    > > port discovering target ports using an FC Name Server, and an iSCSI
    > > initiator port discovering target ports using a "SendTargets" 
    > > command.  In either case,
    > > the initiator SCSI port may not really establish a nexus 
    > > with a discovered target SCSI port, unless the application (or
    > > even the SCSI class driver) really wants to do an I/O 
    > >(starting with an open() system call in Unix systems).
    > 
    > I strongly disagree with that.  Virtually all commercial OS
    implementations
    > of SCSI do aggressive establishments of the nexii as part of the boot
    sequence,
    > as SCSI code tends to expect a "bus walker" to find devices in the boot
    > sequence, and make sure they work.  Put a trace analyzer to work on a boot
    > and watch all the TEST UNIT READY and similar commands flow by.  Is anyone
    > making significant changes to this "bus walker" boot paradigm out there?
    
    I disagree with your notion of the "aggressive establishments" behavior
    being good and desirable.  In practical implementations, it causes many
    problems for FC networks when these over-zealous implementations grab disks
    that aren't intended for them.  Elaborate zoning methods were developed to
    try to alleviate these problems and haven't necessarily helped due to the
    complexity of configuring larger FC networks.  Companies have made money
    developing software that reside between the host SCSI layer and FC drivers
    that "see" all the FC disks, but only establish nexus with those that have
    been properly allocated to this host :-)
    
    When you say "Virtually all commercial OS implementations" you must mean MS
    and Sun OS?  Cause I can think of others that don't establish SCSI nexus in
    this manner.  The whole concept of "plug-n-play" belies the notion that SCSI
    nexus must exist at boot time.
    
    > There's also a subtle issue to watch out for here in that this delays
    > error discovery - if the shared persistent reservations are being
    > used by cluster software, discovering that the alternate path nexus can't
    > be established when trying to fail over to it is way too late.  For things
    > like quorum volumes, I'm fairly sure that cluster software checks all the
    > access paths at initialization time, but can someone familiar with cluster
    > software comment on whether the alternate access paths needed for failover
    > of application volumes are checked in advance, vs. relying on the OS to
    > complain if the SCSI connection to the device didn't set up correctly?
    
    Cluster software is a whole nother ballgame.  I don't see how this is tied
    to the "agressive establishment" idea, except loosely.  I'd expect a network
    with clustering software to be set up much differently than a simpler
    "singly attached storage" environment.  Early iSCSI clustering networks may
    have to have SCSI nexus "preconfigured" in some manner to efficiently boot
    and bring up all alternate paths.
    
    Marjorie Krueger
    Networked Storage Architecture
    Networked Storage Solutions Org.
    Hewlett-Packard
    


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Last updated: Thu Jan 03 15:17:55 2002
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