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    Re: Tape drives and iSCSI



    
    
    Dave,
    
    Thanks a lot for your insights.  We will do our best to have recovery
    flexible enough to offer at least the same level as FCP-2.  The added
    benefits of connection failover and dynamic bandwidth allocation
    should keep iSCSI competitive in bot LAN and WAN.
    
    Regards,
    Julo
    
    "Dave Peterson" <dap@cisco.com> on 12/04/2001 02:06:31
    
    Please respond to "Dave Peterson" <dap@cisco.com>
    
    To:   "Ips@Ece. Cmu. Edu" <ips@ece.cmu.edu>
    cc:
    Subject:  Tape drives and iSCSI
    
    
    
    
    
    Don't believe a tape backup application model will add much value to the
    discussion.
    Here's my view:
    
    Tape Requirements
    -----------------
    1. Data integrity
    2. Data integrity
    3. Data integrity
    4. Perform successful uninterrupted backup (within backup window is a big
    plus)
    
    Tape Specifics
    --------------
    Large record sizes are recommended for performance. The largest (typical)
    record size is 256KB.
    Using a default DataPDULength = 8192 would require 32 data PDU's.
    A typical higher-end head-to-tape transfer rates = 10 MB/sec resulting in a
    backup rate of ~36 GB/hour.
    
    Ramblings& Tidbits
    ------------------
    FCP-2 error detection and recovery is implemented by at least one high-end
    tape drive vendor (and they have a significant advantage over other
    vendors,
    i.e., their backup will not fail due to a FC link-level error).
    
    SCSI level timeout and retry is highly dependent on the backup application.
    One must first determine the state/position of the tape drive before
    proceeding. Tools such as READ POSITION and LOCATE are in place for the
    backup app to attempt recovery from an error. Problem has been few backup
    vendors have yet to implement them (but things are getting better). Thus
    FC-TAPE was born and the error detection and recovery mechanism was rolled
    into FCP-2 as a standard. FCP-2 provides tools to determine the
    state/position of the tape drive below the SCSI level allowing a "best
    effort" attempt to complete the exchange/command and not return an error to
    the application. At this point in time this is what is important, i.e., DO
    NOT RETURN AN ERROR TO THE APPLICATION IF AT ALL POSSIBLE.
    
    Refer to Matt Wakeley's I/O (command) recovery write-up for a description
    of
    iSCSI error recovery that should be used as a starting point. Maybe this
    has
    already been done, I'm not involved in the error recovery group. What we
    need is the ability to detect an error and recovery below the SCSI level
    (i.e., the iSCSI transport level). Any further granularity is not needed
    especially due to the low error rates that will be encountered.
    
    Regarding tape devices and maintaining state, FC-TAPE enabled drives have
    the following requirements:
    For non-tagged command queuing operations, the target shall retain the
    Exchange information until
    a) the next FCP_CMND IU has been received for that LUN from the same
    initiator;
    b) an FCP_CONF IU is received for the Exchange; or
    c) after RR_TOV times out.
    For tagged command queuing operations, the target shall retain Exchange
    information until
    a) an FCP_CONF IU is received for the Exchange; or
    b) after RR_TOV times out.
    
    There is a work in progress for a new tape model in the T10 SSC-2 working
    group. This new model will allow for a simpler error detection and recovery
    precedure and a robust command queuing implementation.
    
    Finally, I strongly agree with the sentiment of getting the first version
    "out the door".
    The issues surrounding CRC's and error recovery need to be put to reset
    asap.
    
    David Peterson
    Lead Architect - Standards Development
    Cisco Systems - SRBU
    6450 Wedgwood Road
    Maple Grove, MN 55311
    Office: 763-398-1007
    Cell: 612-802-3299
    Email: dap@cisco.com
    
    
    
    
    


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Last updated: Tue Sep 04 01:05:01 2001
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