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Calculating capacity of magtape?

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The Question is:

I am attempting to figure the amount of data that is stored on DLTtape III
 tapes. The tapes are 10/20 gb.
My calculations dont seem to add up.  I have a total of 54,691,570 blocks of
 data.  I multiply the total blocks by 512 and then divide by 1000000000.  This
 gives me 1.3 gb of data.  Hmmmm that should fit on one tape.
mb should be (blocks * 512)/1000000
gb should be (blocks * 512)/1000000000
Also what is the compaction ratio using a TZ87 table top unit.
Your help would really assist me.
Edward A. Lucas

The Answer is :

  Please see the OpenVMS FAQ for some details of the problems involved
  with predicting the capacity of a magtape.
  Also please note that 54,691,570 half-kilobyte units is equivalent to
  27,345,785 kilobyte-sized units, or 27,345,785,000 bytes, or roughly
  27 gigabytes, uncompressed.  (The FAQ also has information on units,
  both as used by OpenVMS and as used by the storage device ratings.)
  For reference purposes, here are the uncompressed rated capacities of
  various DLT and DLT-like media and drives:
    TK50            95 MB   CompacTape
    TK70            195 MB  CompacTape II
    TZ85            2.5 GB  CompacTape III (DLT III)
    TZ86            5 GB    CompacTape III (DLT III)
    TZ87            10 GB   CompaqTape III (DLT III)
    TZ88            20 GB   DLT IV, CompaqTape III, DLT III-XT
    TZ89            35 GB   DLT IV, CompaqTape III, DLT III-XT
    SDLT 110/220    110 GB  110/220GB Super DLT
    SDLT 160/320    160 GB  160/320GB Super DLT
  For drives with compaction support and assuming 2:1 compaction ratio
  (compression ratio), double the above values.
  Applications such as BACKUP can add redundancy groupings and can inject
  recovery data into the data stream, and can thus reduce the aggregate
  capacity of the media.  Accordingly, you may or may not choose to use
  this capability of BACKUP and/or the capabilities of the particular
  tape drive.

answer written or last revised on ( 2-FEB-2004 )

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