(tar) Tape Positioning

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 9.5.1 Tape Positions and Tape Marks
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 Just as archives can store more than one file from the file system,
 tapes can store more than one archive file.  To keep track of where
 archive files (or any other type of file stored on tape) begin and end,
 tape archive devices write magnetic "tape marks" on the archive media.
 Tape drives write one tape mark between files, two at the end of all
 the file entries.
 
    If you think of data as a series of records "rrrr"'s, and tape marks
 as "*"'s, a tape might look like the following:
 
      rrrr*rrrrrr*rrrrr*rr*rrrrr**-------------------------
 
    Tape devices read and write tapes using a read/write "tape head"--a
 physical part of the device which can only access one point on the tape
 at a time.  When you use `tar' to read or write archive data from a
 tape device, the device will begin reading or writing from wherever on
 the tape the tape head happens to be, regardless of which archive or
 what part of the archive the tape head is on.  Before writing an
 archive, you should make sure that no data on the tape will be
 overwritten (unless it is no longer needed).  Before reading an
 archive, you should make sure the tape head is at the beginning of the
 archive you want to read.  You can do it manually via `mt' utility
 ( mt).  The `restore' script does that automatically (
 Scripted Restoration).
 
    If you want to add new archive file entries to a tape, you should
 advance the tape to the end of the existing file entries, backspace
 over the last tape mark, and write the new archive file.  If you were
 to add two archives to the example above, the tape might look like the
 following:
 
      rrrr*rrrrrr*rrrrr*rr*rrrrr*rrr*rrrr**----------------
 
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