(tar) Many

 
 Many Archives on One Tape
 =========================
 
    Most tape devices have two entries in the `/dev' directory, or
 entries that come in pairs, which differ only in the minor number for
 this device.  Let's take for example `/dev/tape', which often points to
 the only or usual tape device of a given system.  There might be a
 corresponding `/dev/nrtape' or `/dev/ntape'.  The simpler name is the
 _rewinding_ version of the device, while the name having `nr' in it is
 the _no rewinding_ version of the same device.
 
    A rewinding tape device will bring back the tape to its beginning
 point automatically when this device is opened or closed.  Since `tar'
 opens the archive file before using it and closes it afterwards, this
 means that a simple:
 
      $ tar cf /dev/tape DIRECTORY
 
 will reposition the tape to its beginning both prior and after saving
 DIRECTORY contents to it, thus erasing prior tape contents and making
 it so that any subsequent write operation will destroy what has just
 been saved.
 
    So, a rewinding device is normally meant to hold one and only one
 file.  If you want to put more than one `tar' archive on a given tape,
 you will need to avoid using the rewinding version of the tape device.
 You will also have to pay special attention to tape positioning.
 Errors in positioning may overwrite the valuable data already on your
 tape.  Many people, burnt by past experiences, will only use rewinding
 devices and limit themselves to one file per tape, precisely to avoid
 the risk of such errors.  Be fully aware that writing at the wrong
 position on a tape loses all information past this point and most
 probably until the end of the tape, and this destroyed information
 _cannot_ be recovered.
 
    To save DIRECTORY-1 as a first archive at the beginning of a tape,
 and leave that tape ready for a second archive, you should use:
 
      $ mt -f /dev/nrtape rewind
      $ tar cf /dev/nrtape DIRECTORY-1
 
    "Tape marks" are special magnetic patterns written on the tape
 media, which are later recognizable by the reading hardware.  These
 marks are used after each file, when there are many on a single tape.
 An empty file (that is to say, two tape marks in a row) signal the
 logical end of the tape, after which no file exist.  Usually,
 non-rewinding tape device drivers will react to the close request issued
 by `tar' by first writing two tape marks after your archive, and by
 backspacing over one of these.  So, if you remove the tape at that time
 from the tape drive, it is properly terminated.  But if you write
 another file at the current position, the second tape mark will be
 erased by the new information, leaving only one tape mark between files.
 
    So, you may now save DIRECTORY-2 as a second archive after the first
 on the same tape by issuing the command:
 
      $ tar cf /dev/nrtape DIRECTORY-2
 
 and so on for all the archives you want to put on the same tape.
 
    Another usual case is that you do not write all the archives the same
 day, and you need to remove and store the tape between two archive
 sessions.  In general, you must remember how many files are already
 saved on your tape.  Suppose your tape already has 16 files on it, and
 that you are ready to write the 17th.  You have to take care of skipping
 the first 16 tape marks before saving DIRECTORY-17, say, by using these
 commands:
 
      $ mt -f /dev/nrtape rewind
      $ mt -f /dev/nrtape fsf 16
      $ tar cf /dev/nrtape DIRECTORY-17
 
    In all the previous examples, we put aside blocking considerations,
 but you should do the proper things for that as well.   Blocking.
(tar) Many

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