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:
