(tar)Using Multiple Tapes

Archive Label

Many

Media

Using Multiple Tapes
====================

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   Often you might want to write a large archive, one larger than will
fit on the actual tape you are using.  In such a case, you can run
multiple `tar' commands, but this can be inconvenient, particularly if
you are using options like `--exclude=PATTERN' or dumping entire
filesystems.  Therefore, `tar' supports multiple tapes automatically.

   Use `--multi-volume' (`-M') on the command line, and then `tar' will,
when it reaches the end of the tape, prompt for another tape, and
continue the archive.  Each tape will have an independent archive, and
can be read without needing the other.  (As an exception to this, the
file that `tar' was archiving when it ran out of tape will usually be
split between the two archives; in this case you need to extract from
the first archive, using `--multi-volume' (`-M'), and then put in the
second tape when prompted, so `tar' can restore both halves of the
file.)

   When prompting for a new tape, `tar' accepts any of the following
responses:

`?'
     Request `tar' to explain possible responses

`q'
     Request `tar' to exit immediately.

`n FILE NAME'
     Request `tar' to write the next volume on the file FILE NAME.

`!'
     Request `tar' to run a subshell.

`y'
     Request `tar' to begin writing the next volume.

   (You should only type `y' after you have changed the tape; otherwise
`tar' will write over the volume it just finished.)

   If you want more elaborate behavior than this, give `tar' the
`--info-script=SCRIPT-NAME' (`-F SCRIPT-NAME') option.  The file
SCRIPT-NAME is expected to be a program (or shell script) to be run
instead of the normal prompting procedure.  When the program finishes,
`tar' will immediately begin writing the next volume.  The behavior of
the `n' response to the normal tape-change prompt is not available if
you use `--info-script=SCRIPT-NAME' (`-F SCRIPT-NAME').

   The method `tar' uses to detect end of tape is not perfect, and
fails on some operating systems or on some devices.  You can use the
`--tape-length=1024-SIZE' (`-L 1024-SIZE') option if `tar' can't detect
the end of the tape itself.  The SIZE argument should be the size of
the tape.

   The volume number used by `tar' in its tape-change prompt can be
changed; if you give the `--volno-file=FILE-OF-NUMBER' option, then
FILE-OF-NUMBER should contain a decimal number.  That number will be
used as the volume number of the first volume written.  When `tar' is
finished, it will rewrite the file with the now-current volume number.
(This does not change the volume number written on a tape label (
FIXME: pxref Special Options for Archiving
   ; it *only* affects the number used in the prompt.)

   If you want `tar' to cycle through a series of tape drives, then you
can use the `n' response to the tape-change prompt.  This is error
prone, however, and doesn't work at all with
`--info-script=SCRIPT-NAME' (`-F SCRIPT-NAME').  Therefore, if you give
`tar' multiple `--file=ARCHIVE-NAME' (`-f ARCHIVE-NAME') options, then
the specified files will be used, in sequence, as the successive volumes
of the archive.  Only when the first one in the sequence needs to be
used again will `tar' prompt for a tape change (or run the info script).

   Multi-volume archives

   With `--multi-volume' (`-M'), `tar' will not abort when it cannot
read or write any more data.  Instead, it will ask you to prepare a new
volume.  If the archive is on a magnetic tape, you should change tapes
now; if the archive is on a floppy disk, you should change disks, etc.

   Each volume of a multi-volume archive is an independent `tar'
archive, complete in itself.  For example, you can list or extract any
volume alone; just don't specify `--multi-volume' (`-M').  However, if
one file in the archive is split across volumes, the only way to extract
it successfully is with a multi-volume extract command `--extract
--multi-volume' (`-xM') starting on or before the volume where the file
begins.

Multi-Volume Archives

Tape Files