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(tar.info)Remote Tape Server

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The Remote Tape Server
======================

   In order to access the tape drive on a remote machine, `tar' uses
the remote tape server written at the University of California at
Berkeley.  The remote tape server must be installed as `/etc/rmt' on
any machine whose tape drive you want to use.  `tar' calls `/etc/rmt'
by running an `rsh' or `remsh' to the remote machine, optionally using
a different login name if one is supplied.

   A copy of the source for the remote tape server is provided.  It is
Copyright (C) 1983 by the Regents of the University of California, but
can be freely distributed.  Instructions for compiling and installing
it are included in the `Makefile'.

   Unless you use the `--absolute-names' (`-P') option, GNU `tar' will
not allow you to create an archive that contains absolute file names (a
file name beginning with `/'.) If you try, `tar' will automatically
remove the leading `/' from the file names it stores in the archive.
It will also type a warning message telling you what it is doing.

   When reading an archive that was created with a different `tar'
program, GNU `tar' automatically extracts entries in the archive which
have absolute file names as if the file names were not absolute.  This
is an important feature.  A visitor here once gave a `tar' tape to an
operator to restore; the operator used Sun `tar' instead of GNU `tar',
and the result was that it replaced large portions of our `/bin' and
friends with versions from the tape; needless to say, we were unhappy
about having to recover the file system from backup tapes.

   For example, if the archive contained a file `/usr/bin/computoy',
GNU `tar' would extract the file to `usr/bin/computoy', relative to the
current directory.  If you want to extract the files in an archive to
the same absolute names that they had when the archive was created, you
should do a `cd /' before extracting the files from the archive, or you
should either use the `--absolute-names' (`-P') option, or use the
command `tar -C / ...'.

   Some versions of Unix (Ultrix 3.1 is known to have this problem),
can claim that a short write near the end of a tape succeeded, when it
actually failed.  This will result in the -M option not working
correctly.  The best workaround at the moment is to use a significantly
larger blocking factor than the default 20.

   In order to update an archive, `tar' must be able to backspace the
archive in order to reread or rewrite a record that was just read (or
written).  This is currently possible only on two kinds of files: normal
disk files (or any other file that can be backspaced with `lseek'), and
industry-standard 9-track magnetic tape (or any other kind of tape that
can be backspaced with the `MTIOCTOP' `ioctl'.

   This means that the `--append' (`-r'), `--update' (`-u'),
`--concatenate' (`--catenate', `-A'), and `--delete' commands will not
work on any other kind of file.  Some media simply cannot be
backspaced, which means these commands and options will never be able
to work on them.  These non-backspacing media include pipes and
cartridge tape drives.

   Some other media can be backspaced, and `tar' will work on them once
`tar' is modified to do so.

   Archives created with the `--multi-volume' (`-M'),
`--label=ARCHIVE-LABEL' (`-V ARCHIVE-LABEL'), and `--incremental'
(`-G') options may not be readable by other version of `tar'.  In
particular, restoring a file that was split over a volume boundary will
require some careful work with `dd', if it can be done at all.  Other
versions of `tar' may also create an empty file whose name is that of
the volume header.  Some versions of `tar' may create normal files
instead of directories archived with the `--incremental' (`-G') option.