tar: Remote Tape Server

1 
1 9.2 Remote Tape Server
1 ======================
1 
1 In order to access the tape drive on a remote machine, 'tar' uses the
1 remote tape server written at the University of California at Berkeley.
1 The remote tape server must be installed as 'PREFIX/libexec/rmt' on any
1 machine whose tape drive you want to use.  'tar' calls 'rmt' by running
1 an 'rsh' or 'remsh' to the remote machine, optionally using a different
1 login name if one is supplied.
1 
1    A copy of the source for the remote tape server is provided.  Its
1 source code can be freely distributed.  It is compiled and installed by
1 default.
1 
1    Unless you use the '--absolute-names' ('-P') option, GNU 'tar' will
1 not allow you to create an archive that contains absolute file names (a
1 file name beginning with '/').  If you try, 'tar' will automatically
1 remove the leading '/' from the file names it stores in the archive.  It
1 will also type a warning message telling you what it is doing.
1 
1    When reading an archive that was created with a different 'tar'
1 program, GNU 'tar' automatically extracts entries in the archive which
1 have absolute file names as if the file names were not absolute.  This
1 is an important feature.  A visitor here once gave a 'tar' tape to an
1 operator to restore; the operator used Sun 'tar' instead of GNU 'tar',
1 and the result was that it replaced large portions of our '/bin' and
1 friends with versions from the tape; needless to say, we were unhappy
1 about having to recover the file system from backup tapes.
1 
1    For example, if the archive contained a file '/usr/bin/computoy', GNU
1 'tar' would extract the file to 'usr/bin/computoy', relative to the
1 current directory.  If you want to extract the files in an archive to
1 the same absolute names that they had when the archive was created, you
1 should do a 'cd /' before extracting the files from the archive, or you
1 should either use the '--absolute-names' option, or use the command 'tar
1 -C / ...'.
1 
1    Some versions of Unix (Ultrix 3.1 is known to have this problem), can
1 claim that a short write near the end of a tape succeeded, when it
1 actually failed.  This will result in the -M option not working
1 correctly.  The best workaround at the moment is to use a significantly
1 larger blocking factor than the default 20.
1 
1    In order to update an archive, 'tar' must be able to backspace the
1 archive in order to reread or rewrite a record that was just read (or
1 written).  This is currently possible only on two kinds of files: normal
1 disk files (or any other file that can be backspaced with 'lseek'), and
1 industry-standard 9-track magnetic tape (or any other kind of tape that
1 can be backspaced with the 'MTIOCTOP' 'ioctl').
1 
1    This means that the '--append', '--concatenate', and '--delete'
1 commands will not work on any other kind of file.  Some media simply
1 cannot be backspaced, which means these commands and options will never
1 be able to work on them.  These non-backspacing media include pipes and
1 cartridge tape drives.
1 
1    Some other media can be backspaced, and 'tar' will work on them once
1 'tar' is modified to do so.
1 
1    Archives created with the '--multi-volume', '--label', and
1 '--incremental' ('-G') options may not be readable by other version of
1 'tar'.  In particular, restoring a file that was split over a volume
1 boundary will require some careful work with 'dd', if it can be done at
1 all.  Other versions of 'tar' may also create an empty file whose name
1 is that of the volume header.  Some versions of 'tar' may create normal
1 files instead of directories archived with the '--incremental' ('-G')
1 option.
1