SFINX documents - the tar command and tapes

Types of tapes

There are various types of magnetic tapes that can be used as storage or backup media. At the Sterrewacht we have the following types of tapes:

DAT (Digital Audio Tape) - stores approx. 1.3 GB (or more if compression is possible)
Exabyte (8mm video) - stores approx 4 GB (or more, with compression)
DLT (Digital Linear Tape) - stores approx. 20 GB, and is fast
QIC tape - stores only 60 MB

Currently, DAT tapes seem to be most widely available around the world, so if you are planning on taking your data to another institute, DAT might be the best choice.

The following machines have a tape device:

DAT : rijn, ijssel, schelde, zijl, vecht, vliet, lithium, melian, flow, redshift drecht (2x), dollard [DAT jukebox], merwede
Exabyte: ijssel, vecht, gouwe
DLT : gouwe, wad
QIC : spui

Under the SFINX environment, the device name of the tape device is always set in the $TAPE envirnoment variable, so that you nearly never have to tell a command the device name to work with.

An up-to-date listing of tapes can be obtained using the command devices tape.

compression

On some machines, compressed tape access is possible. Currently this is the case on all Exabyte and DLT units, and on the newest DAT units (DDS-3; on ijssel, melian, redshift, flow, merwede).

To use this feature, make sure you have the proper device name set in the $TAPE environment variable. The device names are indicated at a label on the tape device.

Beware that compressed backups cannot be read on all machines !!

The mt command

This command (whose name stands for "magnetic tape") is used to view the tape status, and spool the tape backward and forward.

mt status : displays tape status.
If all is OK, the status should end with a line file: 0 block: 0 which means the head is positioned at the beginning of the tape. So file: 1 block: 0 means it is positioned just after the first file on tape. If the status should read file: 0 block: 200 this means it is positioned somewhere in file 1. You should then probably skip forward to the end of the file in order not to damage the file.
mt fsf 1 : forward skip file
This skips forward to the next end of file mark on tape, i.e. to after the end of the current file. The number indicates the number of files to skip, which may be bigger than 1.
mt bsf 1 : backward skip file
Skip back to directly after the previous end of file mark. This command is not supported on all tape devices.
mt rewind : rewind the tape completely
mt rewoffl : rewind and eject the tape

See the mt (1) man page for more detail.

The tar command

The most often used command to write or read tapes is tar, which stands for "tape archive". It can be used to make backups of (parts of) your data. There is also GNU tar (gtar) which has more options (including compression).

Files are always saved and restored with their full path name. This is both convenient and inconvenient: it allows a complete backup and restore with all the files in the same location, but one also has to be careful. If a backup is made with the command tar cv /home/strw/myname, then it is almost impossible to restore this backup on a system where no /home/strw disk exists. The smart way to do a backup is therefore to change to the directory you want to save, and then do tar cv ., because the current directory (.) will always exist on every UNIX system.

Note: GNU tar (gtar) is smart enough to correct this "mistake" automatically.

The most often used commands are:

tar cv .
Create (c) a backup of the current directory (.), and give a verbose (v) description of what is being written.
tar tv
List the contents of a tar-file (backup set).
tar xv ./dir/file Extract (x) a certain file (specified with its full path name).

See the tar (1) man page for more detail.

The Backup command

Backup is a SFINX script built around the tar command. Some people may like it, but it is never necessary to use it. Tapes written by Backup can be read by tar and vice versa.

Other commands that work with tapes

There is a couple of other programs that might be useful when dealing with tapes.

dd : this is a low level tape access program. It can be used to copy files from tape to disk if the block size is known. e.g. dd if=$TAPE of=file1.fits bs=28800
tcopy : a utility to copy tapes. Not very useful at the moment, except on machines with more than one tape device. e.g. tcopy /dev/rmt/0mn /dev/rmt/3mn
Another use for tcopy is to list the record structure of a tape. This is what is done if you give tcopy only one tape argument.
dump & restore : utilities for backups; especially useful for the system manager. Don't try it, unless you know what you are doing.
tpalloc & tpdealloc : utilities to allocate and deallocate the tape unit, in order to prevent others from using it. Don't forget to deallocate !!!
tpcheck : utility to check who has allocated a tape unit

See also the section about the automatic backups.

David.Jansen@strw.leidenuniv.nl

Last modified: Thu Jan 14 10:13:37 1999