VMS Usernote 16

Use of Magnetic tape

Introduction
Allocation and Storage
Loading and Unloading
Software
Examples
Other Points
Document History

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1. Introduction

The main uses of magnetic tape are (a) as a medium for transfer of files to another computer and (b) as a means of storage for files to be read back onto the same computer. For the former, the manner in which files are written will depend upon the facilities available for reading them back at the destination site. Users are advised to contact the Systems staff for more advice. This usernote deals with procedures for the second option.

References are made in this usernote to the tape drive Tape$0: Other devices may be available; further details are available on request.

Increasingly, network links between computers are providing a faster and more convenient means of transferring information. The reader is advised to investigate this possibility (for example via ftp between Internet sites before expending significant effort on the creation of tapes.

2. Allocation and Storage

Tapes are issued by the Systems staff. Each tape is allocated a reference number, which corresponds to a position on the storage rack for user magtapes in room G56 on the ground floor in the Department of Mechanical Engineering. Under normal circumstances, tapes need not be removed from this room. (Storage on office window sills is not recommended).

3. Loading and Unloading

Instructions for loading and unloading a tape will be found on the inside of the door of the magnetic tape deck. A (plastic) write permit ring must be placed in the slot on the back of the tape if you intend to write to it. The ring should be removed if you only wish to read information; this prevents accidental overwriting of files from the tape.

Each tape should bear a sticky label (affixed to the tape, not its container). It is suggested that this should contain a minimum of the following:

   Owner's name
   Username
   Date written

Other useful information could be:

   Computer label of tape
   Software utility used (e.g. COPY, BACKUP, FILEX)

However, users are recommended not to put an excessive amount of information on the label. Lists of files on the tape should be kept elsewhere.

4. Software

The use of COPY or BACKUP is recommended. For a description of BACKUP, refer to VMS Usernote 44. Note, however, that files written onto tape in this manner can only be read back by a VAX computer. Users who wish to use FILEX, WRITETAPE or other formats should refer to the manual. The relevant commands for COPY are as follows:

   $ ALL Tape$0:

Allocates the magnetic tape deck to the user. This prevents other users from attempting to access the tape at the same time. (Load the tape at this stage, checking that it is online).

   $ INI Tape$0:  label /DENS=6250

Initialises the tape (you supply a label of up to 6 characters). This command is ONLY issued before the first time that you write to the tape; any information currently on the tape will be overwritten!. The switch indicates that information is to be written at a density of 6250 bits per inch.

 
   $ MOU Tape$0:  label /DATA=WRITE

Mounts the tape. The label must correspond to the one assigned when the tape was initialised. This enables the computer to check that the correct tape has been loaded. The switch (/DATA=WRITE) is included when writing to the tape, but not for reading back. It verifies that each block has been written correctly.

 
   $ COPY list-of-files Tape$0:*.* /LOG

Copies files from disc to magnetic tape, retaining the same names.

   $ COPY Tape$0:list-of-files Disk$1:*.*

Copies files from magnetic tape back to disc.

   $ DIR Tape$0: /OUT=filename

Creates a directory (list of names) of files on the tape, and stores it in the given file.

   $ DISM Tape$0:

Dismounts the tape. (After this stage, it is possible to physically unload the tape).

   $ DEAL Tape$0:

Deallocates the tape deck.

5. Examples

Example 1

   $ ALL Tape$0:
   $ INI Tape$0: MYTAPE /DENS=6250
   $ MOU Tape$0: MYTAPE /DATA=WRITE
   $ COPY *.FOR Tape$0:*.*
   $ COPY *.DAT Tape$0:*.*
   $ DIR Tape$0: /OUT=MYTAPE.LIS/SIZE/DATE
   $ PRINT MYTAPE.LIS
   $ DISM Tape$0:
   $ DEAL Tape$0:

The tape unit is allocated. The tape is initialised with the label MYTAPE and then mounted. If large data files are to be written to the tape, then an extra qualifier /BLOCK=4096 should be included. All FORTRAN and DATA files are copied from the current directory. A directory of the contents of the tape is then created and printed. Finally, the tape is dismounted and the unit is deallocated.

Example 2

   $ ALL Tape$0:
   $ MOU Tape$0: MYTAPE
   $ COPY Tape$0:PFDE.FOR,HAL.DAT Disk$1:*.*/LOG
   $ DISM Tape$0:
   $ DEAL Tape$0:

6. Other Points

The capacity of a 2400 foot magnetic tape is in excess of 10Mbytes (20000 blocks of filestore) if using COPY at 1600bpi. Approximately four times as much data can be stored at a density of 6250bpi, whilst significantly higher amounts of data can be saved using BACKUP.

Note that the block sizes given on a tape directory differ from their disc counterparts by a factor of 4. This is because the default block size for tape is 2048 bytes, compared with 512 bytes for disc. The size of files restored to disc does not differ from the originals.

In general, multiple versions of the same file should not be saved on tape. Also, there is little point in saving .OBJ or .EXE files, as these can quickly be recreated.

Files cannot be transferred to tape via the RENAME command.

Files cannot be selectively overwritten or deleted from the tape.

Unless the tape has been re-initialised, new files are appended after any existing information. The tape will automatically be wound forward to the correct position. Similarly, the tape is automatically rewound where necessary. However, when reading files back from tape, it is more efficient to do so in the order that they were written.

The message %MOUNT-I-WRITELOCK, Volume is write locked will occur if a write permit ring has not been put on the tape. If the intention is to copy information from the tape, this message may be ignored. Alternatively, use the /NOWRITE qualifier for the MOU command.

7. Document History

Use of Magnetic Tape