SATAN magnetic tape handling

Note: this page is heavily under construction.
This section gives basic informations about the structure of data written on a magnetic tape and about the commands for magnetic tape handling.

The structure of information

Data are written sequentially on the tape and can be retrieved only sequentially. Files, blocks, tape marks are elements of the physical structure of the tape. The information written onto a magnetic tape is organized by FILES, each of them consisting of a number of blocks separated by gaps. Files are closed by an END-OF-FILE MARK (tape mark), which is itself a special data block.
A BLOCK is the unit of information transferred in a single I/O operation, consisting of up to 32 Kbytes of data. A GAP is a constant space left unused between two blocks of data (approx. 0.3 inch for 6250 bpi density, 0.6 inch for 800 and 1600 bpi; before writing an end-of-file, much longer gaps are created).
The bigger the block is, the more efficiently the tape is used. For example, consider two different block lengths for a standard tape with a length of 2400 feet (28800 inches) and a density of 6250 bpi. With 4 Kbytes data blocks, the maximum number of blocks is 28800 inch / (4096 bytes / 6250 bpi + 0.3 inch) = 30145 which results in a maximum capacity of approx. 123 Mbytes. Compared to it, about 168 Mbytes can be stored using blocks of 28 Kbytes.
The LOGICAL STRUCTURE (tape format) reflects the data structure and the specific processing necessities. In the EDAS format the first data block of a file is called a file header and contains informations (date, run identifier etc.) about the succeeding data. (The detailed description of this format can be found in the appendix.) A data block consists of one or several RECORDS. A record is the logical unit of information processed during an I/O operation. In the EDAS for- mat each 4 Kbytes record is identified by a unique 'current block number' to allow checking that no data were lost in the I/O process.
The operating system can identify several VOLUME LABELS. They are writ- ten to the tape as special data blocks, the tape may have a 'STANDARD LABEL' (SL), an 'ANSI LABEL' (AL), or a 'NO LABEL' (NL). At GSI the standard format is 'NL'; however, processing of SL- or AL-tapes is sup- ported by several EDAS commands (e.g. INPUT).
SATAN provides the following functions: Additionally the TSO commands ONLTADI and NDTADI transfer data from tapes to sequential data sets, residing on disks. All tapes containig data must be kept and handled without the write-ring in order to prevent them from being overwritten erroneously.
 
enter command : MALOP
enter volume label (external tape label) :  INPUT
enter tape density = 6250 :
tape mount requested
enter command :  MPOS  8
enter command :  MALOP / U(1) EMPTY
enter volume label (external tape label) :  OUTPUT
enter tape density = 6250 :
tape mount requested
enter command :  MCOPY
 
COPY A COMPLETE TAPE
 
enter command : MALOP
enter volume label (external tape label) :  INPUT
enter tape density = 6250 :
tape mount requested
enter command :  MALOP / U(1) EMPTY
enter volume label (external tape label) :  OUTPUT
enter tape density = 6250 :
tape mount requested
enter command :  MCOPY / TAPE
enter command :  EXIT
 
 
o   Beware from further processing the tape  'OUTPUT'. Data may be over-
    written; e.g.,  executing the  command EXIT  an end-of-file  mark is
    written to the current tape position.
 
LIST THE TAPE CONTENTS; DUMP A FILE
 
enter command : MALOP
enter volume label (external tape label) :  OUTPUT
enter tape density = 6250 :
tape mount requested
enter command :  MLIST / FILE(*) BLOCKS(*) LONG
enter command :  MPOS 6
enter command :  MDUMP / BLOCKS(*) LENGTH(4) FORMAT(4) SKIP(40)
MLIST reports the  length of all blocks on all  files. After positioning
with MPOS, the 6th file is dumped to the protocol; the first 40 bytes of
each  block are  skipped,  and 4  bytes are  interpreted  as an  integer
number.

Problems

o   There is an end-of-tape (EOT) mark at the beginning of the tape.
 
    After opening the tape with command MALOP, enter
       MPOS 99
    The tape is  now positioned just after  the EOT mark. To  get a cor-
    rected tape, execute MCOPY.
 
o   The end-of-tape mark is missing.
 
    Execute command MLIST.
    With IBM tapes a read error will occur at the end. Copy the files up
    to this point.
    PDP tapes do not produce an error. Copy file by file to a new tape.
o   An end-of-file (EOF) mark is missing.
 
    Produce a corrected copy by writing an EOF using command $MWEOF. The
    proper positioning is achieved by MPOS and MLIST.
    Example:
    An EOF-mark is missing on tape B720AE  after the second block of the
    sixth file.
 
       MALOP / U(0) D(6250) V(B720AE)
       MALOP / U(1) D(6250) V(B813KF) EMPTY
       MCOPY / I(0) O(1)
 
       Execute the last command six times to copy up to the sixth file.
 
       MPOS 6 / U(0)
       MPOS 6 / U(1)
       MLIST / U(0) BLOCKS(2)
       MLIST / U(1) BLOCKS(2)
       $MWEOF / U(1)
       MCOPY / I(0) O(1) TAPE
       ...
 
o   A tape is not accepted as a no-label tape.
    To check label and  contents, the system has to be  cheated by simu-
    lating a no-label tape. After
       MALOP / U(1)
    put on  any NL-tape. If it  is accepted (indicated by  the prompting
    message 'ENTER COMMAND :'  on the  terminal), unload  the tape  unit
    manually (press the  button 'UNLOAD'). Mount the tape  to be checked
    and  press 'LOAD'  and  'ONLINE'. The  tape  is  now interpreted  as
    NL-tape.  Execute
       MDUMP
    Label and file name are given by the contents of the first blocks.

Converting GOLDA tapes to SATAN tapes

    MALOP / U(1) VOL(B720AE) DEN(6250) EMPTY
    MALOP / VOL(PDP1) DEN(800)
    MFO28 / W G

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Last updated: M.Kraemer@gsi.de, 9-Jun-1999