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:
- Write reduced list mode data during data analysis. The command IOUTPUT defines a logical device and handles this device (start, stop,
free).
- Read standard EDAS formatted data for replay (command INPUT).
- Convert tapes of different external formats into the EDAS format
(command MFO28).
- Perform standard tape handling, as
copy one tape to another (MCOPY),
list the contents of a tape (MLIST, MDUMP),
position a tape (MPOS, MSKBLOCK, MSKFILE).
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
Last updated: M.Kraemer@gsi.de, 9-Jun-1999