5-7 TAPES AND CARTRIDGES
*************************
(Thanks to Yehavi Bourvine for the help and part of the information,
and to Glenn Everhart for the very informational comments)
Magnetic tapes and cartridges are used to store very large amounts of
data that will not be used frequently. Locating a specific piece of
data on a magnetic tape is slow, as tape velocities are relatively low
and you have to read all information preceding it.
Magnetic media types (tapes and cartridges)
-------------------------------------------
Cartridges (of the same size) with shorter tape length hold less data but
may be more reliable because the magnetic tape is thicker and the influence
of one tape winding on the others is less due to the larger separation of
the magnetic coating.
A relatively new innovation is MRC (Media Recognition System) found usually
on 4mm cartridges. Cartridges with MRC have the relevant tape specifications
written in the beginning of the tape so the tape drive can read it, identify
the tape type and use the tape in the correct way.
4mm DAT tapes are considered by some people to be more reliable than 8mm,
in spite of the fact that 8mm recorded on the medium bit density is less
than 4mm.
However, much of the problems folks have had with 8mm, may have been due to
the lower physical quality of some drives (especially older ones), and has
also been driven by the fact that video grade tapes are easy to use and a
lot more easier to get (and a LOT harder on the drives) than data grade tape.
Some video grade 8mm's are said to damage the head after 3 or 4 uses!
4mm audio grade tape is as hard to find as data grade so folks use the data
grade stuff more often.
Comparison of different magnetic media types
============================================
Type | Tape | Capacity | log | Record | Density | Record | Operation
| dims | /compacted | BER | method | (bpi) | format | mode
---------|--------|------------|-----|---------|---------|--------|-----------
4mm DAT | 60m | 1.3/~2.6GB | 15 | Helical | 1869 | DDS | Streaming
| | | | scan R | tr./in. | ANSI |
---------|--------|------------|-----|---------|---------|--------|-----------
| 90m | 2.0/~4GB | 15 | Helical | 1869 | DDS | Streaming
| | | | scan R | tr./in. | ANSI |
---------|--------|------------|-----|---------|---------|--------|-----------
| 120m | 4/~8GB | | | | |
| | | | | | |
---------|--------|------------|-----|---------|---------|--------|-----------
8mm | 60ft | 2.3GB | | | 5400 | | Streaming
| | | | | bpi | |
---------|--------|------------|-----|---------|---------|--------|-----------
8mm | 112m | 5/~10GB | 17 | Helical | 8500 | Vendor | Streaming
| | | | scan | bpi | specif |
---------|--------|------------|-----|---------|---------|--------|-----------
TK25 | 0.25" | 60MB | | | 8000 | | Streaming
| | | | 10 Tr. | bpi | |
---------|--------|------------|-----|---------|---------|--------|-----------
TK50 | 0.5" x | 95MB | | Serpent | 6667 | | Streaming
| 600ft | | | 22 Tr. | bpi | |
---------|--------|------------|-----|---------|---------|--------|-----------
TK70 | 0.5" x | 296MB | | Serpent | 10000 | | Streaming
| 600ft | | | 48 Tr. | bpi | |
---------|--------|------------|-----|---------|---------|--------|-----------
QIC | 0.25" | 60MB | | | | | Streaming
| 425ft | | | 9 Tr. | | |
---------|--------|------------|-----|---------|---------|--------|-----------
QIC | 0.25" | 150MB | | | | | Streaming
| 700ft | | | 18 Tr. | | |
---------|--------|------------|-----|---------|---------|--------|-----------
QIC | 0.25" | 320,525MB | | | 1000 | | Streaming
| | | | | bpi | |
---------|--------|------------|-----|---------|---------|--------|-----------
IBM-3480 | | 200MB | | Linear | | |
TA90 | | | | 18 Tr. | | |
---------|--------|------------|-----|---------|---------|--------|-----------
6150 | | 150MB | | | | |
---------|--------|------------|-----|---------|---------|--------|-----------
6250 | | 250MB | | | | |
---------|--------|------------|-----|---------|---------|--------|-----------
Typical | 0.5" x | 145, 40MB | 11 | Linear | 1600 PE | | Start/Stop
reel | 2400ft | GCR, PE | | 9 Tr. | 6250 GCR| | Streaming
| D10.5" | | | | bpi | |
---------|--------|------------|-----|---------|---------|--------|-----------
Tape reel and magnetic cartridge drives
----------------------------------------
Classical tape reel drives can accelerate and decelerate rapidly,
for example they can read a record, decelerate to a stop in the
Inter-Record Gap and accelerate again and read the next record
(see below for an explanation of these terms).
It is cheaper to have STREAMING DRIVES, these have simpler mechanisms
and weaker motors, so they need more time (and tape length) to accelerate
to reading speed and decelerate to a stop. Streaming drives perform
well only on long reads and writes, if the operating system can keep up
with the read/write rate.
Tapes are used mainly for data backups and physical transfer of large
data files, for these purposes streaming drives are adequate.
Methods of data encoding
------------------------
Tapes are coated with a material that can be easily magnetized in different
directions by the magnetic field produced by the drive's head. The same
drive head may be used to read the magnetization direction.
Drive heads cannot actually detect the magnetization direction, but only
changes in this direction. This problem is solved by having the drive
hardware 'remember' the direction in the previous time unit, if a direction
transition occurred the new direction has reversed, if not it is the same.
Of course we can't know the initial magnetization direction, we can
arbitrarily call it UP or DOWN as we like, it doesn't matter because all
coding methods are based on direction transitions, not directions.
Another fundamental problem at the hardware level is to determine while
reading when each bit starts and ends, so we can properly sample and
decode its value.
The tape speed and recording density are supposed to be constant, so
bits are read at a (more or less) constant rate, but in practice we
can't depend on that, and we must 'resynchronize' frequently.
The synchronization problem may be solved either by having separate
synchronization pulses on a special track, or incorporating them into
the data track.
NRZI (Non Return to Zero - Inverted)
------------------------------------
In this method the magnetization direction is reversed for 1(binary),
and remains constant for 0(binary). NRZI obviously needs a separate
synchronization track, e.g. if a series of 0(binary) is recorded,
the magnetization direction will remain constant, and we need some
means to count the 0(binary) bits.
In NRZx methods we keep on a separate track 'clock' (synchronization)
pulses - transitions made every time unit. Every clock pulse we check
if a transition occurred and compute the bit value accordingly.
Typical density of NRZI recording is 800 bits per inch
Phase encoding (PE)
-------------------
Also called biphase-mark, Harvard, Manchester or split-frequency system.
Here the data contains the clock pulses, a direction transition occurs
every bit. We use the direction transitions as our basic signs, e.g. an
UP/DOWN transition may be our 1(binary) and a DOWN/UP transition will be
0(binary). If there is no direction transition, we get another useful
sign that we will call NO-VALUE (we will not be able to count such
successive signs).
By the way, between any two consecutive transitions of the same type we
will get an extra non-data transition, that artifact can be ignored by
the drive circuitry.
Typical density of phase encoding is 1600 bits per inch.
GCR (Group Coded Recording)
---------------------------
Typical density is 6250 bits per inch.
The internal structure of tapes
-------------------------------
The internal data structures of tapes are very robust, and can withstand
to some degree controller, tape head or magnetic media errors.
We will think of a written tape as nine long sequences that run side by
side and are composed of the three signs: 0, 1 and NO-VALUE, eight of the
sequences convey the information and the ninth is used as a check
(odd parity check). In addition there is the BOT mark (Beginning Of Tape)
a little after the beginning and the EOT mark (End Of Tape) a little
before the end.
The BOT and EOT marks are not written on the tape using the three signs
but are pieces of photo-reflective tape.
A character is eight 0 or 1 signs, one from each sequence. A series of
characters delimited on both sides by a long (about 0.6 inch) series
of NO-VALUE signs is a record. The delimiting region is called the IRG
(Inter Record Gap).
"Mixing" a record and an IRG creates the important TAPE-MARK, that serves
as a general delimiter at the hardware level. A tape-mark is created when
some of the nine sequences are composed of NO-VALUE signs only, and some
are constant having value of 0 or 1. The drive hardware can easily create
or recognize this state.
A series of records delimited on both sides by a tape-mark is a file, the
IRG's near the tape-marks are then made longer.
Two consecutive tape-marks denote a "logical" end-of-volume. You can have
more than one "volume" on the tape, but you have to do some programming to
pass between them (?).
Labels are records (length = 80 characters), they are separated from other
records by a tape-marks. Labels are used to keep security and structure
information. There are several kinds of labels: Volume label, File header
labels and file trailer labels etc.
Scheme of a tape structure
--------------------------
BOT mark
VOL1 label
HDR1 label ----+
HDR2 label |
Tape-mark |
Data record |---- the first file
........... |
Data record |
Tape-mark |
EOF1 label |
EOF2 label |
Tape-mark ----+
...........
...........
...........
Tape-mark
Tape-mark
........... ----+
........... | unwritten tape area
........... ----+
EOT mark
The tape structure is actually more complicated because:
1) More labels can be present.
2) The files can be continued to another tape.
3) Data can be written behind the EOT.
Using tapes (general)
---------------------
The basic tape operations are:
Drive allocation Tell the operating system it should not let
other users do anything with that tape drive
Initialization Build the basic data structures on the tape, all
previous data on the tape will be inaccessible!
Similar to formatting a PC diskette
Mount Connecting the tape drive to the file system,
the system software that controls all accesses
to disk and tape drives.
Rewind Spinning the tape to the beginning
Unload Opening the drive door and ejecting the cartridge
Write Made with FORTRAN's WRITE statement
Read Made with FORTRAN's READ statement
Deallocation Tell the operating system you don't need
the tape drive anymore
Dismount Cancel a mount command, disconnect the tape drive
from the file system.
Data protective measures
------------------------
The safety switch on a cartridge back side, will prevent writing on the
cartridge, when in the write-protect position.
The integrity of each data byte is guarded by the parity bit. Professional
tape drives and utilities like UNIX/TAR and VMS/BACKUP and VMS/DUMP use
better schemes like CRC (Cyclical Redundancy Code) and Reed-Solomon ECC
(Error Correcting Code) that has far better ability to correct errors.
The volume label on VMS gives some protection against writing on the wrong
cartridge or tape. When the tape is initialized you have to supply an
alphanumeric string that is written into the volume label. Other tape
operations may require that you respecify that string, it is compared with
the string written in the volume label and if found different, the operation
is aborted.
Using tapes on VMS
------------------
Device names
------------
Tape drive names on VMS are composed of:
Allocation-class Optional, 1-2 digits enclosed by '$' signs
Type prefix The letters 'M', 'T'
Type K - SCSI
U - Generic
Controller id A - internal IO bus, B - external IO bus
Device number 0, 1, 2, 3, ...
Placeholder If device number is not zero, add '00'
Colon suffix ':'
For example let's analyze the device name $4$MKB500:
The allocation class is 4, this is site-specific information
The 'M' says it's a magnetic tape drive
the 'K' means a SCSI drive
'B' says the controller is external to the computer box
Device number 5 means the SCSI ID is 5
An important note:
The device naming convention may change as VMS is now
supporting a lot more similar devices simultaneously.
Foreign and structured tapes and mounting modes
-----------------------------------------------
VMS supports the ANSI standard for tapes, and provides an extension
to that standard. Tapes initialized on VMS contain some more data
structures that keep security and structure information.
Using the additional data structures it wrote on the tape, VMS let
you use the tape just like an ordinary disk, albeit very slow.
A tape without the additional VMS data structures is ANSI conforming,
in VMS terminology it's called a 'foreign' tape, and you can mount
it in 'foreign' mode with the MOUNT/FOREIGN command.
Tapes written on other operating systems are usually ANSI conforming
and can be read in foreign mode.
A tape initialized by VMS (contains the additional data structures),
can be mounted either in foreign mode using MOUNT/FOREIGN, or in
'structured mode' without the /FOREIGN qualifier.
The following table summarizes the practical aspects of the foreign
and structured mounting modes:
FOREIGN MODE STRUCTURED VMS MODE
-------------------------- --------------------------------
You don't have to know the You need the label (if you don't
volume label to mount it. know it, mount the tape /FOREIGN
then VMS will tell you the label,
dismount and mount again!)
You can't do a DIR, and can You can do a DIR with all switches,
COPY only to another foreign Unrestricted COPY
mounted tape.
Can use BACKUP Cannot use BACKUP
Can use DUMP with only the Can use DUMP with complete file
drive name specifications (and wildcards)
SET MAGTAPE works and you SET MAGTAPE doesn't work, but you
can change drive properties don't need it either!
and do some tape operations
Relevant commands
-----------------
*ALLOCATE drive-name
*INITIALIZE drive-name volume-label
MOUNT drive-name volume-label logical-drive
/NOUNLOAD
*/FOREIGN
/NOASSIST
/NOMOUNT
SET MAGTAPE drive-name
/DENSITY
/END_OF_FILE
/MEDIA_FORMAT
/RETENSION
/REWIND
/SKIP
/UNLOAD
* doesn't require knowing the tape label
Common error messages related to tapes
--------------------------------------
VOLUME IS WRITE LOCKED The safety switch on the cartridge is in
the wrong position, and the cartridge is
now write protected.
MEDIUM IS OFF LINE The drive door is open(?).
DRIVE IS OFF LINE Something happened to the tape drive.
Reading and writing
-------------------
Using tapes on UNIX
-------------------
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