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
-------------------
A much more important factor in the social movement than those already mentioned was the ever-increasing influence of women. This probably stood at the lowest point to which it has ever fallen, during the classic age of Greek life and thought. In the history of Thucydides, so far as it forms a connected series of events, four times only during a period of nearly seventy years does a woman cross the scene. In each instance her apparition only lasts for a moment. In three of the four instances she is a queen or a princess, and belongs either to the half-barbarous kingdoms of northern Hellas or to wholly barbarous Thrace. In the one remaining instance208— that of the woman who helps some of the trapped Thebans to make their escape from Plataea—while her deed of mercy will live for ever, her name is for ever lost.319 But no sooner did philosophy abandon physics for ethics and religion than the importance of those subjects to women was perceived, first by Socrates, and after him by Xenophon and Plato. Women are said to have attended Plato’s lectures disguised as men. Women formed part of the circle which gathered round Epicurus in his suburban retreat. Others aspired not only to learn but to teach. Arêtê, the daughter of Aristippus, handed on the Cyrenaic doctrine to her son, the younger Aristippus. Hipparchia, the wife of Crates the Cynic, earned a place among the representatives of his school. But all these were exceptions; some of them belonged to the class of Hetaerae; and philosophy, although it might address itself to them, remained unaffected by their influence. The case was widely different in Rome, where women were far more highly honoured than in Greece;320 and even if the prominent part assigned to them in the legendary history of the city be a proof, among others, of its untrustworthiness, still that such stories should be thought worth inventing and preserving is an indirect proof of the extent to which feminine influence prevailed. With the loss of political liberty, their importance, as always happens at such a conjuncture, was considerably increased. Under a personal government there is far more scope for intrigue than where law is king; and as intriguers women are at least the209 equals of men. Moreover, they profited fully by the levelling tendencies of the age. One great service of the imperial jurisconsults was to remove some of the disabilities under which women formerly suffered. According to the old law, they were placed under male guardianship through their whole life, but this restraint was first reduced to a legal fiction by compelling the guardian to do what they wished, and at last it was entirely abolished. Their powers both of inheritance and bequest were extended; they frequently possessed immense wealth; and their wealth was sometimes expended for purposes of public munificence. Their social freedom seems to have been unlimited, and they formed combinations among themselves which probably served to increase their general influence.321 The old religions of Greece and Italy were essentially oracular. While inculcating the existence of supernatural beings, and prescribing the modes according to which such beings were to be worshipped, they paid most attention to the interpretation of the signs by which either future events in general, or the consequences of particular actions, were supposed to be divinely revealed. Of these intimations, some were given to the whole world, so that he who ran might read, others were reserved for certain favoured localities, and only communicated through the appointed ministers of the god. The Delphic oracle in particular enjoyed an enormous reputation both among Greeks and barbarians for guidance afforded under the latter conditions; and during a considerable period it may even be said to have directed the course of Hellenic civilisation. It was also under this form that supernatural religion suffered most injury from the great intellectual movement which followed the Persian wars. Men who had learned to study the constant sequences of Nature for themselves, and to shape their conduct according to fixed principles of prudence or of justice, either thought it irreverent to trouble the god about questions on which they were competent to form an opinion for themselves, or did not choose to place a well-considered scheme at the mercy of his possibly interested responses. That such a revolution occurred about the middle of the fifth century B.C., seems proved by the great change of tone in reference to this subject which one perceives on passing from Aeschylus to Sophocles. That anyone should question the veracity of an oracle is a supposition which never crosses the mind of the elder dramatist. A knowledge of augury counts among the greatest benefits222 conferred by Prometheus on mankind, and the Titan brings Zeus himself to terms by his acquaintance with the secrets of destiny. Sophocles, on the other hand, evidently has to deal with a sceptical generation, despising prophecies and needing to be warned of the fearful consequences brought about by neglecting their injunctions. The stranger had a pleasant, round face, with eyes that twinkled in spite of the creases around them that showed worry. No wonder he was worried, Sandy thought: having deserted the craft they had foiled in its attempt to get the gems, the man had returned from some short foray to discover his craft replaced by another. “Thanks,” Dick retorted, without smiling. When they reached him, in the dying glow of the flashlight Dick trained on a body lying in a heap, they identified the man who had been warned by his gypsy fortune teller to “look out for a hidden enemy.” He was lying at full length in the mould and leaves. "But that is sport," she answered carelessly. On the retirement of Townshend, Walpole reigned supreme and without a rival in the Cabinet. Henry Pelham was made Secretary at War; Compton Earl of Wilmington Privy Seal. He left foreign affairs chiefly to Stanhope, now Lord Harrington, and to the Duke of Newcastle, impressing on them by all means to avoid quarrels with foreign Powers, and maintain the blessings of peace. With all the faults of Walpole, this was the praise of his political system, which system, on the meeting of Parliament in the spring of 1731, was violently attacked by Wyndham and Pulteney, on the plea that we were making ruinous treaties, and sacrificing British interests, in order to benefit Hanover, the eternal millstone round the neck of England. Pulteney and Bolingbroke carried the same attack into the pages of The Craftsman, but they failed to move Walpole, or to shake his power. The English Government, instead of treating Wilkes with a dignified indifference, was weak enough to show how deeply it was touched by him, dismissed him from his commission of Colonel of the Buckinghamshire Militia, and treated Lord Temple as an abettor of his, by depriving him of the Lord-Lieutenancy of the same county, and striking his name from the list of Privy Councillors, giving the Lord-Lieutenancy to Dashwood, now Lord Le Despencer. "I tell you what I'll do," said the Deacon, after a little consideration. "I feel as if both Si and you kin stand a little more'n you had yesterday. I'll cook two to-day. We'll send a big cupful over to Capt. McGillicuddy. That'll leave us two for to-morrer. After that we'll have to trust to Providence." "Indeed you won't," said the Surgeon decisively. "You'll go straight home, and stay there until you are well. You won't be fit for duty for at least a month yet, if then. If you went out into camp now you would have a relapse, and be dead inside of a week. The country between here and Chattanooga is dotted with the graves of men who have been sent back to the front too soon." "Adone do wud that—though you sound more as if you wur in a black temper wud me than as if you pitied me." "Wot about this gal he's married?" "Don't come any further." "Davy, it 'ud be cruel of us to go and leave him." "Insolent priest!" interrupted De Boteler, "do you dare to justify what you have done? Now, by my faith, if you had with proper humility acknowledged your fault and sued for pardon—pardon you should have had. But now, you leave this castle instantly. I will teach you that De Boteler will yet be master of his own house, and his own vassals. And here I swear (and the baron of Sudley uttered an imprecation) that, for your meddling knavery, no priest or monk shall ever again abide here. If the varlets want to shrieve, they can go to the Abbey; and if they want to hear mass, a priest can come from Winchcombe. But never shall another of your meddling fraternity abide at Sudley while Roland de Boteler is its lord." "My lord," said Edith, in her defence, "this woman has sworn falsely. The medicine I gave was a sovereign remedy, if given as I ordered. Ten drops would have saved the child's life; but the contents of the phial destroyed it. The words I uttered were prayers for the life of the child. My children, and all who know me, can bear witness that I have a custom of asking His blessing upon all I take in hand. I raised my eyes towards heaven, and muttered words; but, my lord, they were words of prayer—and I looked up as I prayed, to the footstool of the Lord. But it is in vain to contend: the malice of the wicked will triumph, and Edith Holgrave, who even in thought never harmed one of God's creatures, must be sacrificed to cover the guilt, or hide the thoughtlessness of another." "Aye, Sir Treasurer, thou hast reason to sink thy head! Thy odious poll-tax has mingled vengeance—nay, blood—with the cry of the bond." HoME古一级毛片免费观看
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