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This is the 4th part of this series and now we'll finally start looking at how all of this is put together. First we'll examine the QIC tape format, and in I'll explain other types in later parts.
This section will discuss the QIC method - and it's probably a good starting point as it's fairly easy to understand.
The QIC [Quarter Inch Cartridge] is the data cartridge you have seen which measures about 3" by 5", and with a metal bottom plate.
This is a very rugged design and it's more complex mechanically than you'd notice with a casual glance.
This style of backup media has grown from the early days of 20MB per tape - which took between 30 minutes and one hour on the machines I frist saw them running upon - to the current QIC style cartridges which hold several Gigabytes.
The similarity to an audio cassette is that there is one reel the tape feeds from and another reel for take-up. And at that point - the similarity just about ends.
The aluminum bottom plate is used to give dimensional stability, as the cartridge itself guides the tape, while in a cassette machine there are tape guides in the transport but not in the cartridge.
If you have a QIC cartridge you might examine it while I go through most of the basics of the cartridge.
Holding the cartridge metal side down and facing the opening you will see two openings in the plastic. On the right side you will see a round object. Looking at this object from the the top of the cartridge you will see an arrow and a slot. You can turn this by using a coin in the slot, and the round tube will rotate to reveal an opening on the edge of the cartridge. This mates with a sensor in the tape transport and when it is open, the tape will not be able to be written upon.
If someone wished to distribute master tapes that could never be written upon you could remove this - though I have never seen it done. This is a fail-safe type design.
In the center you can see an opening that is only goes down half way. Behind this there is a rubber wheel - which is 'stepped'. The top part of this being of a larger dimension than the bottom. The top part of this wheel protrudes through the opening.
You'll also notice that you can see only part of the magnetic tape and that it lies in a slot below the protruding part of the wheel.
This is another major area where data tapes diverge from the audio format you are used to. The capstan [in a cassette the metal shaft but in the QIC transport a belt drive shaft with a rubber wheel] drives this rubber idler - but never touches the tape. In the audio world the capstan pinches the tape between it and the idler.
Now move this wheel and note that the tape reels move in a direction opposite that of the wheel movement. Don't move the wheel so that the right spool moves clockwise too far or you'll take the tape right off the end of the spool. It is not fastened down. Move it as far in the other direction as you wish. Don't worry about getting it back to where it was, inserting it into the drive will set it back correctly automatically.
The tape is being moved by a thin flexible ribbon. It is this ribbon you see - about 1/8" of an inch wide when you look at the back of the cartridge - and what you see going to the top of the feed reel on the left. At first glance you may mistake this for the tape in the cartridge as it is more visible.
This belt acts much like a conveyor belt as it supplies the movement and supports the tape through its entire path through the cartridge except for the portion crossing the head and for part way up the take up spool just past the head.
The reason for transport belt is guidance. The tape in current machines moves well over 100" per second, or almost 6 miles per hour [or about 9.5 kilometers/hour for the majority of the world]. Just imagine how hard it would be too keep this evenly spooled without the belt. No home reel-reel audio tape recorder moves that fast in home units in fast-forward or rewind. If you've ever had to hand-wind something onto a spool you can appreciate this. [Brings back memories of hand-winding 1200 feet of unexposed motion picture film in a darkroom after dropping a loop while loading a magazine.]
The other guidance in the cartridge is from two tape guides. The moving white plastic wheels in the corner are for the belt. As far as the guides go, you can see one between the write-protect roller and the pinch-roller/idler, while the other is at the left side of the tape cartridge. To see the left guide, just pull gently on the plastic door to the left of the pinch-roller. This exposes the tape and you can see the post to the left.
These two metal posts ensure that the tape crosses this area at the EXACT height above the metal base-plate in very cartridge made. All the guidance and and most of the tolerances are in the cartridge, and the transport is fairly simple in comparison. This measurement from the base-plate is similar to the measurement from the top-plate in professional audio recorders. It is a stable platform of reference.
The tape also has some small holes punched at either end of the tape. If you look at the cartridge from the top between the write-protect device and the pinch-roller, you will see a small clear window and by moving the cartridge around in the light you will see a small mirror at about a 45 degree angle.
There is a light and a sensor in the transport so that when a hole in the tape passes this device, light goes through the hole and is reflected back and signals the transport that it has reached the end of the tape and to stop.
That covers the cartridge in more detail than you had probably guessed, now let us examine the transport.
When you place the cartridge in the drive - even if was not fully rewound - such as might in a power off situation and the tape is removed before the power is applied - the tape will rewind until it sees the end-of-tape holes and then will set itself for proper operation.
When you place the tape in the drive - the resistance you feel - at least in the older units - is from the ejection spring and at the last push the plastic door you opened above is opened.
On the older units you then slide the tape door lock shut, which then moves the tape head - including the erase head - up against the tape. These mechanism are basically quite simple, which makes them quite reliable. The head pushes the tape in by a very small amount, and only the tension of the tape keeps it against the head. There are no pressure pads such as you would find in an audio cassette recorder.
So now that the tape is loaded we are ready to go.
You start your favorite backup program. The more sophisticated programs read a label they have previously placed on the tape so that it can warn you that you are writing over last night's backup for example. They can also keep track of the count of times the tape has been used so you don't exceed the maximum recommend use. This is typically a conservative figure, but a new tape is far cheaper than paying help to recreate data lost because of scrimping on a $20 tape.
Once the tape starts moving, the erase head, which is in the same housing as the read/write head, is sent current. The current causes the FULL WIDTH of the tape to be erased.. At this time the data flows from the computer to the transport electronics until the tape sees the holes as the end of the tape.
At this time we stop the erase head. The mechanism holding the record head is physically moved down, the tape then reverses direction and the data is written on the next position on the tape.
For each direction reversal the tape moves again so that if you drew the data pattern on the tape on a piece of paper it would go from one end to the other, move down and go back, and repeat, in a serpentine fashion.
You should hear the tape stop and restart only as many times as there are tracks on the tape. Any more than this indicated you aren't sending the data to the tape drive fast enough, and that the tape has to wait for more data. This will increase the time it takes to backup a system dramatically. Each time it stops, it then has to go back an reposition itself at the correct position so that the data stream appears to be continuous when you read it back.
This used to be called 'shoe-shining' but lately I've heard the term 'back-hitching' used to describe this. Properly matching the computer, hard drives, tape controller and tape device is required to avoid this. A fully streaming tape that only stops at the end usually surprises those who have only been exposed to backups in the Microsoft environment for example.
The first QIC tape drives of which I used had only 4 data tracks. So the tape would move across the heads four times - two in each direction.
The multiple tracks coupled with the full-track erase is why it is impossible - for an ordinary user - to recover any tape which has been partially written upon by mistake.
The full-track erase will erase the first part of tracks 1 and 3 and the tail end of tracks 2 and 4. So at least part of the data is damaged on all of the tape. In addition, when you accidentally start a write - typing 'tar cvf' when you meant 'tar tvf' or 'tar xvf', the process of stopping the tape and rewinding puts a tape marker at the end of the data, and the firmware of the tape will prevent you from going past that area.
The only way you can write data on a 1/4" tape is starting from the first of a tape - if the tape has data as in the disaster situation above, or skipping past all the file marks to blank tape. Multiple files can be written to tape using the no-rewind feature.
If you accidentally write to a tape you are trying to restore , then your only real resort is a data recovery firm. ALWAYS write-protect tape before you attempt to restore any critical data.
The first tapes had a capacity of about 20MB. To increase this capacity to the point where we now have multi-gigabyte drive capacity, and going from backing up at under 1MB minute to hundreds of megabytes per minute we used evolutionary [as opposed to revolutionary] techniques.
As mechanical manufacturing precision became better, and sophisticated designs became practical we were able to make the tracks on the tape narrower. The first 4 tracks on quarter inch tape were about the same width as the 9-track on 1/2 tape used in the larger computers.
Once we could make the tracks narrower, we now could move the tape head in a smaller vertical increments each time we changed tape direction. I have not used any of the gigabyte tapes and have not looked up the track density - but on the QIC formats I've worked with the number of tracks was about 30 the last time I checked.
Another step is to improve the electronics so we can switch the data on and off faster. At this point we can move the tape faster and increase the speed we write to tape, and thus reduce the amount of time it takes to write it. Since we have not changed tape head nor tape formulation each bit still occupies the same space, but we just put it there faster.
Using the 5MB track capacity of the first 20 MB drives above - a 30 track tape gives you 150M-byte tape capacity by that alone. Faster write means we back up in 1/2 the time we did originally. So this is just a mechanical change and a data rate change.
The next step is to change the basic formulation of the tape. To increase the density we have to have magnetic domains that are smaller. This means we also have to change the tape head design. That in turn means we have to redesign the electronics, as the higher density tapes are higher coercivity and need more current to be able to write the data.
If, in this example, we make the domains 1/2 the size as they were above and make the tape head gaps 1/2 the size, we can now double the data we have on the tape.
However - to make use of this we need to change one of two things. Either we change the speed of the tape by making it move one half the previous speed - or the smart thing is to redesign the interface so we push the data to the tape twice as fast. Since the data bits are smaller - by keeping the tape speed the same - we can double the amount of data sent and written to the tape in the same time frame.
As Ted Nelson likes to say "Everything is intertwingled".
One by one the QIC style tapes are disappearing, and the only major manufacturer I'm aware of in that market is Tandberg. Smaller systems moved to DAT/DDS and larger systems to DLT. I'll look at those next in this series.
As data storage increases the need to make timely removable off-site storage becomes more critical. What was once an easy overnight backup/verify, can now consume all the available spare time.
A friend of mine who worked at an engineering firm backed up everything every night, until the 7PM start of backup didn't end until 7AM when the employees started arriving. It will only get worse.
I noted in a report from Sandia Labs [www.sandia.gov] that in an article on sorting speed, they commented that at that time of the report a typical data storage had about 250GB in use and predicted that it to be at 6.5 terabytes by 2003. It's beginning to look like those guesses might be conservative.
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More Articles by Bill Vermillion © 2010-10-29 Bill Vermillion
The people I distrust most are those who want to improve our lives but have only one course of action in mind. (Frank Herbert)