Tape drive
A tape drive, or streamer, is a device used to store data on magnetic tapes . In the early days of computer technology in the 1950s and 1960s, the large tape stations, each with two tape reels mounted vertically next to each other, epitomized the look of the computer itself, at least in the world of caricatures and commercial graphics, but also in many feature films and reports.
The datasette was a widely used drive for home computers during an interim period in the 1970s and 1980s. Today's tape drives are designed for professional users and special types of magnetic tape cartridges and are generally used for regular data backup of medium to large amounts of data.
Working principle
Like cassette recorders , tape drives have a drive for two reels, a record / playback head, and an erase head . The data are written or read serially. The available storage capacity does not only depend on the tape length. It also depends on the standard used ( AIT , QIC , DAT , DCC , DDS , SLR , DLT , FSK , LTO , VXA , etc.) and extends into the petabyte range. Cases with only one tape reel are called a cartridge (as opposed to a cassette, which has two reels).
Recording types
- Linear recording
- The magnetic tape is written on in the forwards and backwards direction with several longitudinal tracks lying next to one another.
- Helical scan recording
- The magnetic tape runs diagonally past the (rotating) write head.
- Degressive recording
- The magnetic tape is initially written to very quickly, later the recording takes longer.
- Multi-track recording
- The wide magnetic tape was written (like the punched tape ) with several parallel and simultaneously generated data tracks, for example with 7 or 11 tracks. This led to a large number of formats, which in retrospect make it very difficult to read older tapes correctly.
Writing methods
There are basically two methods of writing data to a magnetic tape, the start-stop method and the streaming method (streaming mode).
Start-stop procedure
With the start-stop procedure, individual data blocks are always written to the tape. Because the tape has to run past the read / write head at a certain minimum speed when writing, the tape must first be accelerated for each write process (start phase). The tape is stopped again after writing. Because of these start and stop phases, empty areas must be provided on the magnetic tape between the data records. These are called fissures or interblock gaps in technical terms .
This procedure is only used very rarely.
Blocking
A written magnetic tape looks like this:
X1..........X2..........X3..........X4..........
where X1 to X4 are the user data and "...." the gaps.
This shows that a written tape consists to a large extent of unused gaps and only a small part of user data. The data can be blocked to increase the amount of user data. To do this, several (short) logical data records in the main memory of the computer are combined into a (long) physical data record and this is written on the tape. This has several advantages: The tape is better used because the amount of useful data increases. In addition, the tape unit and the magnetic tape are spared because there are fewer write accesses and the considerable mechanical stress caused by starting and stopping is reduced. The length of the original data records is called the logical record length , the length of the blocked record is called the physical record length and the number of logical records in a physical record is called the blocking factor .
Example: If you want to transfer the data from punch cards to tape, you have a logical record length of 80 characters. If you block 100 punch cards to a physical record, the physical record length is 8000 characters and the blocking factor is 100. It is therefore usually advantageous to choose the largest possible blocking factor. Modern devices (DLT, LTO etc.) work with a block size of 64 kilobytes and more.
redundancy
To protect against write / read errors, there is also the u. a. Redundancy method used by Unisys, in which the data is written something like this:
X1....X1....X2....X2....X3....X3....X4....X4....
where X1 to X4 are the user data and "...." the gaps.
This double or possibly multiple writing of the same blocks one after the other makes it possible, in the event of worn areas or even tearing of the tape, to effectively repair the area with a simple adhesive strip without suffering data loss.
Streaming mode
Streaming mode is the most modern recording method. In streaming mode, the tape is written to continuously (without stopping). The prerequisite is that the data is delivered to the magnetic tape unit quickly enough (for example from a buffer memory). This procedure is much faster than the start-stop procedure, tape and tape device are protected. This also creates fewer or no empty gaps. If it has to be stopped anyway, so-called filemarks are written on the tape. With modern DLT and S-DLT drives, for example, restarting after a stop is almost lossless, but when restarting the tape must be rewound a bit. This start-stop operation (also known as the Shoeshine effect ) not only leads to a drastic drop in the write rate, but also to greatly increased wear and tear on the magnetic tape (the frequent stops stretch it more) and the drive. It is therefore important to ensure that the data buffer is well-filled, especially with modern drives with their high data transfer rates.
The streaming method was also used in the older tape stations from IBM, UNISYS, Siemens etc. by means of vacuum shafts. With these, only the unwinding and receiving rolls are constantly started and stopped, but the writing / reading speed at the head is kept the same thanks to the vacuum shafts. This also apparently leads to the shoeshine effect.
Designs
Tape drives for installation in computers have a front panel with an eject button and some LEDs that indicate the status. They are installed in 5¼ "bays. In addition to the usual drives with the full height (3½") of a bay, drives with half the height (1¾ ") are also available, which are no larger than an optical drive.
Tape drives with their own housings also have their own power supply unit. They can be used when there is no installation space available in the computer. In addition, they can be put into operation more easily on other computers.
Tape drives for installation in tape libraries are matched to their type and, as a rule, cannot be installed in other types.
