Magnetic tape

a magnetic tape is a storage medium. During the similar storage of audio material it also tape is called. It consists nowadays of a long, narrow plastic foil, which is coated with a magnetizable material. Frequently also the back is coated (back matt finish),in order to make cantilever tape windings possible. In former times there was paper and volume from homogeneous magnetic material, also coated. The today's magnetic tape is a development of the AEG and the industrial union colors (BASF) in the years 1935 to 1940.

Magnetic tape

the volume is on coils or Bobbys rolled up, whereby both are built into cartridges often.

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Applications

for write methods with application as storage medium for digital

2 in principle procedures give data to write data on a magnetic tape the asynchronous operation procedure and the Streaming procedure (streaming mode).

asynchronous operation procedures

with the asynchronous operation procedure are written always individual data blocks on the volume. Because during the writing the volume with oneif minimum speed determined run by the write/read head must, is the volume for each recording procedure first to accelerate (starting phase). After the letter the volume is again stopped.

Because of these starting and stop phases empty ranges must to be planned on like-relieved between the data records.These are called in the technical language of gaps or lnterblock Gap.

This procedure is used only very rarely.

blocking

a described magnetic tape looks therefore in such a way:


       XX .......... XX .......... XX .......... XX ..........
       XX are the utilizable data.
       .......... are the gaps.


By it one recognizes,that a described volume consists to a large part of unused gaps and only to a small part of utilizable data.

In order to increase the portion of the utilizable data, one can block the data. In addition one seizes several (short) logical data records inMain storage of the computer to (are enough) a physical data record together and writes these on the volume. This has several advantages: The volume is better used, because the utilizable data portion increases. In addition the tape unit and the magnetic tape are preserved, because fewer write accessesand so the substantial mechanical load via starting and stopping take place are reduced.

One calls the length of the original data records logical record length, the length of the blocked sentence physical record length and the number of logical sentences in a physicalSentence blocking factor.

Example: If one wants to transmit the data from punch cards to a volume, then one has a logical record length of 80 indications. If one blocks 100 punch cards to a physical sentence, then the physical record length 8000 indications and that amount toBlocking factor is 100.

It is thus usually favourable to select as large a blocking factor as possible. Modern devices (DLT, LTO etc.) work with a block size of 64 kilobyte and more.

Streaming mode

of the Streaming mode is the most modern recording technique.With the Streaming mode is described the volume continuously to continue (without). A condition is that the data are supplied fast enough (for example from a buffer memory) to the magnetic tape unit. This procedure is substantially faster than the asynchronous operation procedure, volume and tape-unit is preserved.Also one needs no Gap. Must be continued nevertheless, so-called Filemarks is written on the volume. Remounting after a stop is for example loss-free with the modern DLT and S-DLT drive assemblies almost.

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