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Linear Tape Technology

'Linear' technology is the simplest of tape technologies. It writes data in long tracks down a tape. Its been said that linear tapes are 12mm wide, and a kilometer long.
A simple, old style linear tape wrote 9 tracks straight up a tape, 8 tracks were for data, and the 9th was for simple error correction. Modern technology gets high capacity by writing up and down the tape several times. For example, the LTO7 format will record up to 3584 tracks across a 1/2 inch tape. It can write in either direction, so it writes tracks down the tape, reverses, and writes back up again. It also has embedded 'servo tracks' to allow fast location of specific data on a tape. This is illustrated by the diagram below. The tape head assembly has up to 34 head elements. It always has 2 read elements for the servo tracks and up to 32 data read/write head elements. These heads can read or write up to 32 tracks in a single pass up the tape, and each end-to-end pass is called a 'wrap'. The head assembly is then moved to the side to process the next wrap down the tape. An LT07 cartridge will need 112 wraps to access the whole tape.

ultrium tape format

There are lots of different implementations of linear tape, including Linear Tape Open (LTO), Digital Linear Tape (DLT), and TS1140 Enterprise tapes.

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As more and more sets of tracks are created on a tape, its critical that the tape heads are always correctly positioned over the correct set of tracks. The combination of high track density and side to side tape movement makes this difficult, so modern tapes use some sort of timing based servo.This makes sure the read/write heads are always positioned precisely over the correct tracks. Timing based servo uses Servo tracks or bands to position the tape heads. On the LTO Ultrium tape there are five servo bands, numbered 0 through 4, which make up the servo tracking mechanism. They are each located at specific distances from the tape reference edge.

The tracking system uses two servo bands simultaneously to provide two sources of reference for increased accuracy. This technology will allow increase in capacity, because it uses relative positions between servo bands, rather than fixed track size. The servo band also records longitudinal position (LPOS). The absolute location down the length of the tape and the manufacturer data are recorded in LPOS 'words', approximately every quarter of an inch (.7cm) along the tape. The LPOS word consists of symbols constructed from bit sequences (ones and zeros); these bits are encoded within the servo frames.

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Modern Linear tape also contains information about the cartridge and the tape. An Ultrium tape has a piece of updatable 16KB memory housed inside the cartridge casing. It holds information such as

  • Data and block locations for files
  • The end-of-data location to allow the drive to use the fast locate function to move directly to the recording area.
  • The age of the cartridge
  • How many times it has been loaded
  • How many errors have occurred
  • Calibration information
  • Initialisation information

This data is stored with the cartridge itself, so it can be retrieved wherever the cartridge is mounted. This has advantages over recording the data in a tape management system. The LTO-CM can be read by an external reader without having to insert the cartridge into a drive.

LTO2 tapes need to be handled carefully as they are a bit fragile. If a cartridge is dropped from even a small distance it can break if it lands on the corner where the tape comes out. At the point where the leader pin slips in the plastic casing thins and if dropped it can bend. This causes the cart to widen slightly, enough to prevent the cart from loading, or if the damage is bad enough, the pin may even fall out of place.

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