The oldest and simplest form of tape data recording is the linear format, which records a set of parallel tracks along the full length of the media. The original half inch tape media used a set of 9 parallel tracks to store the data in one direction, 8 for the data and one for simple error correction.
Since then the density of magnetic storage has improved immensely, allowing LTO6 media to record 2176 parallel tracks the full length of the tape, by moving the heads across the tape. For the most efficient recording in the linear tape format, it is best for the media to run continuously, which in modern tape drives is achieved by variable motor speeds.
The recording format is commonly called serpentine, although in reality the tracks are recorded in an outward spiral. Track zero is recorded along the centre of the media, with successive tracks recorded either side, depending upon the direction the tape is moving.
With such high density recordings, it is important that the exact location of each track can be quickly identified. This is achieved by using 5 servo tracks, which are used to determine and ensure that the tape head is perfectly aligned during reading and writing. These are also used to give the precise location of the tape along its length.
Recorded Block Locations
On DLT cartridges and early LTO tapes without a memory cartridge, they recorded the positions of the data blocks at intervals along each track, which was stored at the front of the tape. Since then LTO cartridges store all the information about block positions on the in-cartridge memory chip.
This allows data blocks to be located at high speed without the need to run the tape the full length of the media for each track. This is important during restoration of files by backup software to provide quick access to any given data block.
Additional Data Stored
In addition to the location of the end-of-data, and the block locations on the tape, the following information is also stored:
- Age of the cartridge
- Number of times cartridge has been loaded
- Count of errors encountered
- Calibration information
- Initialisation data