Magnetic storage devices are often included in computer systems to provide high capacity secondary storage or data archival. Magnetic tape systems are a type of magnetic storage. Magnetic tape systems are widely used because they provide high storage densities while employing low cost media.
A magnetic tape system generally comprises a recording medium, i.e., magnetic tape, and a medium access device known as a tape drive or tape transport that reads data from and writes data to the magnetic tape. A magnetic tape typically comprises a flexible narrow ribbon of a base material (e.g., polyester films such as polyethylene naphthalate or polyethylene tereohthalate), and a magnetic material, such as a metal particulate, affixed to the base material by a binding agent (e.g., vinyl chloride polymer). The tape drive accesses the magnetic tape as the tape passes over a set of transducers or sensor elements affixed to a head assembly. Write elements generate magnetic fields which encode data onto the tape as the tape passes over the head. Read elements sense the magnetic fields of the tape's magnetic material to read data from the tape.
Linear tape systems employ multiple data tracks that run parallel to one another over a length of the tape. Such tapes often include more data tracks than the tape drive has head elements. These systems use a serpentine recording method wherein a first set of data tracks is written as the tape moves across the head in a forward direction, and a second set of data tracks is written as the tape moves across the head in the reverse direction. Many direction reversals may be required to write a tape with hundreds or thousands of tracks.
The magnetic tape used in linear tape systems, for example the Linear Tape Open (“LTO”) tape system, includes a servo track written onto the tape at manufacture. The tape heads are positioned to read and/or write the tape's data tracks by aligning the tape heads in accordance with the servo track. The servo track also contains encoded longitudinal position information used to determine the longitudinal position (i.e., the position along the length) of the tape relative to the tape head. Accurate determination of longitudinal tape position in the presence of tape defects and various other inhibitions to servo track access is desirable.