7.6 Magnetic Tape

Magnetic tape is the oldest and most cost-effective of all mass-storage devices. First-generation magnetic tapes were made of the same material used by analog tape recorders. A cellulose-acetate film one-half inch wide (1.25 cm) was coated on one side with a magnetic oxide. Twelve hundred feet of this material was wound onto a reel, which then could be hand-threaded on a tape drive. These tape drives were approximately the size of a small refrigerator. Early tapes had capacities under 11MB, and required nearly a half hour to read or write the entire reel.

Data was written across the tape one byte at a time, creating one track for each bit. An additional track was added for parity, making the tape nine tracks wide, as shown in Figure 7.17. Nine-track tape used phase modulation coding with odd parity. The parity was odd to ensure that at least one "opposite" flux transition took place during long runs of zeros (nulls), characteristic of database records.

The evolution of tape technology over the years has been remarkable, with manufacturers constantly packing more bytes onto each linear inch of tape. Higher density tapes are not only more economical to purchase and store, but they also allow backups to be made more quickly. This means that if a system must be taken offline while its files are being copied, downtime is reduced. Further economies can be realized when data is compressed before being written to the tape. (See Section 7.8.)

The price paid for all of these innovative tape technologies is that a plethora of standards and proprietary techniques have emerged. Cartridges of various sizes and capacities have replaced nine-track open-reel tapes. Thin film coatings similar to those found on digital recording tape have replaced oxide coatings. Tapes support various track densities and employ serpentine or helical scan recording methods.

Serpentine recording methods place bits on the tape in series. Instead of the bytes being perpendicular to the edges of the tape, as in the nine-track format, they are written "lengthwise," with each byte aligning in parallel with the edge of the tape. A stream of data is written along the length of the tape until the end is reached, then the tape reverses and the next track is written beneath the first one (see Figure 7.18). This process continues until the track capacity of the tape has been reached. Digital linear tape (DLT) and Quarter Inch Cartridge (QIC) systems use serpentine recording with 50 or more tracks per tape.

Digital audio tape (DAT) and 8mm tape systems use helical scan recording. In other recording systems, the tape passes straight across a fixed magnetic head in a manner similar to a tape recorder. DAT systems pass tape over a tilted rotating drum (capstan), which has two read heads and two write heads, as shown in Figure 7.19. (During write operations, the read heads verify the integrity of the data just after it has been written.) The capstan spins at 2,000 RPM in the direction opposite of the motion of the tape. (This configuration is similar to the mechanism used by VCRs.) The two read/write head assemblies write data at 40-degree angles to one another. Data written by the two heads overlaps, thus increasing the recording density. Helical scan systems tend to be slower, and the tapes are subject to more wear than serpentine systems with their simpler tape paths.

Tape storage has been a staple of mainframe environments from the beginning. Tapes appear to offer "infinite" storage at bargain prices. They continue to be the primary medium for making file and system backups on large systems. Although the medium itself is inexpensive, cataloging and handling costs can be substantial, especially when the tape library consists of thousands of tape volumes. Recognizing this problem, several vendors have produced a variety of robotic devices that can catalog, fetch, and load tapes in seconds. Robotic tape libraries, also known as tape silos, can be found in many large data centers. The largest robotic tape library systems have capacities in the hundreds of terabytes and can load a cartridge at user request in less than half a minute.