Tape library

In computer storage, a tape library is a physical area that holds magnetic data tapes. In an earlier era, tape libraries were maintained by people known as tape librarians and computer operators and the proper operation of the library was crucial to the running of batch processing jobs. Although tape libraries of this era were not automated, the use of tape management system software could assist in running them.

Subsequently, tape libraries became physically automated, and as such are sometimes called a tape silo, tape robot, or tape jukebox. These are a storage devices that contain one or more tape drives, a number of slots to hold tape cartridges, a barcode reader to identify tape cartridges, and an automated method for loading tapes (a robot). Such solutions are mostly used for backups and for digital archiving. Additionally, the area where tapes that are not currently in a silo are stored is also called a tape library. One of the earliest examples was the IBM 3850 Mass Storage System (MSS), announced in 1974.

In either era, tape libraries can contain millions of tapes.

In the mainframe computer era, especially the IBM mainframe, the most common format in use was the 9-track tape. Some large application systems could require scores of different tapes as part of their batch job runs.

In the data processing applications of the era, the master files for such things as employee payroll information, supplies and stores inventory, or customer accounts were typically kept on tape. Batch jobs to update these master files would take the existing tape master file as input and write out a new tape master file as output. In addition, the set of update transactions themselves might constitute a second input tape. The master file output of one update job would then be the master file input to the next time the job is run, perhaps a day, a week, or a month later. The tapes representing a few past iterations of a master file would typically be retained, in case a problem with the latest version were to be discovered and the job had to be rerun.

Mainframe computer installations often had a separate room, the tape library, to house their racks and cabinets of tapes. The typical workflow for running a batch job was to go into the library, pull certain tapes off the racks there and load them onto a rolling cart, move the cart into the computer area, mount the tapes onto tape drives for a production run, take the tapes off the drives when the run was over, move the cart back to the library, and put the tapes back on the library racks. Such tape libraries existed at most computer installations.

Even a modestly sized computer installation could have hundreds of tapes, and library sizes of several thousand reels of tapes were commonplace. And they could be much larger: by the mid-1970s, the U.S. Census Bureau and NASA each had tape libraries with around one million tape reels in them. The person in charge of all this was typically called the tape librarian.

In this era, there were no automated tape delivery and mounting systems, and so this action had to be done by computer operators. These people were the ones responsible for mounting tapes onto tape drives as part of running a job. Even careful computer operators could sometimes mount the wrong tape as input to a job or present the reels of a multi-tape dataset out of order. Overwriting a tape that was meant to be preserved was another potential mistake.