Open Reel Tapes
When IBM needed to develop a data storage system they decided to use the ferrous-oxide coated half inch magnetic tape similar to that used in recording studios. The first reels were 10.5” (27cm) in diameter, quickly becoming the de facto standard within the industry for use with mainframe systems. Although tape cassettes and tape cartridges capable of recording computer data were introduced during the 1970s, for mini-computer systems, half inch open reels were in common usage well into the 1980s.
Despite IBM introducing the 3480 data cartridge in 1984, capable of storing more data and usable in tape libraries, it took many years for open reel tapes to be phased out.
The tape recording methods used on computer tapes fall into two categories:
Data written using the linear recording method arranges the data in parallel tracks which run the full length of the tape. Drives using this method use a tape head capable of recording multiple data tracks at the same time. This was the method used in the earliest half inch open reel tape drives. It is the simplest recording method and a variation of it remains the most commonly used recording format today.
Linear serpentine recording is a variation of the method, where a set of data tracks are recorded on each successive pass off the tape across the read/write head. After each pass along the tape, the read/write head is shifted slightly up or down before the next pass writes the data in the reverse direction.
By using a large number of passes the linear serpentine method allows a large number of tracks to be written to the tape. In many cases the tracks are not recorded in a serpentine but record the first track in the centre, before moving the read/write head up and down for each successive track, describing an outward spiral. This outward spiral is used on several tapes, such as DLT and LTO data cartridges.
Instead of passing the tape across a fixed head, the scanning recording method moves the tape media slowly across the read/write head which rotates at high speed. The read and write heads are embedded into the drum allowing data to be written across the width of the tape media.
The earliest method recorded the tracks in a transverse manner, with the tape heads at the outer edges of the drum, which was positioned perpendicular to the media. Arcuate scan was another early method, in which the heads are embedded into the face of a spinning disk laid flat against the media. In this method the heads describe an arc.
The helical scan recording method now used in almost all video tape system and many data tape drive, records the data tracks diagonal to the media. This has several advantages, one being that the pair of the read/write heads can be aligned at different angles, allowing adjacent tracks to be overlapped. Another feature is that the any data tracks which have been written can be immediately read by the corresponding read head in order to verify the data was written accurately. This allows the drive to be able to re-write if there was an error, unless the write error threshold has been exceeded.
The earliest drives using the helical scan recording method often suffered from alignment issues due to the guide pins becoming lose, causing the tape not to track across the read/write head accurately. This could lead to a situation where the data could not be read using another drive or in extreme cases each new track could overwrite the previous track, requiring special tools or even rendering the data completely unreadable. This problem now occurs only rarely since the introduction of automatic tracking.
Data blocks are written to the tape in a single operation while the media is running, usually with an inter-block gap recorded as a delimiter. This inter-block gap is a constant size while each data block may be a different length, depending bytes in the block and whether compression has been used, making larger block sizes more efficient. For drives supporting a fixed block size, the data block recorded to the media will normally contain a several fixed data blocks to ensure the efficiency is not effected.
Data may not be transferred to or from the tape drive at a constant rate, which means the drive must be able to compensate, which for most drives involves stopping the tape and moving it backwards before passing it across the head again as phenomenon known as shoe shining, which causing an increase wear on the media. This is undesirable as it shortens the life span of the media and potentially the drive.
Some drives, such as the LTO Ultrium drives are able to dynamically reduce the speed at which the tape physical moving across the head, reducing the number of times the media needs to be stop and restarted. Another method to reduce this shoe-shining is to either repeat or use dummy blocks, but this can significantly reduce the maximum capacity of the tape.
Data is always written to the tape media in a sequential manner, which is the main reason a tape drive is described as a sequential device. It is however possible, when you wish to restore only a selection of files, to seek to a particular block on the tape and start reading data from that point. This same method is also used for performing quick scans of a tape or reading a backup catalog stored on the media. In order to facilitate this, an end of file marker can be written to tape which is easily identifiable when the tape is travelling at high speed.