Magnetic Tapes
Magnetic tape is the most popular data storage medium for big data sets accessed and processed sequentially. Computer systems also use it as backup storage for data stored on storage devices such as a hard disk.
Magnetic tape medium is a plastic ribbon usually ½ inch or ¼ inch wide and 50-2400 feet long. Its surface has an iron oxide or chromium dioxide coating that can record data by magnetization. Here the data is recorded on the coated surface as tiny, invisible magnetized, and non-magnetized spots (representing 1s and 0s). The tape ribbon is stored in reels, a small cartridge, or a cassette.
Like in the case of audio or video tapes, we can erase old data and record new data on magnetic tapes as well. As we record new data, it automatically erases old data in the same area. However, we can read stored data many times without affecting it.
Basic principles of operation
Storage Organization
A tape consists of vertical columns called frames and horizontal rows called channels or tracks. Older tapes had seven tracks and used a 6-bit BCD code format for data recording.
Hence, letter A is represented on this tape by its BCD code 110001. The first six tracks record the 6 bits of BCD code, and the seventh track records parity bit.
Parity bit: A parity bit or check bit enables the detection of errors that may occur due to the loss of a bit from a string of 6 bits or 8 bits during data input or output operations.
If the basic code for a character requires an odd number of 1 bit (such as characters 1,2 or A in BCD), an additional 1 bit is added to the check bit location so that there will always be an even number of 1 bit. It’s an example of even-parity.
Similarly, in odd-parity, the check bit is 0 if the total number of 1 bit for representing a character is odd and it is 1, otherwise. The tape in the figure above uses the parity bit for even parity.
Most modern magnetic tapes have nine tracks and use the 8-bit EBCDIC code format for data recording.
The fourth track records parity bit, which produces an odd parity. Notice that it represents the letter A by its 8-bit EBCDIC code 11000001.
A magnetic tape is a continuous medium that records data serially. There is no addressing.
Q. In this situation, how are different pieces of data (known as records) identified on a tape?
For this, it separates records by blank spaces called inter-record gaps (IRG). A tape drive creates IRGs automatically as it writes data on tape. Each IRG is about 0.5 inches.
While reading data from a moving tape, a tape drive stops tape movement when it encounters an IRG. The tape remains stationary until the record processing is over and then moves again to read the next record. This process repeats for all records in a file.
Records are of varying lengths. When a tape contains a large number of very short records with an IRG after each record, more than half of the tape is unused (occupied with IRGs).
Moreover, tape I/O operation also becomes very slow due to frequent interruptions in tape movement during I/O operations. To overcome these problems, it groups records into blocks and separates blocks into blank spaces called inter-block gaps (IBG).
Blocking: The process of grouping two or more records to form a block of data is known as blocking, and the number of records grouped in each block is called the blocking factor.
The blocking factor generally depends on record length. A deciding block should be at least ten times as long as the IBG to reduce wastage of tape storage space.
Storage capacity
Storage capacity of a tape = Data recording density × Length
Data recording density is the amount of data stored on a given length of tape. We measure it in bytes per inch (bpi), or the number of bytes (characters) stored per linear inch of tape. Tape density varies from 800 bpi in older systems to 77,000 bpi in modern systems.
Hence, if a 2400 feet tape has a data recording density of 800 bpi, its storage capacity will be 2400 × 12 inches × 800 bpi = 23 × 106 bytes = 23 Megabytes. It is the total (raw) storage capacity of the tape.
Its actual storage capacity (storage available for storing user data) is much less due to the use of IBGs. The exact storage capacity of a tape may be anywhere from 35% to 70% of its raw storage capacity, depending on the storage organization.
Data Transfer Rate
The data transfer rate of a tape is the number of characters transmitted per second from the tape to primary storage. We measure it in bytes per second (bps). Its value depends on data recording density and speed of tape movement under the read/write head. Tape speed is typical of the order of 100 inches per second.
Hence, for a tape having a data recording density of 77,000 bpi and its drive having a tape speed of 100 inches per second, the data transfer rate is 77,000 × 100 = 77,00,000 bytes or roughly 7.7 MB per second.
Tape Drive
A magnetic tape drive enables storage/retrieval of data on/from tape. It has read/write heads. As the tape ribbon passes under the read/write heads, it can read/write data from/to the tape by giving suitable commands (read or write) to the tape drive.
Like in the case of an audio tape recorder or videocassette recorder, we must first load a magnetic tape reel, cartridge, or cassette onto a tape drive attached to a computer for processing. Once we load the magnetic tape on a magnetic tape drive, the magnetic tape is online. The computer can now use for storage or retrieval of data.
When processing is complete, we remove the tape from the tape drive for offline storage (away from the computer system), and data on it becomes inaccessible until we load it again on the tape drive.
Tape controller
We must mount a magnetic tape on a tape drive before using it for reading/writing information. A tape controller controls a tape drive connected to it. It interprets the commands for operating the tape drive.
Types of Magnetic Tapes
Data recording density and data transfer rate of magnetic tapes depend largely on the data organization and principles of operation of various magnetic tapes and their tape drives.
Commonly used ones are: –
1. ½-inch tape reel
2. ½-inch tape cartridge
3. ½-inch streamer tape
4. 4-mm digital audio tape (DAT)
Half-inch Tape Reel
A half-inch tape reel uses a ½-inch tape ribbon stored on a tape reel. It is a tape drive on which we have to mount the tape reel for data processing.
During processing, the tape moves from a supply reel to a take-up reel via two vacuum channels and a read/write head assembly. The read/write head assembly reads/writes data from/to the tape. The only unit has one read/write head per tape track.
Hence, the read/write head assembly of a 9-track tape drive will have a set of nine heads. Each head operates alone and stores information on nine tracks parallel to the border of the tape. This mode of storing data in adjacent bit formats is called parallel representation. A tape drive reads/writes data a byte at a time in parallel representation.
Different tape drives use other mechanisms to prevent tape damage from sudden bursts of speed. Users commonly use magnetic tape reels as archival storage for offline storage of data and the exchange of data and programs between organizations.
Organizations with large computing systems (such as mainframe systems) use them because the tape controller and tape drives of magnetic tape reels are bulky and expensive.
Many medium-sized business organizations have vaults containing several thousands of tape reels. For example, a large insurance company may have a storage area containing over 50,000 magnetic tape reels.
However, tape cartridges, streamer tapes, and digital audio tapes are fast replacing magnetic tape reels because these new magnetic tape storage devices are more compact, cheaper, and easier to handle and use.
Half-inch Tape Cartridge
Magnetic tape reels are suitable for use with large and medium size computers. The same magnetic tape is used in smaller computer systems as tape cartridges.
Tape cartridges look similar to audio or video cassettes used in familiar home music or video systems. Their tape drives are also very similar in look and mechanical operation to our home’s audio or video players. We push/eject a tape cartridge inside/from a tape drive just as we push/eject a videotape cassette inside/from a video cassette player.
Tape cartridges provide a more convenient way to packing tape. They also simplify the tape mounting on the tape drive compared to magnetic tape reels.
Moreover, tape cartridges provide protection against dirt and contamination because the tape on which we record data is sealed inside the cartridge.
Quarter-inch Streamer Tape
In addition to storing data for sequential data processing applications, one of the primary uses of magnetic tapes is to serve as a backup storage medium for data stored on online devices such as disks.
During the backup or recovery process, backup mechanisms process backup tapes continuously from beginning to end because there is no need for selective access to records.
Hence, there is no need to start and stop a backup tape after every few records. Based on this idea, researchers came out with a special type of quarter-inch streamer tape, eliminating traditional tape drives’ start/stop operation for half-inch tape reels or cartridges.
As there is no start/stop mechanism, streamer tape drives can read/write data more efficiently than traditional tape drives, which stop and restart tape movement at each IBG (inter-block gaps). These tapes are ¼-inch wide and sealed in a cartridge. Their tape drives are very similar in look and mechanical operation to the tape drives of half-inch tape cartridges. Moreover, streamer tape drives also utilize tape storage areas more efficiently.
Streamer tapes are so called because they read/write data serially as streams of bits. They use a data format known as the QIC (Quarter inch Cartridge) standard. Standard data format helps easily use these tapes for exchanging data between different computers.
Another computer can read data recorded on a streamer tape by one computer if both use the common QIC standard data format.
4mm Digital Audio tape (DAT)
DAT provides a very high data density per inch of tape and uses a tape ribbon of 4mm width enclosed in a cartridge. The tape length is either 60 meters or 90 meters. For very high data recording density, its tape drive uses a helical scan technique for data recording.
DAT drives use a data-recording format called Digital Data Storage (DDS), which provides three levels of error correcting code to ensure excellent data integrity. The storage capacity of DAT cartridges varies from 4 GB to 14 GB, and the data transfer speed of DAT drives is about 366 KB/sec.
Advantages of Magnetic Tapes
1. The storage capacity of magnetic tape storage is virtually unlimited because as many tapes as required can be used for storing large data sets.
2. With the low cost of tape reels and cartridges and high data recording densities, the cost per bit of storage is very low for magnetic tapes.
Additionally, we can erase data on a tape and reuse it many times. In-spite-of recent advancements in many other storage technologies, tapes remain unrivaled because no other technology offers a high-capacity advantage at such a low price.
3. Tape reels and cartridges are comparatively easy to handle and store as they are compact and light in weight. They enable the storage of a huge amount of data in a small storage space.
4. Magnetic tape reels and cartridges are portable as they are compact in size and lightweight. Tape reels and cartridges are often used to transfer data and programs from one computer to another that are not directly linked.
Disadvantages of Magnetic Tapes
1. Magnetic tapes are unsuitable for randomly accessed data storage because of their sequential access nature.
2. We must store them in a dust-free environment having controlled temperature and humidity (to prevent the tape ribbon from twisting due to warping).
3. We must label magnetic tapes properly and logically to remember what data is on which tape and to prevent the erasure of useful data by mistake.
Uses of Magnetic Tapes
Normally, we use magnetic tapes for the following purposes: –
1. It is used for sequential data processing applications.
2. Backing up of data stored on an online storage device such as a disk for its offline storage. However, over time, the tape will become more of an archive medium because users increasingly use disk-based solutions to meet backup requirements.
3. Archiving of data is used occasionally. Magnetic tapes continue to be the preferred medium for data archival.
4. Transferring of data and programs between two computers not linked together.
5. Distribution of software. Vendors often distribute software products or their updates on magnetic tapes.