Tape Drives

A tape drive can be used to store an archive that contains a large amount of data. In older mainframe computers, before the introduction of hard disk drives, magnetic tape was a favourite storage medium. But with the development of removable hard drives, CD-ROM, DVD and other optical drives, tape became less popular, although it’s still viable for storing 1 GB or more. The low cost of tape makes it ideal for archiving huge amounts of material over a long period of time.

Unfortunately, loading data onto a tape is a slow process, although this problem can be mitigated by doing the job at night or during a break period. The capacity of a tape drive can be expanded by means of data compression, although this can make the loading process even slower.

Tape Formats

A range of common tape cassettes and cartridges are used, as detailed below:-

  8 mm Video Cassette

This type of cassette is also used for analogue video recording in some types of camcorder or video recorder. It operates within a standard helical scan tape mechanism, in which two or four heads are mounted on an angled head drum that rotates against the surface of the tape. This gives a very fast writing speed, even though the tape is moving relatively slowly. Unfortunately, it can take several minutes to reach a particular location on the tape.

Different types of data format are used by different devices, although Exabyte is often used. A typical drive accommodates 3.5 GB on a 160 metre tape, increasing to 7 GB when compression is used. Similarly, a 112 metre tape can hold 2.5 GB without compression or 5 GB with compression.

The most common standards are shown below:-

Length (m)Capacity (GB)
1122.5 (5)

  4 mm Digital Audio Tape (DAT)

The DAT format, originally designed for digital audio recording, also employs helical scan technology. Unfortunately, access to a particular part of the tape can take as long as 20 to 60 seconds. The capacity varies from 1.3 to 40 GB, depending on the length of the tape and the type of compression that’s applied.

The data format in most modern drives conforms to the DDS-2, DDS-3 or DDS-4 standards, all derived from the DDS/DAT technology devised by Hewlett-Packard and Sony. The DATA/DAT format is also used in some earlier products.

The following table shows the capacity in GB of typical devices, with the results of compression shown in parenthesis:-

Length (m)DDS-1DDS-2DDS-3DDS-4
902 (4)      
120  4 (8)    
125    12 (24)  
150      20 (40)

A typical DDS-3 drive has a transfer rate of around 2.4 MB/s whilst a DDS-4 machine offers a rate of up to 4.8 MB/s. The following table shows the performance for typical drives:-

Length (m)Size (GB)Rate (MB/min)
Using jukebox or auto-loader with six cassettes

A jukebox or auto-loader can be used to store 64, 72, 144 GB or more.

  Digital Linear Tape (DLT)

This alternative to DAT for digital audio recording employs a high-technology multitrack stationary head and a high tape speed, making it simple, reliable and cost-effective, although the access time is typically 60 seconds.

The most common data formats for DLT are:-

FormatSize (GB)
DLT III10 (20)
DLT IIIXT15 (30)
DLT IV20 (40), 35 (70), 40 (80)
Super DLT110 (220)

The full range of standard DLT tapes accommodate 2.6, 6, 10, 15, 20, 35, 40, 110 or 160 GB increasing to 5.2, 12, 20, 30, 40, 70, 80, 220 or 320 GB with compression. The transfer rate is usually about 6 MB/s, increasing to 11 MB/s for the Super DLT format. Fortunately, Super DLT drives can also read older tapes.

  Advanced Intelligent Tape (AIT)

Developed by Sony, AIT accommodates 25 GB at a rate of 3 MB/s, whilst AIT-2 stores 50 GB at 3 to 6 MB/s and has an access time of around 20 seconds. It takes around three hours to back up 90 GB of data with this kind of device. The following table shows the capacity of standard tapes, with the results of compression shown in parenthesis:-

TypeLength (m)Size (GB)
AIT-117025 (50)
AIT-123035 (70 or 90)
AIT-223050 (100 or 130)
AIT-3-100 (200 or 260) *
* Maximum capacity provided by ALDC compression at 2.6:1

Linear Tape - Open (LTO)

LTO is modern ‘open’ format designed to replace the proprietary DLT and AIT systems. The version currently in use is based on 12 inch tape in a single-reel cartridge. The LTO-1, LTO-2 and LTO-3 standards offer 100, 200 and 400 GB of storage at rates of 20, 40 and 80 MB/s respectively, with the future standards of LTO-4 to LTO-6 offering multiples of these values. The LTO-1 version was originally available in 10, 30, 50 and 100 GB cartridges, whilst LTO-3 Write Once Read Many (WORM) cartridges are available for archiving purposes. Compression is often used to increase capacity.

  Other Formats

Several other stationery head formats exist, including Travan drives with capacities of 400/800 MB, 1.6/3.2 GB, 2.5/5 GB, 8 GB, 14 GB or 20 GB and Iomega Ditto drives holding 2 GB or 3-10 GB.

The traditional quarter-inch cartridge (QIC) includes sizes from 20 to 160 MB, increasing to 250 MB with compression. Sizes include 150 MB, 250 MB, 525 MB, 1.2 GB, 2.0 GB and 2.5 GB, whilst the mini-cartridge version holds 40 or 120 MB.

The VXA-1 format accommodates cartridges giving 12, 20 or 33 GB of storage without compression, increasing to a maximum of 66 GB with 2:1 compression. The transfer rate is 6 MB/s. The later VXA-2 format holds 160 GB and transfers data at 43.2 GB per hour.

The ADR format uses hybrid technology which avoids extracting the tape from the cartridge when reading or writing data. It can store up to 30 or 50 GB with a transfer rate between 2 and 4 MB/s.

©Ray White 2006.