Linear Tape-Open
Linear Tape-Open, also known as the LTO Ultrium format, is a magnetic tape data storage technology used for backup, data archiving, and data transfer. It was originally developed in the late 1990s as an open-standard alternative to the proprietary magnetic tape formats available at the time. Upon introduction, LTO rapidly defined the super tape market segment and has consistently been the best-selling super-tape format. The latest generation as of 2026, LTO-10, defines two unique cartridge types which can hold or each.
Cartridges contain hundreds of meters of half-inch wide tape media wound onto a single reel. The first generation LTO cartridge was released in 2000 and stored of data. With each new generation, the capacity has increased, while maintaining the same physical size cartridge.
Mechanisms extract the tape from the cartridge and spool it up on a second reel in the mechanism, reading or writing data as the tape moves between reels. Robotic libraries exist that can hold hundreds or thousands of LTO cartridges and dozens of mechanisms.
In contrast to other non-tape data storage formats, LTO offers high-capacity removable cartridges with a lower cost per TB and better long term stability. As an overall system, LTO requires significantly less electrical power per TB and includes built-in technologies useful for data interchange and safe-keeping, like LTFS, WORM, encryption and data compression.
Historical context
Half-inch wide magnetic tape has been used for data storage since the 1950s, starting with the open reel formats IBM 7-track and later IBM 9-track.In the mid-1980s, smaller, enclosed, single-reel cartridge formats were developed by IBM and DEC. Although the physical tape was nominally the same width in these new formats and the preceding open-reel formats, the technologies and intended markets were significantly different and there was no compatibility between them. The IBM 3480 tape format was designed to meet the demanding requirements of its mainframe products. DEC's CompacTape was targeted at a broader market, including minicomputers and smaller systems.
Later on, it was renamed Digital Linear Tape and eventually sold to Quantum Corporation.
In the late 1980s, Exabyte's Data8 format, derived from Sony's dual-reel cartridge 8 mm video format, saw some popularity, especially with UNIX systems. Sony followed this success with their own now-discontinued 8 mm data format, Advanced Intelligent Tape.
By the late 1990s, Quantum's DLT and Sony's AIT were the leading options for high-capacity tape storage for PC servers and UNIX systems. These technologies were tightly controlled by their owners and consequently, there was little to no competition between vendors and the prices were relatively high.
Birth of LTO
Seeing an opportunity, IBM, HP and Seagate formed the LTO Consortium, which introduced a more open format focusing on the same mid-range market segment. Much of the technology is an extension of the work done by IBM at its Tucson lab during the previous 20 years.In 2000, and around the time of the release of LTO-1, Seagate's magnetic tape division was spun off as Seagate Removable Storage Solutions, renamed Certance in 2003, and subsequently acquired by Quantum in 2004.
Unrealized variations
Initial plans called for two distinct LTO formats: 1) Ultrium - with half-inch tape on a single reel, optimized for high capacity, and 2) Accelis - with 8 mm tape on dual reels, optimized for fast access. Only Ultrium was ever produced, so in common usage, LTO refers to just the Ultrium form factor.Additionally, the first generation of Ultrium was proposed to be available with 4 different lengths of tape, holding 10 GB, 30 GB, 50 GB, and 100 GB per cartridge. Only full length cartridges were ever produced.
Generations
As of 2025, ten generations of LTO Ultrium technology have been made available and four more are planned. Between generations, there are strict compatibility rules that describe how and which drives and cartridges can be used together.The LTO Consortium publishes a roadmap of future generations, which states that LTO-14 will have a capacity of "up to" 576 TB.
Key specifications
Compatibility
In contrast to other tape technologies, an Ultrium cartridge is rigidly defined by a particular generation of LTO technology and cannot be used in any other way. Ultrium drives prior to LTO-10 have some level of compatibility with older generations of cartridges.The of drives and cartridges are as follows:
- Drives of every generation can read and write cartridges of the same generation.
- Drives from generations 2 through 9 can also read and write cartridges of the prior generation.
- Drives from generations 3 through 7 can also read cartridges of 2 generations prior.
- Drives from generation 8 can reformat unused cartridges from generation 7 with a special, higher-capacity format. Once reformatted as Type M, the cartridge is only compatible with drives from generation 8.
Core technology
Tape specifications
Band layout
LTO Ultrium tape is laid out with four wide data bands sandwiched between five narrow servo bands. A thin edge guard band runs along each edge. The tape head assembly, which reads from and writes to the tape, straddles a single data band and the two adjacent servo bands.Servo bands
The servo bands contain a pattern of angled magnetic stripes permanently written to the tape at the factory. The primary use of the servo signal is to maintain the transverse position of the tape head in relation to the tape. This allows the tape head to precisely follow each track as the tape moves past at high speed. The servo signal can also encode a low bitrate data stream. This data is used to encode the longitudinal position of that point on the tape. In addition to LPOS, manufacturers can encode additional data in the signal. With LPOS and a precise time source, a tape drive can precisely measure the velocity of the tape.Data bands
The actual data stored on the tape is recorded in individual tracks located within the data bands. Depending on the generation, there could be tens, hundreds, or thousands of data tracks per band.The tape head has 8, 16, or 32 data read/write head elements and additional servo read elements. The set of 8, 16, or 32 tracks written in a single, one-way, end-to-end pass is called a "wrap". The tape head shifts laterally to access the different wraps within each band and also to access the other bands.
Writing to a blank tape starts at band 0, wrap 0, a forward wrap that runs from the beginning of the tape to the end of the tape and includes a track that runs along one side of the data band. The next wrap written, band 0, wrap 1, is a reverse wrap and includes a track along the other side of the band. Wraps continue in forward and reverse passes, with slight shifts toward the middle of the band on each pass.
The tracks written on each pass partially overlap the tracks written on the previous wrap of the same direction, like roof shingles. The back and forth pattern, working from the edges into the middle, conceptually resembles a coiled serpent and is known as linear serpentine recording.
Logical structure
The block structure of the tape is logical so interblock gaps, file marks, tape marks and so forth take only a few bytes each. In LTO-1 and LTO-2, this logical structure has CRC codes and compression added to create blocks of 403,884 bytes. Another chunk of 468 bytes of information is then added to create a "dataset". Finally error correction bytes are added to bring the total size of the dataset to 491,520 bytes before it is written in a specific format across the eight heads. LTO-3 and LTO-4 use a similar format with 1,616,940-byte blocks.The tape drives use a strong error correction algorithm that makes data recovery possible when lost data is within one track. Also, when data is written to the tape it is verified by reading it back using the read heads that are positioned just "behind" the write heads. This allows the drive to write a second copy of any data that fails the verify without the help of the host system.
Positioning times
While specifications vary between different drives, a typical LTO-7 drive will take about 15 seconds to load the tape and 20 seconds to unload the tape. These drives have an average rewind time of 60 seconds and an average access time of about 56 seconds. Because of serpentine writing methods, rewinding often takes less time than the maximum. If a tape is written to full capacity, there is no rewind time, since the last pass is a reverse pass leaving the head at the beginning of the tape.Durability
LTO tape is designed for 15 to 30 years of archival storage. If tapes are archived for longer than 6 months they have to be stored at a temperature between and between 20 – 50% RH.Both drives and media should be kept free from airborne dust or other contaminants from packing and storage materials, paper dust, cardboard particles, printer toner dust etc.
Depending on the generation of LTO technology, a single LTO tape should be able to sustain approximately 200-364 full file passes. There is a large amount of lifespan variability in actual use. One full file pass is equal to writing enough data to fill an entire tape and takes between 44 and 208 end-to-end passes. Regularly writing only 50% capacity of the tape results in half as many end-to-end tape passes for each scheduled backup, and thereby doubles the tape lifespan. LTO uses an automatic verify-after-write technology to immediately check the data as it is being written, but some backup systems explicitly perform a completely separate tape reading operation to verify the tape was written correctly. This separate verify operation doubles the number of end-to-end passes for each scheduled backup, and reduces the tape life by half.