Linear Tape-Open

Linear Tape-Open (LTO), 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 standards 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 2025, LTO-10, can hold in one cartridge, or with industry-standard 2.5:1 compression.

Cartridges contain hundreds of meters of half-inch (12.65 mm) wide tape media wound onto a single reel. Mechanisms (a.k.a. tape drives, streamers) 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.

The original version of LTO Ultrium, called LTO-1, was released in 2000 and stored of data in a cartridge; throughout newer generations, the capacity has increased while maintaining the same physical size. They feature built-in encryption for safer storing and transporting of data, and the partition feature enables usage of LTFS, generally having higher capacity, better long-term stability, and lower unit cost than other data storage formats. There are also write once read many LTO cartridges, useful to protect against accidental or malicious deletion.

Historical context

Half-inch (12.65 mm) 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 (DLT) 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 (AIT).

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, later renamed Certance, which was subsequently acquired by Quantum.

Unrealized variations

Initial plans called for two distinct LTO formats:
# Ultrium - with half-inch tape on a single reel, optimized for high capacity, and
# Accelis - with 8 mm tape on dual reels, optimized for fast access.

Despite the initial plans, only Ultrium was ever produced. So, in common usage, LTO refers to just the Ultrium form factor.

Another proposed variation was to have different length tapes. The first generation of Ultrium was going to be available with four types of cartridges, holding 10 GB, 30 GB, 50 GB, and 100 GB. However, only the full length 100 GB tapes were ever produced.

Generations

As of 2020, nine generations of LTO Ultrium technology have been made available and five more are planned. Between generations, there are strict compatibility rules that describe how and which drives and cartridges can be used together.

While data capacity and speed figures vary with ''uncompressed or compressed'' data, most manufacturers list compressed capacities and speeds on their marketing material. Capacities are often stated on tapes assuming that data will be compressed at a fixed ratio, commonly 2:1. See Compression below for algorithm descriptions and the table above for LTO's advertised compression ratios.

The units for data capacity and data transfer rates generally follow the "decimal" SI prefix convention (e.g. mega = 10⁶), not the binary interpretation of a decimal prefix (e.g. mega = 2²⁰).

Minimum and maximum reading and writing speeds are drive-dependent.

Drives usually support variable-speed operation to dynamically match the data rate flow. This nearly eliminates tape backhitching or "shoe-shining", maximizing overall throughput and device/tape life.

The LTO Consortium provide a roadmap of future generations, which state that LTO-10 is 30 TB of storage and LTO-14 potentially 576 TB.

Compatibility

An LTO-2 mechanism, from IBM. This SCSI drive fits in a 5.25 inch, Full-Height drive bay. ">drive_bay.html" ;"title="SCSI drive fits in a 5.25 inch, Full-Height drive bay">SCSI drive fits in a 5.25 inch, Full-Height drive bay.
file:LTODrive HP-448HH.jpg">HP Half-Height LTO-2 drive in an enclosure for desktop use

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 (with the exception of LTO-M8, see below). While Ultrium drives are also defined by a particular generation, they are required to have some level of compatibility with older generations of cartridges. The rules for compatibility between generations of drives and cartridges are as follows:

* Up to and including LTO-7, an Ultrium drive ''can read'' data from a cartridge in its own generation and the two prior generations. Later generations reduce read compatibility to only the previous generation: LTO-8 drives can read LTO-7 and LTO-8 tape, but not LTO-6 tape; LTO-9 drives can read and write from LTO-8 and LTO-9 tapes, but not LTO-7 ones.
* An Ultrium drive ''can write'' data to a cartridge in its own generation and to a cartridge from the one prior generation ''in the prior generation's format''.
* Some LTO-8 drives may write previously unused LTO-7 tapes with an increased, uncompressed capacity of 9 TB (''Type M (M8)''). Only new, unused LTO-7 cartridges may be initialized as LTO-7 Type M. Once a cartridge is initialized as Type M it may not be changed back to a 6 TB LTO-7 cartridge. LTO-7 Type M cartridges are only initialized to Type M in an LTO-8 drive. LTO-7 drives are not capable of reading LTO-7 Type M cartridges.
* An Ultrium drive ''cannot make any use'' of a cartridge from a more recent generation. For example, an LTO-2 cartridge can never be used by an LTO-1 drive; and even though it can be used in an LTO-3 drive, it performs as if it were in an LTO-2 drive.

Within the compatibility rules stated above, drives and cartridges from different vendors are expected to be interchangeable. For example, a tape written on any one vendor's drive should be fully readable on any other vendor's drive that is compatible with that generation of LTO.

Core technology

Tape specifications

Physical structure

LTO Ultrium tape is laid out with four wide data bands sandwiched between five narrow servo bands. The tape head assembly, that reads from and writes to the tape, straddles a single data band and the two adjacent servo bands. The tape head has 8, 16, or 32 data read/write head elements and 2 servo read elements. The set of 8, 16, or 32 tracks are read or written in a single, one-way, end-to-end pass that 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 (BOT) to the end of the tape (EOT) 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 (EOT to BOT) 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.

When the first data band is filled (they are filled in 3, 1, 0, 2 order across the tape), the head assembly is moved to the second data band and a new set of wraps is written in the same linear serpentine manner. The total number of tracks on the tape is (4 data bands) × (11 to 52 wraps per band) × (8, 16, or 32 tracks per wrap). For example, an LTO-2 tape has 16 wraps per band, and thus requires 64 passes to fill.

Logical structure

Since LTFS is an open standard, LTFS-formatted tapes are usable by a wide variety of computing systems.

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 (including statistics and information about the drive that wrote the data and when it was written) 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 (480  KiB) 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 (from beginning of tape) 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 (number of tracks ÷ tracks written per pass is always an even number).

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.

Optional technology

The original release of LTO technology defined an optional data compression feature. Subsequent generations of LTO have introduced new technologies, including WORM, encryption, and partitioning features. These features are built into the drives and/or tapes and can be ignored or enabled. Compression and encryption can also be performed in software prior to the data being sent to the tape drive. However, the partitioning function can only be done in hardware, and the WORM feature requires special WORM tapes.

Compression

The original LTO specification describes a data compression method LTO-DC, also called Streaming Lossless Data Compression (SLDC). It is very similar to the algorithm ALDC which is a variation of LZS. LTO-1 through LTO-5 are advertised as achieving a "2:1" compression ratio, while LTO-6 and LTO-7, which apply a modified SLDC algorithm using a larger history buffer, are advertised as having a "2.5:1" ratio. This is inferior to slower algorithms such as gzip, but similar to lzop and the high speed algorithms built into other tape drives. The actually achievable ratio generally depends on the compressibility of the data, e.g. for precompressed data such as ZIP files, JPEG images, and MPEG video or audio the ratio will be close to or equal to 1:1.

WORM

New for LTO-3 was write once read many (WORM) capability. This is useful for legal record keeping, and for protection from accidental or intentional erasure, for example from ransomware, or simply human error. Standard LTO cartridges do include a write-protect switch in the bottom-left corner, although it is easily overridden by the user and doesn't provide any protection from accidental deletion by, for example, misidentification of a cartridge. An LTO-3 or later drive will not erase or overwrite data on a WORM cartridge, but will read it. A WORM cartridge is identical to a normal tape cartridge of the same generation with the following exceptions: the cartridge memory identifies it to the drive as WORM, the servo tracks are slightly different to allow verification that data has not been modified, the bottom half of the cartridge shell is gray, and it may come with tamper-proof screws. WORM-capable drives immediately recognize WORM cartridges and include a unique WORM ID with every dataset written to the tape. There is nothing different about the tape medium in a WORM cartridge.

Encryption

The LTO-4 specification added a feature to allow LTO-4 drives to encrypt data before it is written to tape. All LTO-4 drives must be aware of encrypted tapes, but are not required to support the encryption process. All current LTO manufacturers support encryption natively enabled in the tape drives using Application Managed Encryption (AME). The algorithm used by LTO-4 is AES- GCM, which is an authenticated, symmetric block cipher. The same key is used to encrypt and decrypt data, and the algorithm can detect tampering with the data. Tape drives, tape libraries, and backup software can request and exchange encryption keys using either proprietary protocols, or an open standard like OASIS

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