Linear Tape-Open (or LTO ) is a magnetic tape data storage technology originally developed in the late 1990s as an open standards alternative to the proprietary magnetic tape formats that were available at the time. Seagate, Hewlett-Packard, and IBM initiated the LTO Consortium, which directs development and manages licensing and certification of media and mechanism manufacturers. The standard form-factor of LTO technology goes by the name Ultrium , the original version of which was released in 2000 and could hold 100 GB of data in a single cartridge. The most recent version was released in 2007 and can hold 800 GB in the same size cartridge. Since 2002, LTO has been the best selling "super tape" format and is widely used with small and large computer systems, especially for backup.
Historical context
Half-inch (1/2") magnetic tape has been used for data storage for more than 50 years. In the mid 1980s, IBM and DEC put this kind of tape into a single reel, enclosed cartridge. IBM called their cartridge 3480 and DEC originally called theirs CompacTape, but later it was renamed DLT and sold to Quantum. In the late 1990s, Quantum's DLT and Sony's Advanced Intelligent Tape (AIT) were the leading options for high-capacity, high speed tape storage for PC servers and UNIX systems. Those technologies were and still are tightly controlled by their owners. Consequently, their availability was fairly limited and prices were relatively high.
IBM, HP and Seagate sought to counter this by introducing a more open format. Much of the technology is an extension of the work done by IBM at its Tucson lab during the previous 20 years.
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, soon after, was acquired by Quantum Corp.
Form factors
LTO technology was designed to come in two form factors, Ultrium and Accelis. As of 2008, LTO Ultrium is very popular and there are no commercially available LTO Accelis drives or media. In common usage, LTO generally refers only to the Ultrium form factor.
Accelis
Accelis was developed in 1997 for fast access to data by using a two-reel cartridge that loads at the midpoint of the 8 mm wide tape to minimize access time. IBM's (short-lived) 3570 Magstar MP product pioneered this concept. The real-world performance never exceeded that of the Ultrium tape format, so there was never a demand for Accelis and no drives or media were commercially produced.
Ultrium
LTO Ultrium was developed as a (more or less) drop-in replacement for DLT and has a similar design of 1/2" wide tape in a (slightly smaller) single reel cartridge. This made it easy for robotic tape library vendors to convert their DLT libraries into LTO libraries.
An Ultrium cartridge's dimensions are 102.0 x 105.4 x 21.5 (mm).
- An Ultrium drive is expected to read data from a cartridge in its own generation and at least the two prior generations.
- An Ultrium drive is expected to write data to a cartridge in its own generation and to a cartridge from the immediate prior generation in the prior generation format.
Generations
LTO-1
- Originally designed to come in 4 lengths of tape: 10, 30, 50, and 100 GB.
- Tape encoding is RLL 1,7
- First commercially available in September 2000.
LTO-2
- Doubled capacity and transfer speed
- Switched to PRML encoding
- First mechanisms approved in February 2003. First media approved in March 2003.
LTO-3
- Doubled capacity and transfer speed again
- Native transfer speed for Full-Height tape drives is 80 MB/s and Half-Height drives are 60 MB/s
- Introduced WORM feature
- Doubled number of write elements in head
- First media approved in November 2004.
LTO-4
- Doubled capacity again to 800 GB.
- Added 256-bit AES-GCM drive level encryption.
- Increase data transfer rate by 50% to 120 MB/s for Full-Height drives and 80 MB/s for Half-Height drives.
- First mechanisms approved in April 2007. First media approved in May 2007.
LTO-5
The current draft has the following specifications:
- Capacity of 1.6 TB (1600 GB)
- Uncompressed data transfer rate of 180 MB/s
Notes
- Data Capacity and Speed figures above are for uncompressed data. Most manufacturers list compressed capacities on their marketing material. Capacities are often stated on tapes as double the actual value; they assume that data will be compressed with a 2:1 ratio (IBM uses a 3:1 compression ratio in the documentation for its Mainframe tape drives. Sony uses a 2.6:1 ratio for SAIT). See LTO-DC below. The marketing material also uses decimal definitions for byte capacities.
- The units for data capacity generally follow the (decimal) SI prefix convention. (eg. mega = 10^6)
- The units for data transfer generally follow the binary prefix convention. (eg. mega = 2^20)
- Minimum and maximum reading and writing speeds are drive dependent.
Positioning times
While specifications vary somewhat between different drives, a typical LTO-3 drive will have a maximum rewind time of about 80 seconds and an average access time (from beginning of tape) of about 50 seconds. Note that due to the serpentine writing, 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.
Tape durability
- 15 to 30 years archival.
- 5000 cartridge loads/unloads
- Approximately 260 full file passes. (One file pass is equal to writing enough data to fill an entire tape.)
The following durability figures are quoted from Imation corporation's 2008 tape specifications:
There is a large amount of lifespan variability in actual use:
- Regularly writing only 50% capacity of the tape results in half as many end-to-end tape passes for each scheduled backup, and 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.
Technical features
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 actually support the encryption process. 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.
Error detection and correction
The tapes contain a strong error correction algorithm that makes data recovery possible when lost data is within one track. 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.
Leader pin
The tape inside an LTO cartridge is wound around a single reel. The end of the tape is attached to a perpendicular leader pin that is used by an LTO drive to reliably grasp the end of the tape and mount it in a take-up reel inside the drive. When a cartridge is not in a drive, the pin is held in place at the opening of the cartridge with a small spring.
A common reason for a cartridge failing to load into a drive is the misplacement of the leader pin as a result of the cartridge having been dropped. The plastic slot where the pin is normally held is deformed by the drop and the leader pin is no longer in the position that the drive expects it to be.
Older tape technologies used different means to load tape onto a take-up reel. Some 9 track tape drives used a burst of air against the spinning reel to automatically separate and grasp the loose end of the tape. This worked without a leader pin. DLT tapes have a hole punched in the end of the tape that a drive can use to grasp the end of the tape.
LTO-CM
Every LTO cartridge has a Cartridge Memory chip inside it.
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