Linear Tape-Open (or LTO) is a magnetic tape datastorage technology originally developed in the late 1990sas an open standards alternative to the proprietary mag-netic tape formats that were available at the time. Hewlett-Packard, IBM and Seagate initiated the LTO Consortium, which directs development and manages li-censing and certication of media and mechanism man-ufacturers.
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 cartridge. LTO version 6 released in 2012 can hold 2.5 TB in a car-tridge of the same size.
Upon introduction, LTO Ultrium rapidly dened the su-per tape market segment and has consistently been thebest-selling super tape format.LTO is widely used with small and large computer systems,especially for backup.
|1 Historical context|
Half-inch (-inch, 12.65 mm) magnetic tape has been used for data storage for more than 50 years. In the mid1980s, IBM and DEC put this kind of tape into a sin- le reel, enclosed cartridge. Although the physical tape was nominally the same size, the technologies and in- ended markets were signicantly dierent and there was no compatibility between them. IBM called its format 3480 (after IBM’s one product that used it) and designed it to meet the demanding requirements of its mainframe products. DEC originally called theirs CompacTape, later it was renamed DLT and sold to Quantum Corpora-tion. 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 8mm 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 (and still are) tightly controlled by their owners. Con- sequently, there was little competition between vendors and the prices were relatively high. IBM, HP and Seagate sought to counter this by intro-ducing a more open format focusing on this market segment. Much of the technology is an extension of the work done by IBM at its Tucson lab during the previ-ous 20 years. Initial plans called for two LTO formats to directly compete with these market leaders. Ultrium was half-inch tape on a single reel, optimized for high-capacity and Accelis was supposed to be 8 mm tape on dual-reels, optimized for low-latency. Around the time of the release of LTO-1, Seagate’s mag-netic tape division was spun o as Seagate Removable Storage Solutions, later renamed Certance, which wassubsequently acquired by Quantum.
LTO Ultrium was developed as a replacement for DLT and has a similar design of -inch wide tape in a (slightlysmaller) single reel cartridge. This made it easy forrobotic tape library vendors to convert their DLT libraries into LTO libraries. Compatibility between generations of drives, formats, and cartridges is as follows: An Ultrium cartridge’s dimensions are 102.0 ×105.4 × 21.5 mm. An Ultrium drive reads data from a cartridge in its own generation and at least the two prior genera-
tions. An Ultrium drive writes data to a cartridge in its own generation and to a cartridge from the immediateprior generation in the prior generation format.
|As of December 2012 LTO-6 was the latest generation; further generations are planned as tabulated below|
|First commercially available in September 2000. Ultrium-1 specication is now Standard ECMA-319 Initial capacity of 100 GB. Initial data transfer speed of 20 MB/s (maximum) Tape encoding is RLL 1,7|
|First mechanisms approved in February 2003. First media approved in March 2003.Doubled capacity to 200 GB.Increased data transfer speed to 40 MB/s (maxi-mum).Switched to PRML encoding|
|First media approved in November 2004. Doubled capacity to 400 GB.Increased data transfer speed to 80 MB/s (maxi-mum).Introduced WORM feature.Doubled number of write elements in head.|
|First mechanisms approved in April 2007. Firstmedia approved in May 2007.Doubled capacity again to 800 GB.Increased data transfer rate to 120 MB/s (maxi-mum).Introduced drive level encryption feature using 256-bit AES-GCM.|
Specications announced January 19, 2010. Therst LTO-5 drives appeared on the market in Q2,2010.Nearly doubled capacity to 1.5 TB (1500 GB)Increased data transfer rate to 140 MB/s (maxi-mum).Introduced partition feature that allows tape to be“split” into two separately writable areas. This fea-
ture is required by LTFS.
|Licensing Specications announced June 11,2012.Increased capacity to 2.5 TBIncreased estimated data-compression ratio due to alarger compression buer.Increased cartridge memory to 16 KiBIncreased data transfer rate to 160 MB/s (maxi-mum).|
|Data capacity and speed gures above are for un-compressed data.Most manufacturers list com-pressed capacities on their marketing material. Ca-pacities are often stated on tapes as double the actualvalue; they assume that data will be compressed witha 2:1 ratio (IBM uses a 3:1 compression ratio in thedocumentation for its mainframe tape drives. Sonyuses a 2.6:1 ratio for SAIT). See LTO-DC below.The units for data capacity and data transfer ratesgenerally follow the “decimal” SI prex convention(e.g. mega = 106) and not the binary prex conven-tion (e.g. mega = 220).Minimum and maximum reading and writing speedsare drive-dependent.|
While specications vary somewhat between dierentdrives, a typical LTO-3 drive will have a maximum
rewind time of about 80 seconds and an average accesstime (from beginning of tape) of about 50 seconds.Because of the serpentine writing, rewinding often takesless time than the maximum. If a tape is written to fullcapacity, there is no rewind time, since the last pass is areverse pass leaving the head at the beginning of the tape(number of tracks ÷ tracks written per pass is always aneven number).
5000 cartridge loads/unloads Approximately 260 full le passes. (One full pass isequal to writing enough data to ll an entire tape.) The following durability gures are quoted from Imationcorporation's Expected Usage Life of Imation Media:
There is a large amount of lifespan variability in actual
Regularly writing only 50% capacity of the tape re-sults 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 technol-ogy to immediately check the data as it is being written but some backup systems explicitly perform acompletely separate tape reading opera-tion to verify the tape was written correctly. This separate verify operation doubles the number of end-to-end passes for each scheduled backup, andreduces the tape life by half.
Files and directories to appear on desktop and di-rectory listings Drag-and-drop les to/from tape File level access to data Supports data exchange With LTFS tape media can be used in a fashion like other removable media (USB ash drive, external hard disk drive, etc.). With LTFS the drive may behave like a disk (drive) but it is still a tape with serial access. Files are al-ways appended to the end of the tape. If a le is removed from the listing the associated tape blocks used are not freed up, they are simply marked as unavailable. Data is only deleted if the whole tape is reformatted. LTFS now supports 'Library mode' facilitating access to multiple tapes in atape library. LTFS was rst introduced with the IBM LTO Gen5 drie.The Linear Tape File System Technical Work Group ofSNIA works on the development of the format for LTFS.
|LTO Ultrium tape is laid out with four wide data bandssandwiched between ve narrow servo bands. The tapehead assembly straddles two adjacent servo bands, with 2servo read heads and 8 or 16 data read/write heads. Eachdata head moves up and down within its own data sub-band the same width as the servo band. First, the headassembly is positioned so all tape heads are at the top oftheir sub-bands and 8 or 16 tracks are written in the for-ward direction. Then the head assembly is moved to thebottom of each sub-band and 8 or 16 tracks are writtenin the reverse direction. The set of tracks written at thesame time is referred to as a “wrap”. Alternating forwardand reverse wraps are written as each data sub-band islled up in 0, 2, 4, ..., 5, 3, 1 order.When the rst data band is lled (they are actually lledin 3, 1, 0, 2 order across the tape), the head assembly ismoved to the second data band and a new set of wrapsis written. The total number of tracks on the tape is (4data bands) × (11–20 wraps per band) × (8 or 16 tracksper wrap). For example, an LTO-2 tape has 16 wraps perband, and thus requires 64 passes to ll.The block structure of the tape is logical so interblockgaps, le marks, tape marks and so forth take only a fewbytes each. In LTO-1 and LTO-2, this logical structurehas CRC codes and compression added to create blocksof 403,884 bytes. Another chunk of 468 bytes of in-formation (including statistics and information about thedrive that wrote the data and when it was written) is thenadded to create a 'dataset'. Finally error correction bytesare added to bring the total size of the dataset to 491,520bytes (480 KiB) before it is written in a specic formatacross the eight heads. LTO-3 and LTO-4 use a similarformat with 1,616,940-byte blocks.|
|New for LTO-3 was write once read many (WORM) ca-pability. This is normally only useful for legal record keeping. An LTO-3 or later drive will not erase or over-write 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 identies it to the drive as WORM, the servo tracks are slightly dierent to allow verica-tion that data has not been modied, the bottom half of the cartridge shell is gray,and it may come with tamper-proof screws. WORM capable drives imme-diately recognize WORM cartridges and include a unique WORM ID with every dataset written to the tape. There is nothing dierent about the tape medium in a WORM cartridge.|
|LTO-2cartridgewiththetopshellremoved,showingtheinternal components. Top right corner: tape access gate. Bottom left corner: write-protect-tab. Bottom right corner: cartridge memory chip|
|Compliance-Veried licensed manufacturers of LTO technology media are EMTEC, Imation, Fujilm, Maxell, TDK and Sony.All other brands of media are manufactured by these companies under contract. Since its bankruptcy in 2003, EMTEC no longer manu-factures LTO media products. TDKwithdrew from the data tape business as of the end of March 2014. Verbatim and Quantegy both licensed LTO tech-nology, but never manufactured their own compliance-veried media.|
|The colors of LTO Ultrium cartridge shells are mostly consistent, though not formally standardised. HP is the|
|HP’s dark red LTO-2 cartridge||HP’s light-blue LTO-5 cartridge|
|notable exception. Sometimes similar, rather than identi-cal, colors are used by dierent manufacturers (slate-blueand blue-grey; green, teal, and blue-green). UCC means universal cleaning cartridge, whichworks with all drives. Dierent manufacturers use dierent names for thesame color sometimes. The names in the table abovecome from each manufacturer’s own documentation. WORM (write once, read many) cartridges are two-tone, the top half of the shell is the normal color ofthat generation for that manufacturer, and the bot-tom half of the shell is a light grey.|
|Every LTO cartridge has a cartridge memory chip insideit. It is made up of 256 (128 on LTO-1, 2 and 3) blocks ofmemory, where each block is 32 bytes for a total of 8 KB(4 KB on LTO-1, 2 and 3). This memory can be read orwritten, one block at a time, via a non-contacting passiveRF interface. This memory is used to identify tapes, tohelp drives discriminate between dierent generations ofthe technology, and to store tape-use information.|
|LTO cartridge memory|
|EveryLTOdrivehasacartridgememoryreaderinit. The non-contactinterfacehasarangeof20mm. External readersareavailable,bothbuiltintotapelibrariesandPC based. Onesuchreader,Veritape,connectsbyUSBtoa PCandintegrateswithanalyticalsoftwaretoevaluatethe quality of tapes. This device is also rebranded as the SpectraMLMReader andtheMaxellLTOCartridge MemoryAnalyzer.|
|Leader pin on the end of a length of LTO tape|
|ThetapeinsideanLTOcartridgeiswoundaroundasingle reel. The end of the tape is attached to a perpendicular leaderpinthatisusedbyanLTOdrivetoreliablygrasp theendofthetapeandmountitinatake-upreelinside the drive. When a cartridge is not in a drive, the pin is heldinplaceattheopeningofthecartridgewithasmall spring. A common reason for a cartridge failing to load into a driveisthemisplacementoftheleaderpinasaresultof thecartridgehavingbeendropped. Theplasticslotwhere thepinisnormallyheldisdeformedbythedropandthe leader pin is no longer in the position that the drive expectsittobe. Oldersingle-reeltapetechnologiesuseddiﬀerentmeans toloadtapeontoatake-upreel.|
|The LTO cartridge label uses the bar code symbology ofUSS-39. A description and denition is available fromthe Automatic Identication Manufacturers (AIM) spec-ication Uniform Symbol Specication (USS-39) and theANSI MH10.8M-1993 ANSI Barcode specication.|
|The magnetic servo tracks on the tape are factory en-coded. Using a bulk eraser (or otherwise exposing thecartridge to a strong magnetic eld) will erase the servotracks along with the data tracks, rendering the cartridgeunusable.|
|IBM Full-Height LTO-2 drive||HP Half-Height LTO-2 drive in an enclosure for desktop use|
|Current Compliance-Veried licensed manufacturers of LTO technology mechanisms are IBM, Hewlett-Packard, Quantum, and Tandberg Storage.|
Internal head cleaning brush from an IBM LTO-2 FH drive. Swipes once for every insert and eject
|Although keeping a tape drive clean is important, nor-mal cleaning cartridges are abrasive and frequent use willshorten the drive’s lifespan. LTO drives have an internaltape head cleaning brush that is activated when a cartridgeis inserted. When a more thorough cleaning is requiredthe drive signals this on its display and/or via Tape Alertags. Cleaning cartridge lifespan is usually from 15 to50 cleanings. There are 2 basic methods of initiating acleaning of a drive: robot cleaning and software cleaning.|
|The LTO specication describes a Data Compression method LTO-DC, also called Streaming Lossless Data Compression (SLDC).It is very similar to the al-gorithm ALDC which is a variation of LZS (a patent-encumbered algorithm controlled by Hi/Fn).The primary dierence between ALDC and SLDC is that SLDC does not apply the compression algorithm to un-compressible data (i.e. data that is already compressed or suciently random to defeat the compression algo-rithm). Every block of data written to tape has a header bit indicating whether the block is compressed or raw. For each block of data that the algorithm works on, it saves a copy of the raw data. After applying the com-pression function to the data, the algorithm compares the “compressed” data block to the raw data block in mem-ory and writes the smaller of the two to tape. Because of the pigeonhole principle, every lossless data compression algorithm will end up increasing the size of some inputs. The extra bit used by SLDC to dierentiate between raw and compressed blocks eectively places an upper bound on this data expansion. LTO-DC achieves an approximately 2:1 compression ra-tio when applied to the Calgary Corpus. This is inferior to slower algorithms such as gzip, but similar to lzop and the high speed algorithms built into other tape drives. It should be noted that plain text, raw images, and database les (TXT, ASCII, BMP, DBF, etc.) typically compress much better than other types of data stored on computer systems. In contrast, encrypted data and pre-compressed data (PGP, ZIP, JPEG, MPEG, MP3, etc.) would nor-mally increase in size, if data compression was applied. In some cases this data expansion could be as much as 15%. With the SLDC algorithm, this signicant expan-sion is avoided.|
The LTO-4 specication added a feature to allow LTO-4. LTO-4 drives must be aware of encrypted tapes, but are
not required to actually support the encryption process. All current LTO manufacturers support encryption na-tively enabled in the tape drives using Application Man-aged 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 re-quest and exchange encryption keys using either propri-etary protocols, or an open standard like OASIS's Key Management Interoperability Protocol.
|The tape drives use a strong error correction algorithmthat makes data recovery possible when lost data is withinone track. Also, when data is written to the tape it isveried by reading it back using the read heads that arepositioned just 'behind' the write heads. This allows thedrive to write a second copy of any data that fails the ver-ify without the help of the host system.|