Although the cost and capacity of other removable magnetic storage devices continues to improve, tape drives remain the best choice to back up data or to transfer very large amounts of data between systems. Tape drives provide a combination of high capacity, speed, low media cost and reliability that no other technology can match.
Two tape technologies compete for the standalone Desktop and the small-network market:
∑ Quarter Inch Cartridge (QIC)
∑ Digital Data Storage (DDS)
For For DLT, SDLT, LTO and others see Section 12.3.4 below.
As agencies and corporations commit more and more mission-critical information to automated systems, electronic storage requirements are increasing dramatically. Imaging, multimedia and other emerging data-intensive applications are major drivers of this growth fuelling the need for better, more cost-effective solutions that deliver higher capacity, increased performance and better data integrity.
This has resulted in both a new generation of tape drive technology such as :
QIC drives use serpentine recording which records many parallel tracks on each tape. The drive records data from the beginning to the end of the first track, reverses direction, writes data from the end to beginning of second track and so on, until all tracks have been written. This means that filling a tape may require 50 or more passes of the tape through the drive. Some recent QIC drives have the extra head required for read-while-write, witch allows the drive to back up and compare data in one pass.
Current QIC drives use Travan technology, a combination of tape and drive technologies developed by 3M/Imation and now implemented by many drive manufacturers. The new generation Travan-NS (Network Solution) drives provide read-while-write verification and hardware compression, which allows the drive itself to compress data as it writes it, rather than depending on compression performed by backup software.
Travan drives are relatively inexpensive, provide high capacity and performance and are available in IDE, SCSI, USB, FireWire and parallel interfaces.
DDS drives use helical-scan recording where the head rotates at an angle relative to tape movement and lays down a series of short diagonal tracks across the full width of the tape. This means that DDS drive can theoretically fill a tape during one pass, although real world drives may require several passes to do so. DDS drives support read-while-write.
DDS drives provide high capacity and performance, but are relatively expensive and require a SCSI interface. DDS drives use relatively (to QIC) inexpensive tapes and are most appropriate to servers and high end Desktops
The 8mm tape technology was originally designed for the video industry. Its original purpose was to transfer high-quality color images to tape for storage and retrieval. Now 8mm technology has been adopted by the computer industry as a reliable way to store large amounts of computer data.
There are two major protocols, utilising different compression algorithms and drive technologies, but the basic function is the same. Exabyte Corporation sponsors standard 8mm and Mammoth while Seagate and Sony represent a new 8mm technology known as Advanced Intelligent Tape (AIT).
Introduced in 1996, Mammoth is a more advanced and reliable technology and represents Exabyte's response to the requirements of this mid-range server market. It offers the following features:
Advanced Intelligent Tape (AIT) was the first multisourced tape standard targeted at the midrange server market, which is typically characterized by systems that support 2 to 129 users in a professional environment. Introduced in 1996, AIT was designed to support this key market segment with a combination of exceptional data integrity, speed and capacity.
Several breakthroughs make this possible including stronger, thinner media that is more stable and has better coatings than previously available, new head technologies, higher levels of integration and a unique Memory-In-Cassette (MIC) feature. The result is multi-Gbyte, high-performance tape drive systems with very low frequencies of error that are perfect for tape libraries and robotic applications associated with midrange systems backup.
Lately was announced the third generation of its AIT drives and media cartridges. AIT-3 offers:
∑ Twice the capacity and performance of the previous generation, offering 100GB of uncompressed capacity (260GB with 2.6:1 compression) and a 12MBps/28MBps transfer rates.
∑ The following generation Super-AIT (S-AIT) based on on a hybrid technology will allow a native capacity of up to 500GB on the same size cartridge as is used in the rival DLT and SuperDLT technologies
The origins of Digital Linear Tape date back to the mid-1980s when Digital Equipment Corporation (DEC) developed a new technology, based on standard half-inch magnetic tape, for use with its highly successful MicroVAX system. The first true DLT system emerged in 1989 and the technology was later acquired by Quantum Corporation in 1994. A number of OEMs have subsequently licensed the technology primarily for the purpose of manufacturing automated tape libraries. Effectively, DLT is an adaptation of the old reel to reel magnetic recording method where the tape cartridge performs as one reel and the tape drive as the other.
∑ DLT drives use half-inch wide metal particle tapes - 60% wider than 8mm tape and the widest tape available
∑ The drives use a computer-controlled dual-motor system that precisely controls tape acceleration, deceleration and write/read speed. Their design is also self-cleaning. This results in a head life specified at 30,000 hours as compared to 2,000 hours for 8 mm helical scan devices.
∑ DLT technology minimizes search time through a file mark index located at the logical end of the tape. The index lists the tape segment address of each file on the tape. Using this index, the drive "steps" to the track containing the file and performs a high speed streaming search to the file. This feature enables DLT products to find any file in a 20-gigabyte capacity tape in an average of 45 seconds.
∑ The prime advantages DLT retains are higher storage capacity; higher data transfer rates and higher reliability, mainly because the media does not physically touch the head in the drive. The following chart lists the principal performance characteristics of the various DLT standards:
Figure 6: DLT Performance Characteristics
Super DLT is the next generation of Digital Linear Tape (DLT), Drives based on Super DLT technology far exceed the 35GB native capacity of the DLT tape IV format - with which it aims to be backwards compatible.
The SDLT 320 - high end model in 2003 - specifications are:
∑ Native sustained transfer rate : 160 MB/s
∑ 2:1 compressed transfer rate 320 MB/s
∑ Burst transfer rate SCSI bus : 80 MB/s
∑ Native formatted capacity : 160 GB
∑ 2:1 compressed capacity : 320 GB
∑ Interface available : LVD, Ultra2 SCSI, HVD, Ultra SCSI
Linear Tape-Open (LTO) is an open format technology that was jointly developed by HP, IBM and Seagate. The "open format" means that tapes and drives from different manufacturers are compatible with each other.
LTO uses linear multi-channel bi-directional tape formats and includes enhancements in the areas of timing based servo (Error correction), hardware data compression and track layouts.
There are two formats based on the LTO technology: Accelis and Ultrium.
The first generation of Ultrium allowed for storage of up to 100GB of data (>200GB compressed) on a single cartridge. The Accelis format utilises all the advantages of LTO except that the capacity has been reduced to improve access times to data.
Ultrium Generation 1: Capacity 100GB native, 200GB compressed; Transfer Rate 20 to 40MBps compressed.
Ultrium Generation 2: Capacity 200GB native, 400GB compressed; Transfer Rate 40 to 80MBps compressed.
Accelis Generation 1: Capacity 25GB native, 50GB compressed; Access Time < 10 sec, Transfer Rate 20 to 40MBps compressed
Consider the following issues when choosing a tape drive for backup:
Capacity: Is the most important consideration. You should get a solution that can back up all data in one operation. Otherwise you use a backup scheme that mixes full and incremental or differential partial backups. (Section 13.1.1 discusses Tape Libraries).
Speed: This may or may not be critical, depending on your environment and practices. Actual throughput depends on the drive mechanism, the interface, the speed of the computer and the data set being backed up.
Media Cost: Travan drives are constructed with loose tolerance and are accordingly inexpensive, bur require expensive tapes built to close tolerances. DDS drive, conversely, are expensive because they are built to tight tolerances, which allows them to use inexpensive, loose-tolerance tapes.
Interface: Tape drives are commonly available in ATAPI/IDE, SCSI and parallel interfaces. Some are available in USB and FireWire
Cross-Drive compatibility: You might reasonably assume that a tape you created in one drive would be readable in a similar drive, but that is not always the case. This problem is seldom encountered on Travan NS or DDS drives.
Unless you use a tape drive to transfer large quantities of data between computers, this may seem a minor issue. It can be critical, however, if your computer is damaged by flood or fire. Even if your backup tapes are safely locked away, you may find that a replacement drive of the same model is unable to read them.