Tape
Instead of allowing random-access to data like hard disk drives, tape drives give sequential-access to data. A hard disk drive can move its read/write heads to any random part of the disk platters in a very short amount of time, but a tape drive must spend a considerable amount of time winding tape across read/write heads to read any one particular piece of data. As a result, tape drives have very slow average seek times. Despite the slow seek time, tape drives can stream data to tape very quickly. For example, modern LTO 4 drives can reach continuous data transfer rates of up to 120MB/s, which is as fast as most hard disk drives.
Tape technology
Tape drives can vary in many ways depending on manufacturer, recording technology and connectivity to host systems. Additionally the type of technology a company will use for their back up & archive strategy will also vary depending on external factors such as historical technology used and time available to them to back up without interruption to other network traffic.
There are and have been many tape technologies since 1951 when the first tape drive was introduced, however today there are three main technologies that dominate the market, DAT or DDS, DLT & LTO.
DAT/DDS (Digital Audio Tape/Digital Data Storage) was developed by Sony in the mid 1980’s for recording professional audio and is closely based on standard video recorder technology. It was adopted by the computer industry in 1990’s as DDS with drives and media specifically designed for data back up. Today it is an established low end format that has a large and loyal user base.
DLT (Digital Linear Tape) was a technology developed by Digital Equipment Corporation in 1984 then sold to Quantum in 1994. DLT uses linear recording and comes in two variants, SDLT/DLT-S (Super DLT) for mid range and enterprise environments and DLT-V which is a value line offering cost effective performance solutions to the SMB market. DLT is a proven technology used by format loyal customers although it has been in decline since the introduction of LTO (Linear Tape Open).
LTO (Linear Tape Open) was developed as an open tape standard in the late 1990’s by a consortium made up of Seagate, HP and IBM. The LTO form factor is also know as Ultrium and was designed as a drop in replacement for the established DLT technology. Being an open standard has lead to its success over other proprietary technologies, now in its 4th generation it has an install base of around 86% in the mid to enterprise market place where it offers best price vs. performance.
Recording technology
There are two types of recording technology used and these can be as important as tape drive brand and format depending on performance requirements.
Helical Scan writes short dense diagonal tracks across the width of the tape media. The tape heads are placed on a rotating drum that spins and writes data as the slower moving tape passes across it. With helical scan capacity per cartridge is limited but higher data rates are achievable. This technology is used by all DAT formats.
Linear (Serpentine recording) recording writes data in tracks along the entire length of the tape media, once the end of the media is reached the recording head will move to the next track and write data back along the length of the media. Unlike reel to reel cartridge technology used in helical scan the media is draw out of the cartridge and around a drum within the tape drive and then back so the tracks can be written. Linear technology is used by both DLT and LTO technology.
