Tape Drives

Tape drives are used universally for backup and restore operations. While there are far fewer tape drives than disk drives, the function they provide is very important. The primary difference between tape and disk storage is that tape media is removable, which means it can be transported for safekeeping from disasters as well as being a mechanism for sharing data over the "sneaker net."

NOTE

Sneaker net is certainly not the only way tapes have been used to share data. Companies have used courier services, air cargo, and even refrigerated trucks to move tapes from one site to another to share data.

One of the less-obvious differences between disk drives and tape drives is their read/write ratios. While disk drives are most often used to read data, tape drives are usually used to write data.

This section discusses topics concerning tapes and tape drives:

Tape media
Caring for tape
Caring for tape heads
Tape drive performance
The tale of two technologies

A Look at Tape Media

The media where data is stored on a tape drive issurprise!magnetic tape. The result of a great deal of chemicals, materials, and manufacturing technology, magnetic tape is pretty amazing and very durable, as long as it is not abused. Magnetic tape is constructed in four basic layers:

Backing
Binder
Magnetic material
Coating

The backing of a tape is the foundation material that gives the tape its inherent flexibility and strength. In addition, backing provides a magnetic barrier so that signals from one section of tape do not "print through" onto adjacent sections of the tape when tapes are rolled up tightly and stored for long periods of time.

Tape binder is the flexible glue-like material that adheres to the backing as well as the magnetic material.

The magnetic materials in tape are where the action is, of course, and where data is written and read. The magnetic properties are provided by fine metal oxides, which are smooth to the human eye but somewhat jagged and rough at a microscopic level.

The coating layer levels the surface of the tape and provides a smoother surface for running over the tape heads. Without the coating layer, wear and tear on tape heads would be excessive.

Caring for Tape

In general, tapes deteriorate slowly over time. They develop cracks in the surface, they tear along the edges, and the metal oxides corrode. It is important to use and store data tapes under conditions of moderate temperature and low humidity. This includes tapes that have not been used yet but are being stored for future use. Tape deterioration that starts before data is written to it can lead to data loss later, even if the tapes are well taken care of after they have been used.

Caring for Tape Heads

Unlike disk drive heads that float at microscopic levels above the platter, tape heads are designed to be in contact with the tape when reading or writing data. As a result, tape heads eventually wear out over time due to the constant friction of regular operations.

Tape heads should be cleaned after every 30 hours of use. Unlike disk drives, which have limited exposure to airborne particles, tape drives are exposed when tapes are removed and inserted. This makes it practically impossible to keep particulate matter away from the tape heads, and that's why it's important to operate tape drives in a clean environment. Also, tapes shed fine pieces inside the tape drive as they are run through the tape transport, especially when they are new. This material sticks to the tape heads and reduces their effectiveness. Dirty tape heads are often reported by backup software as tape failures, even though there might be nothing wrong with the tapes you are using.

Tape Drive Performance

Tape drives have wide performance ranges based on two variables:

A sufficient amount of data being transferred
The compressibility of data

Streaming and Start-Stop Operations

Unlike disk drives, tape drives run at different speeds. A drive's streaming transfer rate represents the speed at which data is written from buffers onto tape. Obviously, this implies that a sufficient amount of data is being written into the drive's buffers by host or subsystem controllers.

Start-stop operations occur when there is insufficient data to maintain streaming mode operations. Start-stop speeds are typically far less than streaming speeds because the tape has to be stopped, rewound slightly, and started up again. Unlike analog recording, where it is no problem to have "dead air," digital recording cannot have undefined gaps. Therefore, if there is no data to record, the tape drive must stop and reposition the tape appropriately for when there is more data to write. This starting and stopping is obviously detrimental to performance. Larger buffers can help, but they do not solve the problem of having too little data to write.

Compression

Tape drives typically incorporate compression technology as a feature to boost data transfer rates. Compression can increase performance several times beyond native (uncompressed) data transfer rates, but that depends on how much the data can be compressed. Different types of data vary a great deal in this respect; multimedia data might not compress at all, and database data might compress many times. Tape drives in storage networks should be able to support native streaming transfer rates of at least 10 Mbps.

The Tale of Two Technologies

The history of tape technology is littered with many obsolete technologies that faded fast. People have protested about the interoperability and compatibility problems of tape technologies for many years, and the situation is no different today. Tape drives used in storage networks can be divided into two broad technology areas, with two contestants in each areaall of them being incompatible with the others.

In the sections that follow, we will briefly look at both technologies.

Linear Tape Technology

Linear tape reads and writes data just as it soundsby placing "lines" of data that run lengthwise on the tape media. Linear tape drives use multiple heads operating in parallel, reading and writing data simultaneously. They tend to have very high transfer rates and capacities.

Tape used in linear tape drives is .5 inches wide, leaving a great deal of room for capacity improvements over time. Linear tape cartridges have only one spool to hold tape. When the tape is loaded, the outside end of the tape is fed into the drive and wrapped around a take-up spool inside the drive.

There are two primary, competing linear tape technologies:

Super digital linear tape (SDLT)
Linear tape open (LTO)

SDLT has its technology roots in the digital linear tape technology developed by Digital Equipment Corporation many years ago for its VAX line of computers. LTO is a relatively new type of tape technology that was jointly developed by IBM, HP, and Seagate.

Helical Scan Tape Technology

Helical scan tape technology was originally developed for video recording applications. Most helical scan tape drives used for data storage applications use 8 mm tape, which is approximately .25 inches wide. Helical scan drives write data in diagonal strips along the tape.

In general, helical scan tape cartridges are much smaller than linear tape cartridges, even though they have two reels, unlike linear tape. Data density with helical scanning technology is typically better, although helical scan tape cartridges usually hold less data than linear tape cartridges.

Two primary helical scan technologies are used in storage networking environments: Mammoth-2 and AIT-3. Mammoth tape technology was developed by Exabyte Corporation in the mid-1990s in response to DLT's growing popularity and success. At this point it seems to have been eclipsed in functionality by AIT, LTO, and DLT as the preferred technologies for storage network environments.

Sony's Advanced Intelligent Tape (AIT) was also developed as a new data recording technology in the mid 1990s in response to the need for higher reliability, performance, and capacity. AIT included many innovations, including a memory chip embedded in the tape cartridge that could be used by storage applications.

Comparing Tape Technologies

Table 4-3 compares the leading tape technologies used in storage networks.

Table 4-3. Comparison of Tape Technologies Used in Storage Networks
Technology	Linear or Helical	Capacity (Native/Compressed)	Maximum Transfer Rate (Native/Compressed)
Mammoth-2	Helical	60 GB/150 GB	12 Mbps/30 Mbps
AIT-3	Helical	100 GB/250 GB	12 Mbps/30 Mbps
SuperDLT	Linear	110 GB/220 GB	10 Mbps/20 Mbps
LTO Ultrium	Linear	340 GB/680 GB	20 Mbps/40 Mbps