Storage and File Structure:Tertiary Storage

Tertiary Storage

In a large database system, some of the data may have to reside on tertiary storage. The two most common tertiary storage media are optical disks and magnetic tapes.

Optical Disks

Compact disks are a popular medium for distributing software, multimedia data such as audio and images, and other electronically published information. They have a fairly large capacity (640 megabytes), and they are cheap to mass-produce.

Digital video disks (DVDs) are replacing compact disks in applications that require very large amounts of data. Disks in the DVD-5 format can store 4.7 gigabytes of data (in one recording layer), while disks in the DVD-9 format can store 8.5 gigabytes of data (in two recording layers). Recording on both sides of a disk yields even larger capacities; DVD-10 and DVD-18 formats, which are the two-sided versions of DVD-5 and DVD-9, can store 9.4 gigabytes and 17 gigabytes respectively.

CD and DVD drives have much longer seek times (100 milliseconds is common) than do magnetic-disk drives, since the head assembly is heavier. Rotational speeds are typically lower than those of magnetic disks, although the faster CD and DVD drives have rotation speeds of about 3000 rotations per minute, which is comparable to speeds of lower-end magnetic-disk drives. Rotational speeds of CD drives originally corresponded to the audio CD standards, and the speeds of DVD drives originally corresponded to the DVD video standards, but current-generation drives rotate at many times the standard rate.

Data transfer rates are somewhat less than for magnetic disks. Current CD drives read at around 3 to 6 megabytes per second, and current DVD drives read at 8 to 15 megabytes per second. Like magnetic disk drives, optical disks store more data in outside tracks and less data in inner tracks. The transfer rate of optical drives is characterized as n×, which means the drive supports transfers at n times the standard rate; rates of around 50× for CD and 12× for DVD are now common.

The record-once versions of optical disks (CD-R, and increasingly, DVD-R) are popular for distribution of data and particularly for archival storage of data because they have a high capacity, have a longer lifetime than magnetic disks, and can be removed and stored at a remote location. Since they cannot be overwritten, they can be used to store information that should not be modified, such as audit trails. The multiple- write versions (CD-RW, DVD-RW, and DVD-RAM) are also used for archival purposes.

Jukeboxes are devices that store a large number of optical disks (up to several hundred) and load them automatically on demand to one of a small number (usually, 1 to 10) of drives. The aggregate storage capacity of such a system can be many terabytes.

When a disk is accessed, it is loaded by a mechanical arm from a rack onto a drive (any disk that was already in the drive must first be placed back on the rack). The disk load/unload time is usually of the order of a few seconds — very much slower than disk access times.

Magnetic Tapes

Although magnetic tapes are relatively permanent, and can hold large volumes of data, they are slow in comparison to magnetic and optical disks. Even more important, magnetic tapes are limited to sequential access. Thus, they cannot provide random access for secondary-storage requirements, although historically, prior to the use of magnetic disks, tapes were used as a secondary-storage medium.

Tapes are used mainly for backup, for storage of infrequently used information, and as an offline medium for transferring information from one system to another.

Tapes are also used for storing large volumes of data, such as video or image data, that either do not need to be accessible quickly or are so voluminous that magnetic disk storage would be too expensive.

A tape is kept in a spool, and is wound or rewound past a read – write head.

Moving to the correct spot on a tape can take seconds or even minutes, rather than milliseconds; once positioned, however, tape drives can write data at densities and speeds approaching those of disk drives. Capacities vary, depending on the length and width of the tape and on the density at which the head can read and write. The market is currently fragmented among a wide variety of tape formats. Currently available tape capacities range from a few gigabytes [with the Digital Audio Tape (DAT) format], 10 to 40 gigabytes [with the Digital Linear Tape (DLT) format], 100 gigabytes and higher (with the Ultrium format), to 330 gigabytes (with Ampex heli- cal scan tape formats). Data transfer rates are of the order of a few to tens of megabytes per second.

Tape devices are quite reliable, and good tape drive systems perform a read of the just-written data to ensure that it has been recorded correctly. Tapes, however, have limits on the number of times that they can be read or written reliably.

Some tape formats (such as the Accelis format) support faster seek times (of the order of tens of seconds), which is important for applications that need quick access to very large amounts of data, larger than what would fit economically on a disk drive. Most other tape formats provide larger capacities, at the cost of slower access; such formats are ideal for data backup, where fast seeks are not important.

Tape jukeboxes, like optical disk jukeboxes, hold large numbers of tapes, with a few drives onto which the tapes can be mounted; they are used for storing large volumes of data, ranging up to many terabytes (1012 bytes), with access times on the order of seconds to a few minutes. Applications that need such enormous data storage include imaging systems that gather data by remote-sensing satellites, and large video libraries for television broadcasters.