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Choices in Backup Devices and Media

Floppy disks

On PCs, diskettes are often used for backups. Their low storage capacity makes this an impractical means of doing a full backup on a Unix workstation. However, this method is better than nothing and can be used in a pinch for individual files or directories. They are inexpensive and can be fairly reliable if stored correctly.

1. How should floppy disks be stored?

Magneto optical and floptical disks

Magneto optical disks are the same length and width as a 5 1/4 inch floppy disk, but thicker. They can store 128MB-1.2GB of raw data. They can be more stable than magnetic media, such as tapes, because they are written magnetically and read optically. This means that reading does not degrade the stored data. The drives are quite expensive as are the disks themselves.

Floptical disks utilize the same technology as magneto optical disks. They have a smaller storage capacity of 21-120MB of raw data. Most floptical drives can read and write to basic floppy diskettes.

Optical disks

Optical disks have a storage capacity from 128MB to 2GB on a 4.6-inch compact disk. They use a laser for writing and reading to the disk. They offer high capacity storage but are 2 to 3 times slower than hard drives. There are three primary types used for storage:
CD-ROM (compact disk - read only memory)
CD-ROM is the most common optical disk type, and is used by the music industry as well. This is not useful for backups but is a good choice for archiving large pools of static data.
WORM (write once read many)
A write-once CD-ROM drive is another viable, although expensive, backup option. Recordable CDs are usually less expensive than optical disks. Once written, the data is permanently recorded. Current raw storage capacities are from 540 to 640 MB.
Rewritable optical disks
Rewritable optical disks typically are commonly used for data backup and archiving data. The drives and disk are generally fairly costly but both fast and reliable.

Hard drives and disks

A hard drive can also be used to create a disk image backup, where all the data on one hard disk is simply copied to another hard disk. The second disk can be used as a backup if the first drive should fail. With the recent reduction in the cost of hard drives this option is more attractive. However, there are a couple problems with this method. For example, since it is difficult to store (multiple) disk drives off-site, this would not be a good backup method in a disaster-recovery situation. Previous versions of the same file would be unavailable.

Magnetic tapes

Magnetic tape is the most realistic medium for creating Unix backups. The tape is actually a mylar film strip on which information is stored. It is the traditional backup medium that has been in use for years. Magnetic tapes are a sequential storage device. Since tape drives cannot randomly access data like other storage devices, such as disk drives, they are much slower. However, high storage capabilities and low cost make magnetic tapes the storage medium of choice for archiving large amounts of data.

9-track tape (also called half-inch tape) is the old standard in magnetic tape storage. It consists of half-inch tape wound on a circular reel. Although these tapes are still in use, they are extremely bulky and the storage capacity is small by today's standards. A 9-track tape will only hold around 225MB at the highest density.

QIC (quarter inch cartridge) tapes are reliable and were widely used several years ago. The drives are inexpensive, but slow. Current storage capacity of QIC tapes is up to 2GB, however, more common capacities are 150MB, 320MB, and 525MB.

DAT (digital audio tape) or helical scan devices come in two standard sizes, 8mm and 4mm. 4mm DAT's support storage capacities from 1-8 GB, while 8mm DAT's support storage capacities from 2-10 GB. 8mm and 4mm tapes are most common on newer systems. 4mm tapes are physically the smallest of the magnetic tapes and therefore take up less storage room. The only disadvantage of these tapes seems to be that they are more sensitive to heat damage than other types of tape. 8mm and 4mm tapes come in two grades; one for video/audio recording and one for binary data. The video/audio tapes may work for making backups, but they are less reliable in terms of retaining data. The binary grade tapes are a better choice. The 4mm is currently the most widely used but is being replaced by DLT.

DLT (digital linear tapes) have a storage capacity of up to 40GB with compression. The drives are quite fast and are the newest standard backup media technology. For recommended reading on the Quantum Corporation's DLT technology, see, DLT Technology -- Delivering Data Protection You Can Depend On and DLT Frequently Asked Questions.

2. Which type of magnetic tape is the best choice for unattended backups?

Jukeboxes, Stackloaders, etc

Jukeboxes and stackloaders are designed to automate the handling of media to single or multiple DAT, DLT, or optical drives. They are also known as tape or optical libraries. These devices are able to load and unload tapes into removable media drives on an as needed basis.

Terms used: floptical, magneto optical disk, jukebox, stackloader, compression.

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