Backup Media

Selecting backup media is as critical as selecting the program to perform the backup. Your backup media should be removable so that the information can be taken to another site for safe storage in case of fire, flood, or other natural disaster. In some cases, the backup medium is simply another system on the network that's located in a different building from the primary data. Most backup systems, however, use tape media. Magnetic tape still provides the lowest cost per megabyte for storing data. Table 7.1 lists some typical tape devices that are used for storing backed-up data.

Table 7.1. Tape Device Types

Media Type	Capacity
1/2-inch reel tape	140MB (6250BPI)
1/4-inch cartridge tape (QIC)	8GB
8mm cartridge tape	40GB70GB
4mm DAT cartridge tape (DDS-DDS4)	1GB40GB
DLT (Digital Linear Tape) 1/2-inch cartridge tape	20GB400GB
SDLT (Super Digital Linear Tape) cartridge tape	160 GB320GB
LTO (Linear Tape Open) cartridge tape	100GB200GB

To achieve high capacity, one or more of these tape drives are often combined into cabinets called tape libraries or tape silos, which are capable of storing several terabytes of data, spread across tens or even hundreds of tapes. Robotic arms are used to locate, retrieve, and load tapes into a tape drive automatically to eliminate human intervention.

Tape Device Names

Chapter 1, "Managing File Systems," describes disk device names. Tape drives are also accessed through their logical device names. In fact, for each tape drive, you see 24 different logical device files assigned to each tape drive. These device files are located under the directory /dev/rmt and are comprised of numbers and letters:

/dev/rmt/#cn

The following are the numbers and letters in the device files:

• #This number refers to the tape drive's logical device number. The first tape drive found by the system is given the designation 0, the second is 1, the third is 2, and so forth. These numbers do not correspond to SCSI ID numbers.

• cThe letter following the device number is the tape density. This can be l (low), m (medium), h (high), c (compressed), or u (ultra compressed).

• nIf an n is present after the tape density letter, it means "no rewind." Sometimes after a tape drive is finished, you do not want the tape to automatically rewind. If the n is not present in the device name, the tape automatically rewinds when the backup is complete.

Table 7.2 describes 24 device files for a tape drive with a logical device number of 0. Beside each device file listed is a description of what the letters mean. All these device files contain different attributes but refer to the same physical tape drive, 0, and can be used at will.

Table 7.2. Tape Logical Device Files

Device Name	Description
0	SystemV (SysV)-style rewinding device with no compression. This is the standard tape device. When you use this device name, the tape rewinds when complete.
0b	Berkeley-style rewinding tape device with no compression.
0bn	Berkeley-style nonrewinding tape device with no compression.
0c	SysV-style rewinding tape device with compression.
0cb	Berkeley-style rewinding tape device with compression.
0cbn	Berkeley-style nonrewinding tape device with compression.
0cn	SysV-style nonrewinding tape device with compression.
0h	SysV-style rewinding tape device with high density.
0hb	Berkeley-style rewinding tape device with high density.
0hbn	Berkeley-style nonrewinding tape device with high density.
0hn	SysV-style nonrewinding tape device with high density.
0l	SysV-style rewinding tape device with low density.
0lb	Berkeley-style rewinding tape device with low density.
0lbn	Berkeley-style nonrewinding tape device with low density.
0ln	SysV-style nonrewinding tape device with low density.
0m	SysV-style rewinding tape device with medium density.
0mb	Berkeley-style rewinding tape device with medium density.
0mbn	Berkeley-style nonrewinding tape device with medium density.
0mn	SysV-style nonrewinding tape device with medium density.
0n	SysV-style nonrewinding tape device with no compression.
0u	SysV-style rewinding tape device with ultra compression.
0ub	Berkeley-style rewinding tape device with ultra compression.
0ubn	Berkeley-style nonrewinding tape device with ultra compression.
0un	SysV-style nonrewinding tape device with ultra compression.