Pros and Cons of Tape

Tape might be the most common backup medium. Tape drives vary in capacity from a few megabytes to 160GB uncompressed, so a single tape is often large enough to hold a backup of an entire computer. The tapes themselves are also usually fairly inexpensive, ranging in price from a few dollars to a few tens of dollars, depending on the type and capacity. The initial prices for tape drives can range from about $250 to several thousand dollars. Many of the multithousand-dollar drives are actually changers. They can automatically swap tapes in and out, effectively increasing the drive's capacity to several times the capacity of a single tape, often in the terabyte range. Even aside from the multi-tape needs of a changer, a typical tape backup system is likely to need between half a dozen and several dozen tapes, depending upon tape capacity, backup frequency, and how long you plan to keep the backups. Between a drive and enough tapes to last for a year or two, you should plan to spend the better part of $1,000 on a low-end tape system capable of storing 10-40GB uncompressed. The total cost goes up as the system's capacity does, but the cost to store 100GB or so probably won't be substantially over $1,500.

Warning Tape drive manufacturers often quote their drives' estimated capacities with compression enabled. The amount of compression you get varies depending on the data you back up, so take these estimates with a grain of salt. Uncompressed capacity is usually about half the claimed compressed capacity.

A variety of tape formats are available. These formats differ in capacity range, price, reliability, and features. Table 17.1 summarizes some of the characteristics of some of the more popular formats. The prices are typical of what I found advertised by several web merchants in early 2003; they might change by the time you read this. The capacities and prices (including media prices) are for drives that were available when I checked websites; most format families include older lower-capacity variants. Some drives can use these older media, and these media may cost less than indicated in Table 17.1. This table summarizes the prices for single-tape units; changers for most formats are available, but they cost more than indicated.

Table 17.1: Vital Statistics for Several Popular Tape Formats

Drive Type

Uncompressed Capacity

Drive Cost

Media Cost

Speed

8mm

20-60GB

$2,000-3,500

$40-80

3-12MB/S

AIT

35-100GB

$700-3,500

$55-120

3-12MB/S

DAT

4-20GB

$450-1,200

$6-30

1.5-5MB/S

DLT and SuperDLT

40-160GB

$1,000-$4,500

$50-170

3-16MB/S

Travan

10-20GB

$250-550

$35-50

0.6-2MB/S

VXA

33-80GB

$550-1,300

$70-100

3-6MB/S

Travan and its predecessor, Quarter-Inch Cartridge (QIC), have traditionally dominated the low end of the tape drive backup arena, in part due to the low cost of the drives. Although it's traditionally been aimed at a slightly higher market, Digital Audio Tape (DAT) is now attractive for low-end uses, due in part to the low cost of DAT media. Formats such as AIT and DLT, which are available in 100GB or higher capacities, are attractive for network backup servers and even workstations with unusually large storage needs, but the cost of these drives and their media can be prohibitive for smaller workstations.

Most tape drives use SCSI or ATA interfaces. Typically, higher-end units use SCSI, and a few low-end models use ATA. Some external drives also use USB or IEEE-1394. In the past, low-end drives sometimes used the floppy or parallel-port interfaces, but such devices are now relics. SCSI drives use two device files; /dev/stO causes the drive to rewind after each use and /dev/nstO doesn't. ATA drives use /dev/htO and /dev/nhtO, respectively, for these functions. In both cases, if your system has more than one tape drive, the trailing number increases from 0 to 1 and higher for the second and later drives. External USB or IEEE-1394 drives look like SCSI drives to Linux.

One of the problems with tapes is that they aren't extremely reliable. Individual experiences differ, but most people who use tapes find that the media fail on occasion. Drives sometimes have problems reading tapes written on other drives, too. Tape is also a sequential-access medium, meaning that you must read through all of a tape's contents to get to data toward the end of the tape. (Some formats provide shortcuts in some situations, though.)

Despite tape's problems, it remains a popular backup medium because of the high capacity of individual tapes and the low cost of tapes on a per-gigabyte basis. Given hard disk pricing trends, though, tape is starting to lose ground to removable disks.

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