9.1 Tape Technologies

Four tape technologies compete in the PC and small server markets:

Quarter Inch Cartridge (QIC)

Originally developed in the early 1970s, two styles of QICs have been made. The physically larger DC600 cartridge is obsolete. Recent QIC tape drives use DC2000 minicartridges, which have been made in capacities from 400 MB to 20 GB. QIC drives use serpentine recording, which records many parallel tracks on each tape. The drive records data from the beginning to the end of the first track, reverses direction, writes data from the end to beginning of the second track, and so on, until all tracks have been written. This means that filling a tape may require more than 100 passes of the tape through the drive, which increases wear and tear on both drive and tape. Some recent QIC drives have the extra head required for read-while-write, which allows the drive to back up and compare data in one pass. Doing a compare on a single-head drive doubles the number of passes required, and extends backup time significantly.

Current QIC drives use Travan technology, a combination of tape and drive technologies developed by 3M/Imation, and now implemented by many drive manufacturers. Travan-NS (Network Solution) drives provide read-while-write verification and hardware compression, which allows the drive to compress data as it writes it, rather than depending on compression performed by the backup software. The most recent Travan technology, called Travan 40, is targeted at desktop PCs and small servers, and does not include read-while-write verification or hardware compression. Travan drives are relatively inexpensive, provide high capacity and performance, and are available in IDE, SCSI, USB, FireWire, and parallel interfaces. Travan drives are well-supported by all major operating systems, including Linux. The major drawback of Travan is the relatively high cost of tapes?$25 to $50, depending on capacity.

Advanced Digital Recording (ADR)

Advanced Digital Recording, or ADR, is a proprietary technology based on a patent portfolio held by Philips, and is best known for its use in tape drives made by OnStream. ADR writes eight tracks simultaneously, which allows it to provide high throughput while running the tape very slowly. That in turn means ADR drives are quieter and minimize tape wear relative to other serpentine tape technologies such as Travan. ADR equals or betters Travan in most other respects as well, including capacity, throughput, reliability, and operating system support. Unfortunately, the price of ADR tapes is also comparable to that of Travan tapes.

Digital Data Storage (DDS)

Digital Data Storage, or DDS, is often incorrectly called Digital Audio Tape (DAT). Actually, there is a technology called DataDAT, but it's nonstandard, and nearly all drives use the DDS standard instead. DDS drives use helical-scan recording similar to that used by a VCR. The recording head rotates at an angle relative to tape movement and lays down a series of short diagonal tracks across the full width of the tape. This means a DDS drive can theoretically fill a tape during one pass, although real-world drives may require several passes to do so. Relative to serpentine drives, the lower tape speed and smaller number of passes mean that DDS drives incur much less wear on both drive and tape during a backup pass, but the more complex tape path offsets this advantage somewhat. Nearly all DDS drives support read-while-write. DDS drives provide high capacity and performance and are well-supported by Windows and Linux, but are relatively expensive and require a SCSI interface. The major advantage of DDS drives is that they use relatively inexpensive tapes, typically $3 to $15. DDS drives are most appropriate for servers and workstations that use a tape rotation scheme that requires many tapes. We also consider DDS drives appropriate for desktop PCs that store large amounts of high-value data.

Advanced Intelligent Tape (AIT)

Advanced Intelligent Tape, or AIT, was developed by Sony and is a proprietary technology that uses 8mm tape in a 3.5-inch form factor cartridge. Sony and Seagate produce AIT tape drives, which are more expensive than DDS drives but provide higher capacity and sustained transfer rates, better error correction, and faster random access. AIT drives are self-cleaning, which is a significant advantage in server environments, albeit less so for desktop systems. AIT is well known in the server and workstation markets, but the relatively high cost of AIT drives limited their acceptance in the PC market until recently.

Many AIT advantages result from MIC (Memory In Cassette), a technology that is built into AIT drives and tapes. DDS and other older tape technologies store the Table of Contents (TOC) and other tape information on the tape itself, which means the tape must be rewound each time the TOC needs to be written or read. MIC is a 64 KB EEPROM embedded in the AIT tape. MIC contains the TOC, which can be read or written without moving the tape, which allows an AIT drive to load, unload, and search tapes much faster. MIC also stores data about the number of times the tape has been loaded and how frequently each area of the tape has been written, which contributes to increased reliability.

AIT is available in three variants, known as AIT-1, AIT-2, and AIT-3, that differ in capacity and throughput. AIT drives are well-supported by Windows and Linux. AIT-1 drives are now inexpensive enough that they are a reasonable choice to back up workstations, small servers, and high-end PCs if a less expensive DDS-3 or DDS-4 drive has insufficient capacity.

Some readers point out that the cost of a tape drive and tapes may exceed the cost of a PC. That is true, but immaterial. What matters is not the cost of the PC or the tape drive, but the value of the data that tape drive protects. The key issues that determine if tape is an appropriate choice for backing up are how much data needs to be protected, how often the data changes, and the cost of reconstructing the data if it were lost. If you have relatively little data that changes infrequently and is of little value, backing up to CD-R or writable DVD is a less expensive alternative. But if you have a lot of data that changes frequently and is of high value, tape is by far the best choice. Not necessarily the cheapest, but the best.

Table 9-1, Table 9-2, and Table 9-3 list key selection criteria for typical tape drives that use these four technologies. Capacity and sustained transfer rates are native, and assume no compression. Uncorrectable error rate is the minimum number of bits the drive is expected to read successfully before encountering an unrecoverable error. An unrecoverable error is one for which the drive cannot reconstruct the data from ECC data. When that happens, the original data is irretrievably lost. For example, a typical tape drive has an uncorrectable error rate of 10^-15. That means the drive on average reads 10¹⁵ bits (more than 100,000 GB) before it encounters a read error that cannot be corrected using the ECC data stored on the tape. For comparison, typical hard drives have an uncorrectable error rate of 10^-14 and typical CD writers 10^-12. That means in the course of reading the same amount of data the hard drive generates 10 times as many uncorrectable errors as the tape drive, and the CD writer 1,000 times as many. Items listed as "NLA" are no longer readily available.

In the preceding edition we said we didn't expect to see Travan TR-6 drives because they would cost as much as DDS-4 drives and require very expensive tapes. Seagate proved us (partially) wrong by shipping Travan 40 tape drives in mid-2002, just as that edition hit the stores. Oh, well.

Travan 40 drives are about half the price of DDS-4 drives, and store 20 GB (40 GB compressed) on a $50 tape. The good news is that those hideously expensive tapes are guaranteed for life, although we wouldn't use any tape heavily for more than a couple of years even if our own mothers guaranteed it. Travan 40 drives are obviously targeted at high-end desktop systems rather than servers. They do not support read-while-write or hardware data compression, and are available only with an ATAPI interface.

Table 9-2 lists the key characteristics of ADR tape drives. First-generation ADR drives use the original ADR30 or ADR50 tapes. ADR30 models are still sold, targeted at price-sensitive buyers and desktop PCs. Second-generation ADR² drives are considerably more expensive, use higher-capacity ADR2.60C or ADR2.120C tapes, and are marketed as an inexpensive, high-capacity alternative to DDS libraries or AIT drives for network servers.

Table 9-3 lists the key characteristics of DDS and AIT drives. Other than one off-brand ATAPI DDS model, all DDS and AIT drives we know of use some form of SCSI interface. DDS-1 was originally designated simply DDS. When DDS-2 drives became available, DDS drives were renamed DDS-1 to differentiate them. DDS-1 drives come in two variants. The original DDS-1 drives did not support hardware compression. DDS-1 drives with hardware compression added are called DDS-DC drives. All DDS-1 drives can use 60-meter DDS-1 tapes, which store 1.3 GB natively, and 90-meter DDS-1 tapes, which store 2.0 GB natively. Other than non-DC DDS-1 drives, all DDS and AIT drives support read-while-write and hardware compression. All DDS and AIT drives can read and write tapes based on earlier standards, except that DDS-4 drives cannot use 60-meter DDS-1 tapes.