Chapter 9. Tape Drives

Although the cost and capacity of other removable magnetic storage devices continues to improve, tape drives remain the best choice to back up data or to transfer very large amounts of data between systems. Tape drives provide a combination of high capacity, speed, low media cost, and reliability that no other technology can match. Two tape technologies compete for the standalone PC and small-network market:

Quarter Inch Cartridge (QIC)

Originally developed in the early 1970s, two styles of QIC cartridges exist. The DC600 is physically larger and is now obsolescent. Recent QIC drives use DC2000 mini-cartridges, which are available in a wide variety of incompatible types and capacities. 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 50 or more 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. The new generation Travan-NS (Network Solution) drives provide read-while-write verification and hardware compression, which allows the drive itself to compress data as it writes it, rather than depending on compression performed by the backup software. Travan drives are relatively inexpensive, provide high capacity and performance, and are available in IDE, SCSI, USB, FireWire, and parallel interfaces. The major practical drawback of Travan is the relatively high cost of tapes, typically $25 to $40, depending on capacity.

Digital Data Storage (DDS)

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 that a DAT drive can theoretically fill a tape during one pass, although real-world drives may require several passes to do so. The lower tape speed and smaller number of passes means 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, 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 used to recommend OnStream tape drives, which offer a unique combination of high capacity and performance with low price. OnStream drives use ADR, a proprietary technology developed by Philips. ADR writes eight tracks simultaneously, which allows it to provide high throughput while running the tape very slowly, which in turn means ADR drives are quieter and minimize tape wear relative to other drive technologies. For complete details on ADR, see http://www.onstream.com/adr/index.html. Unfortunately, OnStream declared bankruptcy and, although the parent company has since reincarnated the company under a slightly different name, OnStream drives and tapes now have very limited distribution. Accordingly, although we like the technology and are impressed with the OnStream drives we have in-house, we are unable to recommend these drives in good conscience. We hope that OnStream's fortunes will improve and that they will again become a major player in the low-end tape drive segment. But unless that happens, we suggest you avoid OnStream drives.

Table 9-1 and Table 9-2 list the important characteristics of Travan and DDS tape drives, the two major tape technologies suitable for backing up standalone PCs and small networks. All figures are native, and do not include the 2:1 compression assumed by all tape manufacturers, which doubles capacities and transfer rates. Other tape technologies (e.g., 8mm, Mammoth, DLT, and AIT) are available, but are quite expensive, targeted at large-scale networks, and beyond the scope of this book. For data density, "ftpi" means flux transitions per inch, and is a measure of how closely packed the data is on the tape. Items marked "NLA" are no longer readily available.

Table 9-1. Key characteristics of Travan drives for typical drive models

	TR-1	TR-2	TR-3	TR-4	NS8	TR-5	NS20
Capacity (MB)	400	800	1,600	4,000	4,000	10,000	10,000
Maximum transfer rate (MB/min)	3.75	3.75	7.5	30	36	60	60
IDE/SCSI interface	/	/	/	/	/	/	/
USB/Parallel interface	/	/	/	/	/	/	/
Read-while-write
HW compression
Native QIC format	3010	3020	3080	3095	3095	3220	3220
Tracks	36	50	50	72	72	108	108
Data density (ftpi)	14,700	22,125	44,250	50,800	50,800	50,800	50,800
QIC-40 read/write	/	/	/	/	/	/	/
QIC-80 read/write	/	/	/	/	/	/	/
QIC-3010 read/write	/	/	/	/	/	/	/
QIC-3020 read/write	/	/	/	/	/	/	/
QIC-3080 read/write	/	/	/	/	/	/	/
QIC-3095 read/write	/	/	/	/	/	/	/
QIC-3220 read/write	/	/	/	/	/	/	/
Drive cost (US$)	NLA	NLA	NLA	~ 150	~ 200	~ 250	~ 325
Tape cost (US$)	NLA	NLA	25	24	24	35	35
Tape cost/GB (US$)	n/a	n/a	15.63	6.00	6.00	3.50	3.50

Travan TR-6 drives were to have shipped by early 2001, but so far we've seen no sign of them and we don't expect to. TR-6 drives were to have had up to twice the capacity and throughput of TR-5 drives, but both technical and marketing considerations seem to have put TR-6 on the back burner. In short, TR-6 drives would likely cost as much as DDS drives with similar capacity and performance, but would require much more expensive tapes than comparable DDS-4 drives.

Table 9-2. Key characteristics of DDS drives for typical drive models

	DDS-1	DDS-2	DDS-3	DDS-4
Capacity (GB)	2	4	12	20
Maximum transfer rate (MB/min)	36	36	72	144
Tape length (metres)	90	120	125	150
Drive cost (US$)	~ $ 400	~ $ 500	~ $ 650	~ $ 900
Tape cost (US$)	4	4	8	17
Tape cost/GB (US$)	2.00	1.00	0.67	0.85

This is a simple table because DDS drives are well standardized. With the exception of one off-brand model that uses an ATAPI interface, all DDS drives we know of use some form of SCSI interface and support read-while-write and hardware compression. All DDS drives can read and write tapes based on earlier DDS standards. DDS-5 drives are in development. We could tell you about them, but then we'd have to kill you.