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 The physically larger DC600 cartridge is obsolete.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.
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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.
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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.
Table 9-1. Key selection criteria for Travan drives|
Capacity (MB)
|
400
|
800
|
1600
|
4000
|
4000
|
10,000
|
10,000
|
20,000
|
Sustained transfer rate (MB/min)
|
3.75
|
3.75
|
7.5
|
30
|
36
|
60
|
60
|
120
|
Uncorrectable error rate
|
10-14
|
10-14
|
10-14
|
10-15
|
10-15
|
10-15
|
10-15
|
10-15
|
IDE/SCSI interface
|
/
|
/
|
/
|
/
|
/
|
/
|
/
|
/--
|
USB/Parallel interface
|
--
/
|
--
/
|
--
/
|
/
|
/--
|
/--
|
/--
|
/--
|
Read-while-write?
|
--
|
--
|
--
|
--
|
|
--
|
|
--
|
HW compression?
|
--
|
--
|
--
|
--
|
|
--
|
|
--
|
Native QIC format
|
3010
|
3020
|
3080
|
3095
|
3095
|
3220
|
3220
|
Travan 40
|
Can read/write QIC-40 tapes?
|
/--
|
--
/--
|
--
/--
|
--
/--
|
--
/--
|
--
/--
|
--
/--
|
--
/--
|
Can read/write QIC-80 tapes?
|
/
|
/--
|
/--
|
--
/--
|
--
/--
|
--
/--
|
--
/--
|
--
/--
|
Can read/write QIC-3010 tapes?
|
/
|
/
|
/
|
--
/--
|
--
/--
|
--
/--
|
--
/--
|
--
/--
|
Can read/write QIC-3020 tapes?
|
--
/--
|
/
|
/
|
/--
|
/--
|
--
/--
|
--
/--
|
--
/--
|
Can read/write QIC-3080 tapes?
|
--
/--
|
--
/--
|
/
|
/
|
/
|
--
/--
|
--
/--
|
--
/--
|
Can read/write QIC-3095 tapes?
|
--
/--
|
--
/--
|
--
/--
|
/
|
/
|
/--
|
/--
|
--
/--
|
Can read/write QIC-3220 tapes?
|
--
/--
|
--
/--
|
--
/--
|
--
/--
|
--
/--
|
/
|
/
|
/--
|
Can read/write Travan 40 tapes?
|
--
/--
|
--
/--
|
--
/--
|
--
/--
|
--
/--
|
--
/--
|
--
/--
|
/
|
Drive cost (US$)
|
NLA
|
NLA
|
NLA
|
NLA
|
NLA
|
NLA
|
~ 225
|
~ 325
|
Tape cost (US$)
|
NLA
|
NLA
|
25
|
24
|
24
|
35
|
35
|
50
|
Tape cost/GB (US$)
|
n/a
|
n/a
|
15.63
|
6.00
|
6.00
|
3.50
|
3.50
|
2.50
|
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 $50 tape. The good news is that those
hideously expensive tapes are guaranteed for life, although we
wouldn't use any tape heavily for more than a couple
of years even if our own mothers guaranteed it. Travan 40 drives are
obviously targeted at high-end desktop systems rather than servers.
They do not support read-while-write or hardware data compression,
and are available only with an ATAPI interface.
Table 9-2 lists the key characteristics of ADR
tape drives. First-generation ADR drives use the original ADR30 or
ADR50 tapes. ADR30 models are still sold, targeted at price-sensitive
buyers and desktop PCs. Second-generation
ADR2 drives are considerably more
expensive, use higher-capacity ADR2.60C or ADR2.120C tapes, and are
marketed as an inexpensive, high-capacity alternative to DDS
libraries or AIT drives for network servers.
Table 9-2. Key selection criteria for ADR drives|
Capacity (MB)
|
15,000
|
25,000
|
30,000
|
60,000
|
Sustained transfer rate (MB/min)
|
120
|
120
|
240
|
480
|
Uncorrectable error rate
|
10-19
|
10-19
|
10-19
|
10-19
|
IDE/SCSI interface
|
/
|
--
/
|
/
|
--
/
|
USB/Parallel interface
|
/
|
--
/--
|
/--
|
--
/--
|
Read-while-write?
|
--
|
--
|
--
|
--
|
HW compression?
|
--
|
|
--
|
--
|
Can read/write ADR30 tapes?
|
/
|
/
|
--
/--
|
--
/--
|
Can read/write ADR50 tapes?
|
--
/--
|
/
|
--
/--
|
--
/--
|
Can read/write ADR2.60C tapes?
|
--
/--
|
--
/--
|
/
|
/
|
Can read/write ADR2.120C tapes?
|
--
/--
|
--
/--
|
--
/--
|
/
|
Drive cost (US$)
|
225 - 350
|
NLA
|
400 - 650
|
750 - 900
|
Tape cost (US$)
|
35
|
50
|
55
|
80
|
Tape cost/GB (US$)
|
2.33
|
2.00
|
1.83
|
1.33
|
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.
Table 9-3. Key selection criteria for DDS and AIT drives|
Capacity (GB)
|
1.3/2
|
4
|
12
|
20
|
25/35
|
36/50
|
100
|
Sustained transfer rate (MB/min)
|
36
|
36
|
72
|
144
|
240
|
360
|
720
|
Uncorrectable error rate
|
10-15
|
10-15
|
10-15
|
10-15
|
10-17
|
10-17
|
10-17
|
Can read/write DDS-1 tapes?
|
/
|
/
|
/
|
/
|
--
/--
|
--
/--
|
--
/--
|
Can read/write DDS-2 tapes?
|
--
/--
|
/
|
/
|
/
|
--
/--
|
--
/--
|
--
/--
|
Can read/write DDS-3 tapes?
|
--
/--
|
--
/--
|
/
|
/
|
--
/--
|
--
/--
|
--
/--
|
Can read/write DDS-4 tapes?
|
--
/--
|
--
/--
|
--
/--
|
/
|
--
/--
|
--
/--
|
--
/--
|
Can read/write AIT-1 tapes?
|
--
/--
|
--
/--
|
--
/--
|
--
/--
|
/
|
/
|
/
|
Can read/write AIT-2 tapes?
|
--
/--
|
--
/--
|
--
/--
|
--
/--
|
--
/--
|
/
|
/
|
Can read/write AIT-3 tapes?
|
--
/--
|
--
/--
|
--
/--
|
--
/--
|
--
/--
|
--
/--
|
/
|
Drive cost (US$)
|
NLA
|
NLA
|
~ 425
|
~ 550
|
~ 700
|
~ 1,000
|
~ 2,000
|
Tape cost (US$)
|
~ 4
|
~ 4
|
~ 6
|
~ 12
|
~ 50
|
~ 60
|
~ 75
|
Tape cost/GB (US$)
|
~ 2.00
|
~ 1.00
|
~ 0.50
|
~ 0.60
|
~ 1.43
|
~ 1.20
|
~ 0.75
|
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