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A Superior Tape Path
A Superior Tape Path: The Industry-Leading Reliability, Speed, and Scalability of the Quantum DLTtape System
One of the critical advantages that distinguish Quantum DLTtape systems from
competing products is the drive's tape path. Multiple tape path components
work together to provide many of the benefits associated with the DLTtape
system, including th industry's highest reliability, fastest transfer rate,
and highest capacity.
Background: The Path To Mission Critical Backup
When it comes to server, network, enterprise, and other mission-critical backup
situations, organizations today can no longer afford to cut corners. With data
storage capacity at mid- to large-scale companies doubling every 12 to 18 months,
these organizations need fast, reliable tape backup that matches their current
needs and provides a path for future growth. The DLTtape system was designed for
demanding, mission-critical backup applications. For instance, DLTtape drives
are tested to perform at 100 percent duty cycle rather than the usual 10-20
percent duty cycle used by other drive manufacturers. In addition, DLTtape
drives provide 200,000 hours MBTF (mean time between failures) and up to
30,000-hour head life. At the high end, the Quantum DLT7000 drive offers an
industry-leading capacity of 35 GB native (70 GB at 2:1 compression) with a
data transfer rate of up to 5 MB/s in native mode (10 MB/s at 2:1 compression).
The DLTtape system tape path is the cornerstone of DLTtape drive reliability and
performance. The elements that comprise DLTtape drive's tape path include the
single-reel cartridge design, dual-motor tape transport system, fixed head guide
assembly, fixed read/write head, and adaptive control techniques. These elements,
together, help give DLTtape drives the best reliability, highest capacity, and
fastest data transfer rate of any mid-range tape backup system on the market
today.
Patented Head Guide Assembly Provides Strong Foundation
The inherent stability and accuracy of the DLTtape system path begins with a solid
foundation: the head guide assembly (HGA; see Figure 1). The patented DLTtape head
guide is a boomerang-shaped aluminum plate with six large bearing-mounted rollers.
These rollers are free-wheeling guides, with the last roller acting as both a guide
and a digital tachometer. The tachometer controls the system¹s two drive motors to
regulate tape position, speed, and tension.
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Figure 1: Patented Head Guide Assembly (HGA) used in all DLTtape drives provides a solid tape path for industry-leading reliability and accuracy. |
The precise head/tape alignment achieved at the factory is maintained by the HGA.
Other tape drives, particularly helical scan systems, have a much longer and complex
tape path than that found in the DLTtape system.
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Figure 2: Helical Scan Tape Path requires a moving head guide assembly that must pull the tape from the cartridge and around a spinning read/write drum. This system results in acute wrap angles and multiple points of tape contact that can adversely affect tape and head wear. |
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Figure 3: The DLTtape System Tape Path through the head guide assembly is a gentle curve that reduces tape wear and ensures tracking accuracy. |
A helical scan system must pull its tape out of the cartridge and nearly all the way
around a cylindrical head (see Figure 2). In addition, the tape path rollers used on
helical scan systems are small and result in acute tape wrap angles that stress the
tape. The DLTtape system tape path rollers are large and are arranged in a gentle arc
along the HGA (see Figure 3). This roller arrangement results in much less wear on the
head and media than in helical scan systems.
The DLTtape head guide assembly is one of the key components that give DLTtape drives
an advantage over other drives on the market today. The DLTtape HGA has evolved to
meet the ever-increasing need for higher capacity drives and higher data density.
The six-roller DLTtape head guide design was first introduced in 1989. The new head
guide provided the accuracy required to increase track density from 48 tracks to 128
racks across the half-inch wide tape. Today, the Quantum DLT7000 drive records 208
tracks across the same half-inch tape.
Single Reel Cartridge
All DLTtape systems feature a single-reel cartridge design that works to enhance tape
path stability. Tape drives such as DAT and 8mm use a two-reel cassette that houses a
supply reel and a take-up reel. In these drives, the tape is pulled from the cassette
by a moving head guide assembly and wrapped around a spinning read/write cylinder as
described earlier. The portion of the tape that needs to be accurately aligned with
the read/write heads is actually suspended between two moving points, and held in place
by a movable guide system. This large number of moving parts make it difficult to
maintain accurate tape alignment.
The DLTtape cartridges, on the other hand, house only the supply reel. The take-up reel
is mounted permanently inside the DLTtape drive. When the DLTtape cartridge is inserted
in the drive, it is automatically held securely in position by two cartridge locks. A
leader from the internal take-up reel is mated with the leader from the supply reel by
means of a patented buckling mechanism and the tape is threaded through the tape path.
This process is controlled by the drive's firmware and two drive motors.
The tape is drawn from a fixed supply reel through a fixed head guide assembly, past an
immovable (during read/write operations) read/write head, to a fixed take-up reel. This
high degree of stability provides a tape path that enhances tracking accuracy and drive
reliability.
An added advantage of the DLTtape drive's single-reel design is the elimination of wasted
space in the tape cartridge. Unlike competing drives that use a two-reel cassette, the
DLTtape cartridges contain only tape and no wasted air space. This design maximizes media
capacity per cartridge and lets users store more data in less space than other tape
systems.
Head Assembly
The characteristics of the read/write head assembly used on DLTtape drives is another
important reason why DLTtape drives have one of the most stable and accurate tape paths
in the industry. As mentioned earlier, unlike the continuously spinning read/write head
used in helical scan systems, the DLTtape drive head is moved only enough to establish
exact track position for each tape pass. At end of each tape pass, a precision stepper
servo is used to move the DLTtape drive's read/write head to the next track position and
to optimize read/write signal strength. This feature enhances tracking accuracy and
improves head wear. In the DLT 7000 drive, the head assembly is also tilted forward or
back at the end of each tape pass. This allows the DLT 7000 drive to record data at an
angle along linear tracks thereby eliminating cross-track interference and increasing
data density.
It's important to note that the DLTtape drive's fixed read/write head is the only point
of contact with the magnetized side of the DLTtape media. The six tape path rollers in
the DLTtape drive's HGA touch only the back of the tape, so wear is kept to an absolute
minimum. The DLTtape drives are one of the only drives on the market that provide this
level of contact-free tape transport. In helical scan systems, the recording side of
the tape makes contact with numerous smaller rollers and with the large cylindrical
read/write head that spins at up to 5,000 rpm's. This large area of contact and high
relative tape speed in helical scan drives result in wear to both tape and head.
The DLTtape drive's read/write head further enhances head/media wear by providing a
contoured profile that minimizes tape deflection at the contact point. Tape/head
contact is further reduced by elevating the DLTtape drive's vertically-aligned
read/write elements on three ìislandsî (see Figure 4). This reduces the point of
contact with the tape recording surface to a bare minimum. Two additional "outrigger"
islands help align the tape with the read/write elements and provide a self-cleaning
function. These non-energized islands continuously wipe contaminants from the tape
as it passes. In fact, this system is so reliable that regularly scheduled head
cleaning is not required for DLTtape drives.
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Figure 4: Self-Cleaning Contoured Head on all DLTtape drives includes two fixed "outriggers" that wipe any debris from tape, reducing head wear. |
A low-stress tape path, contoured head, and self-cleaning design provide DLTtape drives
with 1,000,000 tape passes per cartridge and a head life of 30,000 hours. All DLTtape
cartridges provide a shelf life in excess of 30 years with just a five percent loss in
magnetic strength.
Adaptive Features
The DLTtape drives feature adaptive control techniques that help optimize drive operation
and head/media life by dynamically aligning tracks, optimizing head signal amplitudes, and
controlling tape tension. For instance, a predefined algorithm dynamically controls the
drive motors and adjusts tape tension and speed. One input to this algorithm is a calibrated
write-then-read signal at the beginning of the tape in the "scratch" area. The write
magnetization current is varied over a pre-determined range and the resulting signals are
sampled throughout the range as tension is adjusted. This determines the optimum tape tension
and speed.
A second input to the tension/speed algorithm is the tape linear speed, which is measured
by the tachometer built into the HGA. The DLTtape drive's tachometer-controlled drive
motors act in tandem.
One motor pulls the tape through the tape path while the opposing motor applies just enough
drag to maintain optimal tape tension. When the tape reverses direction, the roles of the
motors reverses, too. The tachometer also keeps track of how much tape is wound onto each
reel and adjusts the speed of the two motors accordingly. To prevent spacing losses, the
tape-to-head contact is precisely controlled through constant tape speed, while the tension
is adjusted to control contact pressure. This produces the optimum tape write/read signals
at a constant linear velocity and at the lowest possible contact pressure. Low tape tension
contributes to the long head life of DLTtape drives.
Another adaptive technique used in DLTtape drives is cache buffering. This feature helps
reduce stops and starts, thereby reducing drive wear and tear. Adaptive techniques enable
the DLTtape system to adjust block sizes to match the host data rate. If compression is on,
data is compacted as it enters the cache buffer at a rate that matches as closely as possible
the rate at which it is written to tape. While the total throughput rate is dependent on how
fast the host can supply data to the DLTtape drive, adaptive cache buffering helps keep the
drive streaming as much as possible, and reduces delays due to repositioning.
The DLTtape System Tape Path Benefits
The combination of tape path components in the DLTtape system provides users with the most
reliable, highest performance mid-range tape drive available. The tape path supports the
DLTtape system's:
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