Studies show that the amount of data being recorded is increasing at 30 to 40
percent per year. At the same time, the capacity of modern hard drives,
which are used to store most of this, is increasing at less than half
that rate. Fortunately, much of this information doesn’t need to be
accessed instantly. And for such things, magnetic tape is the perfect
solution.
Seriously? Tape? The very idea may evoke images of reels rotating fitfully next to a bulky mainframe in an old movie like Desk Set or Dr. Strangelove. So, a quick reality check: Tape has never gone away!
Indeed,
much of the world’s data is still kept on tape, including data for
basic science, such as particle physics and radio astronomy, human
heritage and national archives, major motion pictures,
banking, insurance, oil exploration, and more. There is even a cadre of
people (including me, trained in materials science, engineering, or
physics) whose job it is to keep improving tape storage.
Tape has
been around for a long while, yes, but the technology hasn’t been frozen
in time. Quite the contrary. Like the hard disk and the transistor, magnetic tape has advanced enormously over the decades.
The first commercial digital-tape storage system, IBM’s Model 726,
could store about 1.1 megabytes on one reel of tape. Today, a modern
tape cartridge can hold 15 terabytes. And a single robotic tape library
can contain up to 278 petabytes of data. Storing that much data on
compact discs would require more than 397 million of them, which if
stacked would form a tower more than 476 kilometers high.
It’s
true that tape doesn’t offer the fast access speeds of hard disks or
semiconductor memories. Still, the medium’s advantages are many. To
begin with, tape storage is more energy efficient: Once all the data has
been recorded, a tape cartridge simply sits quietly in a slot in a
robotic library and doesn’t consume any power at all. Tape is also
exceedingly reliable, with error rates that are four to five orders of
magnitude lower than those of hard drives. And tape is very secure, with
built-in, on-the-fly encryption and additional security provided by the
nature of the medium itself. After all, if a cartridge isn’t mounted in
a drive, the data cannot be accessed or modified. This “air gap” is
particularly attractive in light of the growing rate of data theft
through cyberattacks.
The offline nature of tape also provides an
additional line of defense against buggy software. For example, in 2011,
a flaw in a software update caused Google to accidentally delete
the saved email messages in about 40,000 Gmail accounts. That loss
occurred despite there being several copies of the data stored on hard
drives across multiple data centers. Fortunately, the data was also
recorded on tape, and Google could eventually restore all the lost data from that backup.
The
2011 Gmail incident was one of the first disclosures that a
cloud-service provider was using tape for its operations. More recently,
Microsoft let it be known that its Azure Archive Storage uses IBM tape storage equipment.
All these pluses notwithstanding, the main reason why companies use
tape is usually simple economics. Tape storage costs one-sixth the
amount you’d have to pay to keep the same amount of data on disks, which
is why you find tape systems almost anyplace where massive amounts of
data are being stored. But because tape has now disappeared completely
from consumer-level products, most people are unaware of its existence,
let alone of the tremendous advances that tape recording technology has
made in recent years and will continue to make for the foreseeable
future.
All this is to say that tape has been with us for decades and will be here for decades to come. How can I be so sure? Read on.
Tape has survived
for as long as it has for one fundamental reason: It’s cheap. And it’s
getting cheaper all the time. But will that always be the case?
You
might expect that if the ability to cram ever more data onto magnetic
disks is diminishing, so too must this be true for tape, which uses the
same basic technology but is even older. The surprising reality is that
for tape, this scaling up in capacity is showing no signs of slowing.
Indeed, it should continue for many more years at its historical rate of
about 33 percent per year, meaning that you can expect a doubling in
capacity roughly every two to three years. Think of it as a Moore’s Law
for magnetic tape.
That’s great news for anyone who has to deal
with the explosion in data on a storage budget that remains flat. To
understand why tape still has so much potential relative to hard drives,
consider the way tape and hard drives evolved.
Both rely on the
same basic physical mechanisms to store digital data. They do so in the
form of narrow tracks in a thin film of magnetic material in which the
magnetism switches between two states of polarity. The information is
encoded as a series of bits, represented by the presence or absence of a
magnetic-polarity transition at specific points along a track. Since
the introduction of tape and hard drives in the 1950s, the manufacturers
of both have been driven by the mantra “denser, faster, cheaper.” As a
result, the cost of both, in terms of dollars per gigabyte of capacity,
has fallen by many orders of magnitude.
These cost reductions are
the result of exponential increases in the density of information that
can be recorded on each square millimeter of the magnetic substrate.
That areal density is the product of the recording density along the
data tracks and the density of those tracks in the perpendicular
direction.
Early on, the areal densities of tapes and hard drives
were similar. But the much greater market size and revenue from the sale
of hard drives provided funding for a much larger R&D effort, which
enabled their makers to scale up more aggressively. As a result, the
current areal density of high-capacity hard drives is about 100 times
that of the most recent tape drives.
Nevertheless, because they
have a much larger surface area available for recording,
state-of-the-art tape systems provide a native cartridge capacity of up to 15 TB—greater
than the highest-capacity hard drives on the market. That’s true even
though both kinds of equipment take up about the same amount of space.
With the exception of capacity, the performance characteristics of
tape and hard drives are, of course, very different. The long length of
the tape held in a cartridge—normally hundreds of meters—results in
average data-access times of 50 to 60 seconds compared with just 5 to 10
milliseconds for hard drives. But the rate at which data can be written
to tape is, surprisingly enough, more than twice the rate of writing to
disk.
Over the past few years, the areal density scaling of data
on hard disks has slowed from its historical average of around 40
percent a year to between 10 and 15 percent. The reason has to do with
some fundamental physics: To record more data in a given area, you need
to allot a smaller region to each bit. That in turn reduces the signal
you can get when you read it. And if you reduce the signal too much, it
gets lost in the noise that arises from the granular nature of the
magnetic grains coating the disk.
It’s possible to reduce that
background noise by making those grains smaller. But it’s difficult to
shrink the magnetic grains beyond a certain size without compromising
their ability to maintain a magnetic state in a stable way. The smallest
size that’s practical to use for magnetic recording is known in this
business as the superparamagnetic limit. And disk manufacturers have reached it.
Until
recently, this slowdown was not obvious to consumers, because
disk-drive manufacturers were able to compensate by adding more heads
and platters to each unit, enabling a higher capacity in the same size
package. But now both the available space and the cost of adding more
heads and platters are limiting the gains that drive manufacturers can
make, and the plateau is starting to become apparent.
There are a
few technologies under development that could enable hard-drive scaling
beyond today’s superparamagnetic limit. These include heat-assisted magnetic recording (HAMR) and microwave-assisted magnetic recording
(MAMR), techniques that enable the use of smaller grains and hence
allow smaller regions of the disk to be magnetized. But these approaches
add cost and introduce vexing engineering challenges. And even if they
are successful, the scaling they provide is, according to manufacturers,
likely to remain limited. Western Digital Corp., for example, which
recently announced that it will probably begin shipping MAMR hard drives in 2019, expects that this technology will enable areal density scaling of only about 15 percent per year.
In
contrast, tape storage equipment currently operates at areal densities
that are well below the superparamagnetic limit. So tape’s Moore’s Law
can go on for a decade or more without running into such roadblocks from
fundamental physics.
Still, tape is a tricky technology. Its
removable nature, the use of a thin polymer substrate rather than a
rigid disk, and the simultaneous recording of up to 32 tracks in
parallel create significant hurdles for designers. That’s why my
research team at the IBM Research–Zurich lab
has been working hard to find ways to enable the continued scaling of
tape, either by adapting hard-drive technologies or by inventing
completely new approaches.
In 2015, we and our collaborators at FujiFilm Corp. showed that by using ultrasmall barium ferrite particles oriented perpendicular to the tape,
it’s possible to record data at more than 12 times the density
achievable with today’s commercial technology. And more recently, in collaboration with Sony Storage Media Solutions,
we demonstrated the possibility of recording data at an areal density
that is about 20 times the current figure for state-of-the-art tape
drives. To put this in perspective, if this technology were to be
commercialized, a movie studio, which now might need a dozen tape
cartridges to archive all the digital components of a big-budget
feature, would be able to fit all of them on a single tape.
To enable this degree of scaling, we had to make a
bunch of technical advances. For one, we improved the ability of the
read and write heads to follow the slender tracks on the tape, which
were just 100 or so nanometers wide in our latest demo.
We also
had to reduce the width of the data reader—a magnetoresistive sensor
used to read back the recorded data tracks—from its current micrometer
size to less than 50 nm. As a result, the signal we could pick up with
such a tiny reader got very noisy. We compensated by increasing the
signal-to-noise ratio inherent to the media, which is a function of the
size and orientation of the magnetic particles as well as their
composition and the smoothness and slickness of the tape surface. To
help further, we improved the signal processing and error-correction
schemes our equipment employed.
To ensure that our new prototype
media can retain recorded data for decades, we changed the nature of the
magnetic particles in the recording layer, making them more stable. But
that change made it harder to record the data in the first place, to
the extent that a normal tape transducer could not reliably write to the
new media. So we used a special write head that produces magnetic
fields much stronger than a conventional head could provide.
Combining
these technologies, we were able to read and write data in our
laboratory system at a linear density of 818,000 bits per inch. (For
historical reasons, tape engineers around the world measure data density
in inches.) In combination with the 246,200 tracks per inch that the
new technology can handle, our prototype unit achieved an areal density
of 201 gigabits per square inch. Assuming that one cartridge can hold
1,140 meters of tape—a reasonable assumption, based on the reduced
thickness of the new tape media we used—this areal density corresponds
to a cartridge capacity of a whopping 330 TB. That means that a single
tape cartridge could record as much data as a wheelbarrow full of hard
drives.
In 2015,the Information Storage Industry Consortium,
an organization that includes HP Enterprise, IBM, Oracle, and Quantum,
along with a slew of academic research groups, released what it called
the “International Magnetic Tape Storage Roadmap.” That forecast
predicted that the areal density of tape storage would reach 91 Gb per
square inch by 2025. Extrapolating the trend suggests that it will
surpass 200 Gb per square inch by 2028.
The authors of that road
map each had an interest in the future of tape storage. But you needn’t
worry that they were being too optimistic. The laboratory experiments
that my colleagues and I have recently carried out demonstrate that 200
Gb per square inch is perfectly possible. So the feasibility of keeping
tape on the growth path it’s had for at least another decade is, to my
mind, well assured.
Indeed, tape may be one of the last
information technologies to follow a Moore’s Law–like scaling,
maintaining that for the next decade, if not beyond. And that streak in
turn will only increase the cost advantage of tape over hard drives and
other storage technologies. So even though you may rarely see it outside
of a black-and-white movie, magnetic tape, old as it is, will be here
for years to come.
