Every enterprise needs archival storage to meet compliance
requirements and address litigation issues, but "deep" archiving
remains a challenge. Nobody wants to keep discs powered, spinning
and serviced for up to 50 years or more. Tape is removable and
securable, but tape carries its own long-term readability and
reliability concerns.
Optical storage is emerging as an attempt to
fill this gap, and
holographic storage may emerge as the next vehicle for
long-term offline archival storage, bringing a mix of large
capacity and decades of media stability.
However,
holographic storage technology is far from being the next big
thing. It has been on the drawing boards for years, and even though
most of its technological components are well-founded in current
CD/DVD devices, practical holographic storage systems are still in
development. In fact, there are really only two principal
suppliers. This article examines holographic storage technology,
highlights its anticipated deployment and considers the potentially
rocky road ahead for this high-capacity optical storage scheme.
What is holographic storage?
Holographic storage works by storing a sequence of discrete data
snapshots within the thickness of the media. The storage process
starts when a laser beam is split into two signals. One beam is
used as a reference signal. Another beam, called the data-carrying
beam, is passed through a device called a spatial light modulator
(SLM) which acts as a fine shutter system, passing and blocking
light at points corresponding to ones and zeroes. The reference
beam is then reflected to impinge on the data-carrying beam within
the media. This creates a three-dimensional refraction pattern (the
"hologram") that is captured in the media. Holographic storage uses
circular media similar to a blank CD or DVD that spins to accept
data along a continuous spiral data path. Once the media is
written, data is read back using the reference beam to illuminate
the refraction.
This three-dimensional aspect of data recording is an important
difference between holographic storage and conventional CD/DVD
recording. Traditional optical media uses a single laser beam to
write data in two dimensions along a continuous spiral data path.
In contrast, prototype holographic storage products save one
million pixels at a time in discrete snapshots, also called pages,
which form microscopic cones through the thickness of the
light-sensitive media. Today's holographic media can store over 4.4
million individual pages on a disc.
Today, holographic storage is a Worm technology that relies on
light-sensitive media housed in removable protective cartridges.
Although rewritable media and drives will appear in the next few
years, much like the progression from CD-R to CD-RW or from DVD-R
to DVD-RW, experts note that the most likely application for Worm
media is for long-term archiving.
What are the benefits and drawbacks of holographic
storage?
The argument in favour of holographic storage is quite limited
at the moment, and the value proposition is challenging at best. On
the plus side, long-term media stability and reliability is a
compelling advantage for deep archiving purposes -- discs and tape
simply cannot assure reliability out to 50 years. "Discs are very
impervious to the elements," says Brian Garrett, at Enterprise
Strategy Group (ESG). "I've seen demonstrations where they dip the
platters into something boiling and freeze them and roll them
around in the mud, clean them up and they're still usable."
Holographic technology also provides portability, allowing the
distribution of dense data that cannot be sent conveniently over
networks, such as broadcast or high-definition video. The
technology should also become more appealing for shorter term
backups and archives as companies continue to rely less on tape
backups. For example, holographic storage attached to a virtual
tape library (VTL) system might be an excellent tape
replacement.
On the downside, early holographic storage drives will run in
the £10,000 range, with media costing about £100 per disc.
Holographic media capacity is also limited to about 300Gbytes.
While this capacity is expected to grow substantially over time,
it's hard to make a case for a 300Gbyte optical disc against
readily available 1Tbyte hard drives or 1.6Tbyte (compressed) LTO-4
tapes without a specific application. Furthermore, the long-term
reliability and readability of holographic drives is still
unproven.
Holographic recording is also very data sensitive. "With
holographic, you have to keep the data streaming," says Greg
Schulz, founder and senior analyst at the StorageI/O Group, noting
that it's not yet appropriate for partial recordings. This is
similar to early CD-R or DVD-R systems that required constant data
in the drive's write buffer. If the buffer emptied during a write
process, the CD-R or DVD-R recording would fail and the disc would
be ruined. It wasn't until much later in the technology's lifecycle
that "multi-session" and "burn-proof" techniques were added.
Lesser-known drawbacks to holographic storage include light
sensitivity and limited shelf life of unexposed (unrecorded)
holographic media. Blank optical CD/DVD media is forgiving in its
handling and unrecorded shelf life. On the other hand, blank
(unrecorded) holographic media behaves more like unexposed
photographic paper. Prematurely exposing the holographic discs to
light can expose and ruin them, and the unexposed media only has a
shelf life of about three years.
Standards are also a concern. The European Computer
Manufacturers Association (ECMA) has published two standards in
mid-2007 to address Holographic Versatile Disc (HVD) products,
dubbed ECMA-377 and ECMA-378. But holographic storage in general
has no substantial standards endorsed by the International
Standards Organisation (ISO). This lack of standardisation can work
against holographic storage by complicating interoperability
between media and drives.
How are holographic drives specified and deployed?
Ultimately, any discussion of holographic storage deployment is
theoretical because there are no commercial products available
today. Beta products are being evaluated, but manufacturers, like
InPhase Technologies, are keeping their beta users under wrap.
Consequently, there is no word from the field about value,
performance, reliability or any application of holographic
products. Still, there are important trends worth noting.
As with most storage devices, the key issues to consider are
capacity and data transfer rates. Although holographic storage
capacity and performance are currently below current disc and tape
systems, they compare favorably to existing optical storage
devices. Today, holographic storage media holds 300Gbyte
(uncompressed), and beta drives from suppliers, like InPhase, are
expected to utilise that media. The InPhase product roadmap touts
uncompressed capacities up to 1.6Tbyte over the next few years.
Holographic drives, such as the fledgling InPhase Tapestry 300r,
cite data rates of 160Mbps. Seek time can be a lengthy 250
milliseconds, and you can expect almost two seconds to load or
unload the disc cartridge.
The SLM is a critical part of the overall drive capacity and
performance. SLMs in today's early drives use a 1,000 x 1,000 pixel
matrix (1 million bits) to modulate laser light and encode each
data page. In order to increase storage capacity, SLMs must
eventually become finer (offering more bits) and switch faster.
This will fit more and larger data pages on each disc and allow the
drive to write and read more data per second.
Early generation holographic drives appear positioned as
single-disc external products connected to the local area network
(Lan) or storage area network (San). As an example, the Tapestry
300r is expected to provide SCSI, 4 Gbps Fibre Channel optical,
Gigabit Ethernet, SAS, and iSCSI Ethernet connectivity options,
allowing the drive to reside on a wide range of Lan/San
architectures. When used with a server, holographic storage devices
will invariably require device drivers that correspond to the
operating system in use.
Optical storage technologies use lasers for noncontact
read/write operations, and holographic drives should also be
maintenance free. This is a substantial advantage over tape drives,
which require frequent cleaning to remove accumulations of magnetic
particles from the read/write heads.
What is the future of holographic storage technology?
The future of holographic storage is fraught with unknowns.
"This technology is very promising. I've been hearing about it for
years," Garrett says. "But at this point, the No. 1 concerns are
[high] cost and [product] immaturity." Experts agree that capacity
and performance will only increase over time, moving from 300Gbytes
to 800Gbytes and finally on to 1.6Tbytaes over the next 48 months
or so. But the pace of improvements will ultimately rest heavily on
industry acceptance. Given that holographic technology is currently
geared toward a niche in the storage market, it may be years before
early product releases give way to more capable and cost-effective
systems that appeal to a larger storage audience.
Experts also note the possible introduction of "hybrid"
holographic media. Just as magnetic hard drives are starting to
incorporate significant quantities of flash or Ram within the disc,
near-term holographic storage media may add some amount of flash
memory in the cartridge to provide a degree of rewritability until
a suitable rewritable media is developed and productised.
Backward compatibility also remains a significant unknown. No
tape drive in your enterprise today is capable of reading a tape
written 50 years ago, and the same specter is in the cards for
holographic storage. For example, the InPhase product roadmap
suggests a third generation of holographic drives in roughly four
years and promises backward compatibility with the previous two
generations. (Back to first generation in this case.) Well, then
what? If the fifth or sixth or 10th generation drives cannot read
the holographic discs written today, you'll need to either retain
the older drive software and hardware, assuming that it still
functions, or rewrite the older discs to the newer media later --
defeating the purpose of such long retention. "The same corner case
that justifies holographic storage also works against it," Schulz
says.