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Information that must be kept in long-term storage includes medical charts, building design plans, and reports of stock transactions. Previously stored on paper, such information is now being digitized at a rapid rate. We want to continue to store digital information for the long term and at low cost. To meet this demand, Hitachi has developed the Blu-ray Disc Archive System, built on the research of optical disc that has been done over the years. This system allows users to store data reliably for 50 years or more, giving them peace of mind.

Archiving massive amounts of digital data

First of all, what does "archiving" mean?

OHNOArchiving means storing data for the long term. The need for archiving has grown tremendously in recent years. This is because almost all information is being digitized. More and more information that had been stored for the long term on paper is now being converted to digital data.

As shown in Figure 1, there are a variety of data that need to be archived, depending on the business or operation—for example, health records in hospitals and clinics, and design plans of buildings and machines. In North America, where lawsuits are common, laws and regulations require that companies keep evidence of their business activities for a long period of time—for example, five, ten, or 15 years.

Furthermore, when it comes to research data in a university—genomic analysis data, for example—each set of data is huge, and must be stored semi-permanently. As you can see, data requiring long-term storage are multiplying.

Figure 1: Background of data archiving

When we speak of long-term storage of data, what usually comes to mind is the use of tapes…

KOBAYASHIYes, for the most part, conventional archives have used tapes. However, compared with hard disks, tapes are very inconvenient. They have to be rewound regularly, and reading them took time because they have to be fast forwarded to the right position to be read.

Unfortunately, until now there has not been much advancement in storage technology beyond storing data on PC hard disk drives. Hard disks are very well suited for storing data that is read frequently. However, when it comes to storing data that is accessed less frequently, like archives, the cost of hard disks is too high. There was no storage technology that combined the advantages of both tapes and hard disks. We sensed a gap in the lineup of storage products.

OHNOFigure 2 shows the lifecycle of data, and the products targeted for stages from "reading" to "discard." When we considered what kind of product is optimal for data with low read frequency, we understood what we needed was a product that provided outstanding long-term storage, yet at low-cost.

To create such a product, we decided to use Blu-ray Disc technology. Blu-ray Disc technology has already gained widespread use among consumers. The lifespan of Blu-ray Discs is more than 50 years, and they are robust against temperature changes and water damage. These are major advantages when it comes to long-term, reliable data storage.

Figure 2: Data lifecycle and storage products

How long-term storage and low cost were achieved

How does the Blu-ray Disc Archive System work?

Photo: OHNO Chiyo

OHNOThe Blu-ray Disc Archive System is made up of units of libraries. Each library is composed of Blu-ray Discs and an optical disc drive. In the library there are two cartridges. Each cartridge holds 250 Blu-ray Discs. Each cartridge is pulled out smoothly, and inserted again when needed, so all 250 Blu-ray Discs can be swapped at once.

The pull-out cartridges are stored in a specialized case. Instead of books lined up on shelves, cartridges are placed in specialized cases and stacked. Because data can be stored offline, the entire system reduces energy consumption.

Figure 3: Configuration of Blu-ray Disc Archive System

Exchanging 250 Blu-ray Discs at once—that's amazing, isn't it?

OHNOWe're using Blu-ray Discs in a way completely different from consumer products that have been developed, like Blu-ray Disc recorders.

KOBAYASHIIf 250 Blu-ray Discs were swapped out one by one, the labor cost of operators would be huge. What's more, the operators are idle when no new data is written to the discs. The appeal point of this system is that it can automatically change discs while recording new data 24 hours a day.

OHNOWe seek to differentiate our system from consumer products not only in terms of use, but also reliability. Because this is an enterprise archive system, we need to create a system that can store data safety for 50 years.

We therefore took two approaches to improve reliability. First, to prevent the loss of quality due to degradation over time and satisfy the requirement of long-term storage, we optimized the way data is written. Second, to prevent surface flaws and dust from reducing the quality of the discs, we improved the level of error correction.

Data writing that slows down degradation over time

Tell us about how data writing is optimized.

OHNOAlthough optical disc media have a lifespan of over 50 years, the recorded data degrade over time and cannot be read. To prevent this, we optimized the power of the laser output for writing data to optical discs.

Photo: KOBAYASHI Masayuki

KOBAYASHIWhat is new about our technology is that it not only improves quality, it changes how quality is perceived. Until now, the goal has been to read the data properly immediately after it has been written. However, the goal of our product is to be able to read the data 50 years later. We therefore sought to be able to read the data without problem not immediately after it is written, but over the course of 50 years and beyond.

To figure out what we should do to slow the progress of data's degradation over time, we conducted research with the cooperation of media manufacturers. In actuality, because we cannot test for 50 years, we conducted experiments to accelerate degradation to simulate 50 years' worth of degradation. For example, we placed the discs in hot tanks and highly humid space. As we studied the trends of degradation over time with these experiments, we also investigated how the degradation can be slowed by controlling laser power.

How can you slow down degradation?

KOBAYASHIFor optical discs, data is written as patterns on surface regions by laser heat. These patterns are called "recording marks." For the best conditions immediately after writing, it is better to create big and clear marks.

However, recording marks degrade as a result of physical and chemical changes over time, and noise increases. As time passes, the marks break down and begin to blur. As blurring becomes larger, neighboring marks overlap, and they cannot be read. As a result, data is lost. Because of this, the gaps between marks must be protected as best as possible. Big marks are great for reading data immediately after it is written. However, the tradeoff is that they become hard to read because of breakdown over the years.

We therefore painstakingly explored what are the best conditions for writing data that can be read 50 years later, even if blurring occurs due to degradation over time. As shown in Figure 4, laser output is increased at the start and finish of writing the mark. However, there are millions of combinations involving the ratio of this parameter. So, testing was hard work.

Figure 4: Laser power control technology to achieve long-term storage

You performed writing tests repeatedly?

KOBAYASHIThat's right. The tests took anywhere from several months to about half a year. However, we could finish the tests because we've been continuously researching this technology and gotten the know-how on estimating trends and quantities on paper. Without this research experience, we would have to repeat the tests over and over, and they wouldn't end even after years of experimenting.

Handling errors when reading data over the next 50 years

How do you improve error correction with error correcting code?

OHNOError correcting code is a technology that allows data to be read even if error occurs by inserting redundant codes into the data. We succeeded in implementing a redundant code system called Redundant Recovery Code (RRC) so that data can be read after 50 years.

KOBAYASHIConventional Blu-ray Discs withstand errors just enough to support industrial use. However, 50 years down the road, there may be data that couldn't be read once out of several thousand times or million times. This cannot be permitted in an enterprise setting. We therefore included redundant codes to further support the quality of Blu-ray Discs that we surmised to be 50 years later based on the accelerated tests.

In our technology, redundant codes are inserted into the data area on the disc, separate from the customer's user data. When these codes are shown to the customers, they often say that it looks as if unknown data is inserted. So, we need to hide the codes. However, if the method for hiding the data is one not yet known to customers, they may not know 50 years later how the data is written to the discs. We therefore devised a new method to insert redundant codes that conforms to the Universal Disk Format (UDF), an international Blu-ray Disc standard.

Figure 5: Implementation of RRC

Conventional Blu-ray Discs don't have redundant codes, do they?

KOBAYASHIActually, they do. Blu-ray Discs have always had redundant codes. What we did was to keep these codes, and add new RRC on top of them. So, first, error recovery is attempted using the original redundant codes. If that doesn't work, then the final step is to use RCC. With RCC, error recovery could deal with disc damages that are greater than what Blu-ray Disc's specifications established.

Creating even more future storage systems

How will archive systems evolve from here on?

Photo: OHNO Chiyo

OHNOThe archive system we have developed uses Blu-ray Discs. As standards for Blu-ray Discs further develop, they will gain even greater capacity.

Because archive systems will handle even more data in the future, we need to improve software and processing performance.

KOBAYASHIWe won't run out of ideas for improving reliability and performance that we seek. With the Blu-ray Disc Archive System, we took the opportunity to leverage both a recording method suited for long lifespan and redundant code technology. In the future, I would like to apply a variety of new technologies to archive products.

You can contribute to social infrastructure with even more powerful Hitachi storage products, can't you?

OHNOThat's right. In devoting energy to the cloud business, Hitachi is offering a lineup of storage products that can cover the entire data lifecycle so that we can provide optimal services to customers. Because of this, I want to conduct R&D that supports the cloud business by making further use of next-generation media.

Storage systems are one of society's lifelines, so they can be used in a variety of fields. I want to overcome the challenges in each field one by one.

Photo: KOBAYASHI Masayuki」

KOBAYASHIOvercoming the challenges of each field being targeted—that's so representative of Ms. OHNO's thinking, shaped by her background in the consumer electronics department.

In consumer electronics, you narrow down targets—for example, developing TV for people living by themselves. By narrowing down, further specialized products are created. Enterprise products like storage systems tend to be universal products that can be used anywhere.

However, I think it would be fun to create storage products that are different from the rest of the field, in the way that consumer electronics try to be. By narrowing down the target, we may be able to do things not possible before, and make future visions like science fiction.

(Publication: July 4, 2014)


  • Publication: July 4, 2014
  • Professional affiliation and official position are at the time of publication.
Blu-ray Disc and BDXL are trademarks of the Blu-ray Disc Association.
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