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6. DIGITAL PRESERVATION2007 Revised Edition |
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6.1 Digital Preservation Challenges
The preservation of cultural heritage materials is a cornerstone of our work as professionals. Ensuring the longevity of the materials entrusted to our care is a concern no matter what other activities you undertake. Clay tablets, stored properly, will last thousands of years. Good rag paper, if kept away from pests and in the right environment, will last hundreds of years. The proof of their longevity lies in museums and rare book collections around the world. Someday, future curators, archivists, and librarians will know how long a floppy disk, a CD, or hard drive will last, but until then we have only estimates. The creation of digital surrogates in no way alleviates these concerns. Instead, it presents a new arena with preservation concerns. This chapter discusses the preservation of digital materials. It does not cover issues associated with traditional preservation. Those measures will not be discussed in light of the voluminous material already available to meet those challenges. The life of inadequately stabilized and housed original documents, artifacts, published works, or works of art will not be extended through the process of digitization. Digitization will do nothing to help the condition of the original. In short, digitization is not a preservation measure for originals, even though it may mitigate against further damage by providing access to surrogates. Institutions can invest a great deal of effort and expense in the digitization of their collections and in the presentation of these digitized collections online. Until recently, the digital world has not given high priority to the preservation and storage of its content. The very nature of the Web is one of impermanence and fluidity, and digital information presented via the Internet has grown with little regard for preservation. Fortunately, cultural institutions by their very nature think long-term, and preservation of digital material has emerged as a major concern in the cultural heritage community. It is necessary that digital objects created remain accessible for as long as possible both to for intended users and the wider community. Digitization projects are costly and time consuming and the digitization process can subject original materials to potentially damaging light and handling. Doing it right the first time, following the "scan once methodology, "and properly preserving the digital items saves money, time, and originals from additional and unnecessary wear and tear. More than any other aspect of a digitization project, digital preservation, by its very nature, requires vigilance on the part of cultural heritage professionals. Digital preservation is a new and developing field. Cultural heritage professionals must monitor research being conducted in the fast-paced world of digital technology. In addition, the issues surrounding the preservation of digital objects is of a more immediate imperative than traditional materials. This is due to the lack of stability in digital storage media and the necessary equipment required to interpret and access digital materials. This chapter seeks to equip you with the ability to do both. It introduces the challenges of digital preservation and provides information and recommendations to guide the decision making for digital preservation strategies. Digital Preservation ChallengesAt its most fundamental level, digital material is comprised of ones and zeros. Software programs are written to create and interpret this binary structure into a variety of forms that we use be they images, text documents, databases, video, sound files or a combination of those formats. Without appropriate tools, even the most simplistic of these are impossible to decipher. Consider the variety of computer hardware and software available readily and how fast popular computers and programs become obsolete. Hardware obsolescenceHardware obsolescence refers to the maintenance of the appropriate equipment to read the digital storage media selected. For instance, just five years ago, a 3 1/2-inch floppy disk was used for virtually everything digital anyone wanted to save or transport. Yet in today's computers, 3 1/2-inch floppy disk drives need to be specially ordered in order to be installed on a standard PC. Even if your storage media remains unharmed over the ravages of time, eventually there will be no computer extant that has the appropriate mechanism to read that medium. Without the appropriate hardware, files contained on that floppy disk are lost - irretrievable. In order to prevent this kind of preservation problem, institutions and companies are maintaining working machines that will read the variety of medium that have been supplanted with more up to date or advanced technologies. This kind of access, though, can be expensive. Maintaining the hardware requires space and expertise and outsourcing obsolete storage media for access requires funds that may not be readily available. Software obsolescenceClosely connected to the issues of hardware obsolescence is the development of software applications. In interacting with those ones and zeros, software programs use their own systems for creation and interpretation. Often times, a file created in one software program cannot be interpreted by another because the program does not know how to read this particular configuration of ones and zeros. Updates to newer versions of software or changes from one to another can create obstacles to access for those digital objects. FragilityBoth hardware and software obsolescence underscore the fragility of digital objects. Their very nature requires machines that are supplanted as technology advances and they require programs that are updated as new functionality becomes available. However, there is another kind of fragility that needs to be considered as well, for fragility is no just an operational mater. As with our more traditional objects, digital media have their own specific environmental concerns, and like the material it stores, degradation to the digital media can be subtler and not identifiable by the naked eye. Keeping CurrentDigital preservation is a fast-growing area in research for information science and technologies. The Commission on Preservation and Access (CPA), the Digital Preservation Consortium, CLIR (Council on Library and Information Resources), and the Digital Library Federation (DLF) are working to develop methodological frameworks, and to ensure continuing research and development in the little understood areas of digital preservation. A series of publications and additional initiatives can be found on the preservation pages of DLF, available at: http://www.diglib.org/preserve.htm. Cornell University has created a thorough tutorial on digital preservation management available at: http://www.library.cornell.edu/iris/tutorial/dpm/eng_index.html. Additional resources can be found in the Further Reading section of this chapter and the Resources section at the end of these Guidelines. Creating a Digital Preservation StrategyHow can an institution keep up with all the various types of digital files, programs, and computers being used in-house much less keep abreast of emerging technologies? How much preservation is enough? How much is too much? What are the deciding factors for your institution? These are all issues that need to be considered when devising a digital preservation strategy. Here are four places to start:
The Digital Preservation ProcessAn institution can easily become overwhelmed by the avalanche of issues that impact the process of planning for digital preservation. This section seeks to addresses several issues at the core of digital preservation including digital storage during the digitization process, migration of digital material, storage media for the short term and long term, and trends for the future. It includes guidelines on what is currently considered best practice for digital preservation with the understanding that these standards are fluid and require revisiting often. The very best result that cultural institutions can hope to achieve for the long-term sustainability of digital material will be accomplished through good digital preservation planning and vigilant management. There are essentially five main storage applications that occur during the digitization process: production, data transportation, presentation to the public, backup or archiving, and migration. ProductionThe production or creation of digital material generally requires sufficient hard disk capacity to store working files while they are being manipulated and developed. If the collection is considerable and there is a large production environment, a Redundant Array of Inexpensive Disks (RAID) may be the most appropriate however storage for active files can generally be handled by a large hard drive. Be warned that determining what will reside on your hard drive and what will be forwarded to a server in networked environments can often be difficult because multiple versions of files can become confusing. It is important to outline the various processes you need to perform on the same images and then determine how many "active" files you need at any one time. This demonstrates the role of file management and file naming in the preservation process. Data TransportationGenerally, moving digital information is handled by portable storage devices such as recordable CDs (compact disks) and more recently by DVDs (Digital Video Disks or Digital Versatile Disks). The capacity of the CD and DVD is greater than that of the tape drive, an early favorite for data transportation, though its transfer rate is slower than tape. Another feature of the CD is its compatibility across platforms. The CD-R (a CD which can only be written upon once) is a secure format; its "write once mechanism" does not allow overwriting. CD-RW (CD Read/Write) is less secure but more versatile. Presentation to the PublicMost institutions making images of their collections available to the public via the Internet make use of in-house servers or rent space on commercial servers. Backup/ArchivingDigital collections should be backed up in a format that is easily accessible and stored remote from the original source on a routine basis. When evaluating storage for backup, the inevitable dilemma is between speed and cost. Most managers prefer tape for backup, as it may be used at non-peak hours, when speed is not an issue. For small networked systems, tape backup is the common practice. Remember that no digital media is considered permanent. MigrationIn the context of digital preservation, migration refers to the shifting of digital objects from old media formats and software programs to newer ones. Migration of backed-up digital material needs to be as easy and cost-effective as possible for institutions to buy into a system. The continual drain on fiscal resources to repeatedly upgrade equipment and software can be borne by some institutions, but others will find it difficult to stay abreast of continual migration. Decisions must be made in every institution concerning what information will be saved and migrated and what will not based on a combination of cost effectiveness, intellectual necessity, and moral and professional obligation. When making these tough decisions, refer to the four preservation strategies discussed above. Types of Digital Storage MediaLongevity of a digital medium depends on many factors - the type of media (CD, DVD, tape, etc.), how often and the way in which the media is handled, and how the media is stored. It is important to keep in mind that even with proper maintenance and great luck, no digital format is permanent or archival. The very best result that cultural institutions can hope to accomplish is long-term sustainability of digital material through good preservation planning and vigilant management. The storage media is an essential part of that process. There are two types of digital storage media - portable and non-portable. Each has advantages and disadvantages for long-term storage. Portable MediaCD (CD-R and CD-RW) CD-R or Compact Disk Recordable is a format that requires a CD-ROM drive to read and to write. The CD-R format is an inexpensive way to store digital object masters, which typically require many megabytes of storage. Currently the CD-R disk will store 650 MB, though approximately 100 MB of those bytes should remain free to allow for manipulation of the data. These disks, unlike the common music CDs, are more susceptible to scratches, to fingerprints and to extremes in temperature and light. They should be handled and stored with great care. They are also susceptible to other destructive agents. If writing on the disk, only a water-based felt-tip pen should be used. An alcohol-based felt-tip pen can migrate through the protective layer and possibly affect the integrity of the data. CD-R conforms to the ISO standard 9660, which is an established standard that allows a file system to be used under a variety of operating systems. The standard applies only to the data track of a CD-ROM and not to audio tracks or any other media, such as erasable-optical drives. Thus CD-Rs may be read by any of a variety of operating systems such as UNIX and MS-DOS. CD-RW or Compact Disk-Rewritable format allows the re-writing of information on the disk. Unless an institution requires the convenience of re-write, the CD-R format is a better choice. Currently many prefer the CD-R (Compact Disk-Recordable) format for archival storage, though there is debate regarding its archival quality. To store data on a CD-R generally requires that the data be gathered on a hard drive and then "written" to the CD-R. DVD (DVD-R and DVD-RW) DVD technology (Digital Video Disk or Digital Versatile Disk) is a recent addition to the growing optical disk technology market. DVD is backwardly compatible, so it may be used to read CD disks. But CD-R and CD-RW drives cannot read a DVD disk. A DVD-ROM drive will be needed to read DVD-R disk, but some DVD-R disks do not play on some machines. DVD-ROM is different than DVD-VIDEO. The former handles data, while the latter is reserved largely for the commercial video market. A DVD-R disk will hold approximately 4.7 gigabytes. The enormous storage available on a DVD makes it appealing as a storage device; gold standard DVDs have been developed to meet archival standards. Improving the lifespan of CD's and DVD's
DAT Tape, DLT Tape, ZIP® and JAZ® drives Tape, ZIP® and JAZ® drives are all magnetic media, and magnetic media is NOT recommended for long-term storage. Tape is, however, an excellent intermediate medium, particularly for transport of data and for backup. Improving the lifespan of DLT's
The above charts, modified from tables in the NINCH Guide (available at http://www.nyu.edu/its/humanities/ninchguide/XIV/), can assist in making sure digital storage media lasts as long as possible. Non-portable MediaNetwork Servers (drives) The minimum storage space recommended for network servers changes every three to six months. Suffice it to say that if a server is required, it should be purchased to be adequate for the first two years of the project. Depending upon the size of their digital holdings, larger institutions may need to upgrade on an annual basis, especially if production levels of digital materials are high. Hard Drives (PC disk drives) It is recommended that institutions purchase the largest hard drive they can afford. If it is possible to purchase two hard drives, this will provide a more flexible storage system. If managers of digital projects use hard drives for image storage, they should defragment them on a regular basis to maintain optimum performance. Hard drives are not recommended for long-term storage. The following recommendations are based on available hardware in the medium price range:
Other Digital Storage ConcernsThe amount of storage required depends on a number of inter-related issues including but not limited to the size of your digital holdings, your institution's budget, and your institution's digital preservation strategy. There are a number of storage issues other than cost, amount, and media permanence that cultural repositories should factor into decisions regarding the "long term" storage of digital materials. These include labeling, file management, and metadata issues. As soon as digitization projects get going, the number of images piles up. Labeling CD cases alone will not help you manage your digital assets. As noted above, CDs and DVDs hold immense amounts of data, and it is unlikely that contents lists on the jewel cases will be palatable in the long-run, as more and more storage devices are used. Therefore, in addition to the labeling that would occur on the outside of a CD storage case, managers of digital projects should maintain "preservation metadata" for each image. The information necessary is explained in the NC ECHO Preservation Metadata for Digital Objects. It is also recommended that the file naming conventions follow these standards:
NC ECHO recommended storage standards:
ConclusionDigital preservation seeks to achieve longevity of the digital object with all its original properties intact. Many questions in the field of digital preservation remain unanswered, and many more questions will emerge as technology relentlessly forges ahead with new developments. Whether your institution has only the means to preserve the minimum content of your digital creations or can afford to preserve the whole discovery and display system, policies should be put in place to ensure the long-term sustainability and accessibility of the digital content you have chosen to be preserved. Further ReadingBenford, Gregory. Deep Time: How Humanity Communicates Across Millennia, New York: Avon, 1999. Conway, Paul. "The Implications of Digital Imaging for Preservation." In Preservation of Library and Archival Materials, 2nd ed. Edited by Sherelyn Ogden. Andover, MA: Northeast Document Conservation Center, 1994. Conway, Paul. Preservation in the Digital World, available at: http://www.clir.org/pubs/abstract/pub62.html Development of a Testing Methodology to Predict Optical Disk Life Expectancy Values (Summary) http://palimpsest.stanford.edu/byorg/nara/nistsum.html Digital Preservation Coalition http://www.dpconline.org Digital Projects Guidelines. Attachment 11, Arizona State Library, Archives and Public Records, available at http://www.lib.az.us/digital/dg_a11.html "Long-Term Usability of Optical Media - The National Archives and Records Administration and the Long-Term Usability of Optical Media for Federal Records: Three Critical Problem Areas" http://palimpsest.stanford.edu/bytopic/electronic-records/electronic-storage-media/critiss.html Rothenberg, Jeff. "Avoiding Technological Quicksand: Finding a Viable Technical Foundation for Digital Preservation", (January 1998) http://www.clir.org/pubs/reports/rothenberg/contents.html
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