Published on Nov 14, 2015
Digital Preservation is defined as: long-term, error-free storage of digital information, with means for retrieval and interpretation, for the entire time span the information is required for. Long-term is defined as "long enough to be concerned with the impacts of changing technologies, including support for new media and data formats, or with a changing user community. Long Term may extend indefinitely".
"Retrieval" means obtaining needed digital files from the long-term, error-free digital storage, without possibility of corrupting the continued error-free storage of the digital files. "Interpretation" means that the retrieved digital files, files that, for example, are of texts, charts, images or sounds, are decoded and transformed into usable representations.
This is often interpreted as "rendering", i.e. making it available for a human to access. However, in many cases it will mean able to be processed by computational means. This constant input of effort, time, and money to handle rapid technological and organizational advance is considered the main stumbling block for preserving digital information.
Digital preservation can therefore be seen as the set of processes and activities that ensure continued access to information and all kinds of records, scientific and cultural heritage existing in digital formats. This includes the preservation of materials resulting from digital reformatting, but particularly information that is born-digital and has no analog counterpart. In the language of digital imaging and electronic resources, preservation is no longer just the product of a program but an ongoing process. In this regard the way digital information is stored is important in ensuring its longevity. The long-term storage of digital information is assisted by the inclusion of preservation metadata.
The unique characteristic of digital forms makes it easy to create content and keep it up-to-date, but at the same time brings many difficulties in the preservation of this content. Digital preservation faces challenge, that the media on which digital contents are stored are more vulnerable to deterioration and catastrophic loss than some analog media. The recording media for digital data deteriorate at a much more rapid pace, and once the deterioration starts, in most cases there is already data loss.
Preservation Data Stores
As the world becomes digital, we are in ever greater danger of losing business, scientific, artistic, cultural, and personal assets. The threat of such a digital dark age stems from the fact that—unlike physical records that may survive decades, centuries, or even longer without advanced planning—digital records will not survive without planning and diligence. Everything needed to keep digital records viable will become obsolete, including hardware, software, processes, and formats. Consequently, digital preservation environments are needed to ensure the ability to access valuable digital records decades from now and, more significantly, to ensure the interpretability of the records once accessed. We describe Preservation Data Stores, an innovative storage architecture that facilitates robust and optimized preservation environments. It is a layered architecture that builds upon open standards, including Open Archival Information System, XAM (Extensible Access Method), and Object-based Storage Device.
Digital preservation systems aim to ensure that long-lived data will be usable in the distant future. Digital preservation comprises two aspects: bit preservation, which is the ability to access the bits of the digital record, and logical preservation, which is the ability to use and understand the data in the future. In addition, logical preservation must support tracking the provenance of a record and ensuring its authenticity and integrity. Bit preservation issues are mostly well understood and are supported by existing products used to migrate data between different generations of storage technologies. In contrast, logical preservation is still mostly an unsolved problem.
Due to the constant growth in the amount of long-lived data, as well as new compliance legislations that are emerging , this problem has been gaining increasing attention from the research community, government and semi government agencies. The Open Archival Information System (OAIS) ISO standard , developed by NASA, is one example. It targets the preservation of knowledge rather than the preservation of bits, and provides a set of concepts and guidelines for building preservation systems. At the heart of every OAIS preservation system lies a preservation storage component. This is the portion of the system that manages the long-term storage and maintenance of digital material entrusted to the OAIS.
The unique design of object storage differs from standard storage devices with a traditional block-based interface. Object storage is an intelligent evolution of disk drives that can store and serve objects rather than simply place data on tracks and sectors. This task is accomplished by moving low-level storage functions into the storage device and accessing the device through an object interface. Systems using object storage provide the following benefits, which are highly desirable across a wide range of typical IT storage applications:
* Intelligent space management in the storage layer
* Data-aware prefetching and caching
* Robust, shared access by multiple clients
* Scalable performance using an offloaded data path
* Reliable security
Digital Preservation Standards
To standardize digital preservation practice and provide a set of recommendations for preservation program implementation, the Reference Model for an Open Archival Information System (OAIS) was developed. The reference model (ISO 14721:2003) includes the following responsibilities that an OAIS archive must abide by:
• Negotiate for and accept appropriate information from information Producers.
• Obtain sufficient control of the information provided to the level needed to ensure Long-Term Preservation.
• Determine, either by itself or in conjunction with other parties, which communities should become the Designated Community and, therefore, should be able to understand the information provided.
• Ensure that the information to be preserved is Independently Understandable to the Designated Community. In other words, the community should be able to understand the information without needing the assistance of the experts who produced the information.
• Follow documented policies and procedures which ensure that the information is preserved against all reasonable contingencies, and which enable the information to be disseminated as authenticated copies of the original, or as traceable to the original.
• Make the preserved information available to the Designated Community .
The PDS architecture (Figure 1) is based on the open standards OAIS, XAM, and OSD; each standard is implemented as a separate layer. At the top, the preservation engine layer, which is based on OAIS, provides an external interface to PDS and implements preservation functions. Additionally, it maps between the OAIS and XAM levels of abstraction, as detailed in . XAM  serves as the storage mid-layer. It contains the XAM library, which provides the XAM interface, and a vendor interface module (VIM) to communicate with the underlying storage system. XAM provides a logical abstraction for data containers.
It was chosen as the midlayer abstraction because of its support for bundling extensive metadata with the data, its built-in support for import and export services, and its integrated storage management capabilities. The bottom layer of the PDS architecture suggests two alternative back-end storage systems: a standard file system or an OSD. A higher-level application programming interface (API), labeled HL OSD, on top of the OSD provides abstraction and simplification to the object-based storage device (or object store) interface, which resembles a SCSI (small computer system interface).
Using a file system as the object layer is especially attractive when incorporating PDS with existing file systems and archives. However, there are advantages to the use of OSD, which is designed to support an object store layer . Being able to manage space allocation and associate attributes with objects enables optimizations such as placing key OAIS attributes (e.g., a link to the RepInfos) close to the data in a persistent manner. Furthermore, in cases in which the actual disks are network attached, an OSD provides the networked disks with secure object-level access control. The scalability offered by OSD is also an important feature for preservation systems.
The PDS API can be triggered either directly or via Web services to enable flexible and platform-independent use of PDS. According to OAIS, the AIP is the basic object that interacts with the archival storage, and therefore, it is the main object to be passed between PDS and a client. PDS is designed to run two processes: a high-level process and a low-level process. The high-level process is responsible for the PDS interface, the preservation engine, and the XAM implementation.
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