Longevity of archival media

No matter which storage medium is selected, special measures must be taken to counteract loss of data in the archive. The reason for the danger of loosing data can be attributed to two reasons, namely

• media deterioration
• technical obsolescence of storage media

There is widespread awareness of the fact that any storage media will, over time, degrade to a point where the data becomes permanently irretrievable. Even though many studies have been conducted by agencies having a good reputation, controversy persists over the practical physical life-time [Har95]. A fact that is generally agreed upon is that the quality of the medium varies considerably between different vendors. Obviously, the longevity of the medium can be improved when handled with care. Deterioration can be slowed down by storage under controlled environmental conditions, since media are susceptible to damage from high humidity, rapid and extreme temperature fluctuations, and other influences.

The National Media Laboratory [VB95] estimates the durability of a magnetic tape at 10 to 20 years. Yet, archives holding important, perhaps at some point in time unique data, have to warrant data in good condition. For this reason, digital magnetic tape should be copied up to once a year to guarantee that none of the information is lost [Rot95].

Differing numbers are also presented for optical disks. Adding to it is a far broader variety of types. The diversity in technology concerns the material the medium is made of and how the information is written with the main categories being read-only, write and rewrite. For instance, the CD-ROM, whereupon data is printed only once, has a predicted physical life-time of 50 to 100 years according to manufacturers, whereas refreshing the data on the disks is recommended to be done after 10 years [Web93].

The primary strategy for preventing information loss purely because of physical degradation of storage media is refreshing the data on the same medium at regular time intervals, by reading and rewriting it combined with a failure detection. Also, migrating the data to a new medium has to be done in time, before the carrier is physically corrupted such that the stored data is permanently irretrievable.

However, the physical life-time of media is rarely the constraining factor for ensuring the retention of digital data. Devices are needed to read the data available on the storage media. These peripherals and their connection to a system able to process the data need to be maintained along with the record carrier.

Yet, since technology develops in a rapid pace new media formats having increased capacity and higher speed at a lower price are introduced in the market at relatively quick succession. Consequently, the outdated technology is no longer available. Currently, this can be followed at the demise of the floppy drive. Phasing out the 3,5-inch floppy drive has already been incited, by the latter half of 2002 supply will be stopped [Smi01]. New formats superseding old technology can be followed with numerous examples. Punch cards, old magnetic tapes, 8-inch, and 5,25-inch floppy disks, among others, those generations of storage media are already antiquated after the relatively short life-time of information technology, all in all. Thereby, the speed is demonstrated with which technology advances and vanishes.

The dual problems of short media life-time and the media format becoming obsolete rapidly calls for migrating the data to new types of storage media in periodic intervals. Since copying digital material is possible without loss of information this method can be realised without misgivings. This straightforward solution must be carried out in short cycles adjusted to the very technology, frequently enough to preempt degradation of the storage media in use.

As an additional safety measure, it can be considered to keep multiple copies of the data collections. If material is found to be corrupted in the course of a regular check, it can be replaced with a pristine version. Moreover, spreading those duplicate versions to different locations counterworks fires, earthquakes, or other hardly foreseeable disaster [CGM01a]. Thereby, the reliability of the remote archive has to be taken into account. Furthermore, cooperations with other institutions spreads costs and raises reliability in providing preservation. Yet, those vantages have to be balanced with the local efforts to be invested. Thus, trading networks between autonomous sites should not become to big. Still, distributing digital collections is highly beneficial in the desire for a high reliability of their retention.

As a matter of fact, however, deterioration of the physical media and technological obsolescence present only one aspect in ensuring longevity of the information and accessibility of the documents. Besides the raw data to be adequately preserved, software that made the documents accessible runs danger to be outdated and become obsolete. For a thorough discussion on this issue refer to Section 2.4 dealing with long-term preservation strategies.