White paper - Alternative Storage Technologies

Alternative Storage Technologies Whitepaper

OPTICAL DISCS What it is: Optical discs are flat circular disks that encode digital data on one of its flat surfaces. They are usually made of aluminium, but for preserva- tion purposes it is preferred that the reflective lay- er of the optical disk is gold 4 . Optical disc drives (ODD) with laser beams are used for engraving the data onto the disc, and the laser light is also used for reading back the data whilst the disk spins at high speeds. CDs and DVDs: The two main types of optical discs used by archivists are CDs and DVDs. These can be further split into: • CD-R and DVD-R: Write once, read many • CD-RW and DVD-RW: Rewritable • DVD-RAM: Rewritable discs formatted for random access (like a computer hard disc) Blu-Ray is an optical disc designed to supersede the DVD format with a higher capacity. However, it is more popular among households than professional archivists. M-Disk is an optical disc which was introduced in 2013, claiming a lifetime of 1000 years. However, a Blu-Ray reader is needed for data retrieval, and the future accessibility of these is a challenge the manu- facturer does not give an answer to.

Ease of use: Optical discs such as CDs and DVDs are easy to use, and both the media and the hardware are relatively inexpensive to purchase. In addition, the laser-based device reader is not in contact with the disc which makes mechanical failure less likely, and most of the readers are backward compatible. These factors have made optical discs widely used by archivists. High failure rates: Originally developed primarily as a mass consumer product, elaborate measures are required when using optical discs for long term preservation. Optical discs have a relatively high risk of failure 5 , and it is strongly recommended to always do thorough testing to ensure required standards are met. Scratching and environmental factors such as dust, heat or UV light can cause severe damage to the disks. Optical discs also offer low data capacity compared to alternative tech- nologies. Frequent migrations: It is challenging to reliably estimate the real lifetime of optical discs. They are fragile, and the life span depends on factors such as manufactured quality, how well it is recorded and its physical handling and storage. To lower the risk of failure, best practice is to frequently migrateontonewer formats. This is in sharp contrast to manufacturers who tend to claim an expected lifetime of up to several hundred years.

4 DPC; “Handbook in Digital Preservation”; www.dpconline.org/advice/preservationhandbook/media-and-formats/media

5 “Risks Associated with the Use of Recordable CDs and DVDs as a Reliable Storage Medium in Archival Collections – Strategies and Alternatives”, Memory of the World Programme, UNESCO, 2006; http://unesdoc.unesco.org/images/0014/001477/147782E.pdf