Number 5: Managing Media Migration in a Deposit System
Written by: R.J. van Diessen, B.J. van Rijnsoever
Storage technology obsolescence makes media migration a necessity. Data has to be copied from one storage medium to another on a regular basis. However, the fact that storage technology becomes obsolete is not the only trigger for rewriting previously stored digital objects. All storage media degrade over time and have to be rewritten either on the same medium (refreshing) of on another medium (migration).
Ordinarily media refreshment / migration would be a straightforward process. However, the large amounts of storage associated with an electronic deposit system introduce certain volume-specific requirements. Most electronic deposit systems define their storage capacity needs in several TeraBytes (10¹² Bytes). Take a deposit system with 100 TeraBytes of information stored on tape, for example. Lets assume that you want to migrate all this information to an optical storage medium. Current optical storage media have a capacity of around 5 GigaBytes and a write speed of around 4 MegaBytes/second. A quick calculation shows that a complete migration to optical storage would take at least 290 days (100 TeraBytes/ 4 MegaBytes per second)!
This report descibes the actions to be taken to manage media migration / refreshment effectively within an electronic deposit system, focussing specifically on the media migration issues within DIAS. Potential additional capacity required for media migration might be created by redundancy and parallelism.