[HarryT]

I guess it depends whether you have to write it all at once, or if it can be gradually filled up, as the need is required.

The fact that data lasts a long time doesn't necessarily mean that anyone will be able to read it, of course. A classic example is the BBC's "Domesday Project" in 1986. A huge nationwide project to record the lives of everyday people, intended to be a "time capsule" for the future, and written as a laserdisk, which was seen at the time to be the technology of the future. In 2011 they had to issue an appeal for help to save it, because there were no computers around, a mere 25 years later, that could read the information.