Why is a cylinder always the symbol for a database?

Question

Why is a cylinder always the symbol for a database?

Answer 1

Because now all flowchart's basic shapes has become standard in all business & academic documentation and presentations.

Answer 2

A true database, like Access, is a relational object with 3 dimensions.  Some consider an Excel sheet a 'flat' database, that is 2 dimensional... rows and columns.  A cd-rom is kind of like an Excel sheet.  A stack of cd-roms could be considered like an Access database.

A stack of cd-roms resembles....... a cylinder.