This emerged today and that i could not find any historic answer why a database is definitely symbolized like a cylinder. I'm wishing someone within the stack world knows why and possess a link or something like that backing up.
I am reasonably sure that it predates disk drives, and dates back to some substantially older technology: drum memory:
Edit: doing a little more searching around, here is a better picture:
It is because people notice a DB as basic storage, similar to a disk. And disk storage happens to be symbolized with a cylinder because of, well, the physical qualities of spinning magnetic disks.
I haven't any proof however i always assumed the cylinder of database symbolized a tough-drive disk.
It comes down in the days of old (pre 1960) when data was analog, i.e. round. Nowadays with digital (square) data, databases aren't round but regrettably the convention has stuck.