> You’d think that -r option usage forces tar application to append files to the end of the archive, getting the position of the archive’s end from archive’s index. It doesn’t. Tar format is designed in a way that it has no index.

Um, yes. A tar file is just a blob of 512 byte aligned files with 512 byte headers in front of them + an optional 1k null-bytes signifying the end. Technically all there is to appending at the end is to slap on another header and a file blob?

EDIT: Ok, on second thought, you do need a linear scan. There could be ancillary data after the 1k termination blob. Even if not an intentional polyglot nonsense, but if you actually use tar on a main frame tape drive as originally intended, simply seeking to the end doesn't work.

Yes because the name ‘tar’ means ‘Tape ARchive’. Appending to the end is how you would add a file to a tape. In fact tar will silently overwrite an existing file because that is how you would update a file on a tape (that functionality is arguably useful today for other reasons)