I'm currently collecting some archive structure ideas to decide which way to go for BIT (or maybe to invent a better way). Today I try to understand multi-volume archive capabilities of WinRAR and 7-zip separately. And made some small tests. Here is what I've observed:
WinRAR is fully aware of each volume number even volume names damaged (including extension). That's possible with storing volume number in each volume. WinRAR also stores a metadata for each file which has a part in current volume. It's basically a flag that indicates to how to decode a stream. I mean, using previous, next or both volumes.
7-Zip volumes are basically "raw" parts of a single archive. In another word, 7-Zip is not aware volume's information when volume names changed Actually I've a bit shocked how it can use such a basic method to implement multi-volume archives.
As a summary, minumum volume size in WinRAR is [Archive Headers]+[N] while in 7-Zip it's basically [N].
Now, question is that do you have any idea for implementing multi-volume archives? I prefer to hear about multi-volumes archives which support modification.