RAR Support

Ve zkratce odpověď zní, že je to tak navržené a má to tak fungovat. Dlouhá odpověď anglicky přímo od ER:

Indeed, RAR can store a file with zero compression if file has grown in size after archiving. But it happens only for non-solid non-volume archives, because it would be more complicated to implement it for solid archives and volumes. I've checked RAR 5.00 and it doesn't switch to store mode just like RAR 6.10 with switches provided by user.

Now I do not remember exactly what difficulties are associated with implementing this for volumes. I suppose, it might be possible to implement it for volumes as well. But I seriously doubt it really worths the effort even if technically possible.

RAR5 compression overhead is a quite small for non-compressible files. It is less than 0.2% in listing provided by user. Reading again a large file, which doesn't fit into disk cache, can be a quite time consuming. It is questionable if 0.2% gain is worth reading a file again and waiting.

I implemented storing incompressible files long before RAR5 format. Older RAR formats had noticeably larger overhead in this case and this feature was reasonably useful. RAR5 format inherits this code from older versions. But if I had implemented it from scratch, I am not sure I would care about mere 0.2%.

Initially I thought he used -m0 switch. In this case, of course, RAR must respect the switch and store files without compression as requested by user. But repeated processing in store mode for incompressible files without -m0 switch was not guaranteed in older versions too and decision is made by RAR based on the archive type and other factors.