It's nice when things fit in RAM. But very often they don't. When you want to sort arbitrary-sized binary records on disk look no further than bsort: https://github.com/pelotoncycle/bsort
When sorting on disk, there are actually limitations in sorting linear time. Has to do with being able to read blocks in memory and write them back to disk. As everything is in blocks, you are limited to O(n/B log(n/B)) with B being the amount of items per block. For more speed, a merge sort that keeps in mind the block size works quite well. Search for external sorting.
