Have been using gzip for a few years and usually I always
gzip -9 out of habit as it is the highest level of compression. I understand that lower levels means less compression but quicker compress rate.
What is the real difference between the levels of compression?
From what I have read regarding DEFLATE (what gzip uses), the levels specify the amount of time the algorithm spends finding repetition in bytes and encoding the data properly. I don't quite understand how this would differ between one pass of a file or multiple passes of a file. Wouldn't all bytes that are replicated be compressed on the first pass (all the repetition removed)?