compress video for backup I want to compress a video to store it for backup purpose.

By that, I mean that I don't need to play video when it is compress format. So I don't want to lose quality or something like that, just compress it.

I have tried to compress with 7zip, but size not decrease. Is it possible to compress a video ? 

For example, I use 7zip on a 3.92Go mkv video. I try :


zip format : 3.92Go
gz format : 3.88Go
Generally no. Compression algorithms search for repetitive data and replace it with a shorter code and keep a dictionary with all codes. Compressing text works excellent because in longer texts a lot of words repeat. 

With video this work has been done most of the time already. Usually the encoder is  performing a similar operation: looking for repeating patterns and replacing them with a shorter code. Most video files you see are already compressed, most repetitive patterns already encoded and therefore any further compression won't do much.

Sometimes re-encoding videos with a much newer encoder saves a lot of space, but this process is always a bit lossy and requires more work.

You could check if the files have any additional unwanted audio tracks and remove them, like tracks in languages you don't speak anyway.