What is a download server?
In Ubuntu, the software that makes up the system itself, as well as many of the extra applications you can install, are downloaded from repositories by a package manager (probably APT). Ubuntu folks upload the packages they maintain and updates them to these repositories on Canonical's main server.
The main server is one of the many download servers available for the Ubuntu repositories. Download requests vary, servers can go down, and also, physical distance from the server has some effect on speed, so to help everyone get better download speeds, there are download servers with regularly updated copies of the Ubuntu repositories all over the world. These are commonly called "mirrors".
What does this mean for my system?
Sometimes APT is slow because of a problem with your internet connection, but sometimes it is the fault of the server.
By default, the APT configuration that gets set up when you install Ubuntu uses the mirror that it thinks is closest to your location. Sometimes, the local mirror may be less reliable or slower than the main server, so some people prefer to switch to the main server to try to get better speeds (or back again if there's no improvement).
In the Software Settings, you can see whether your system is configured to use the main server or a local one, or some other mirror. You can also take a look at the file that configures this, /etc/apt/sources.list (you can use less /etc/apt/sources.list to read the file in a terminal). If you are using the main server, it will have lines like this:
deb http://archive.ubuntu.com/ubuntu/ ...
and if you're using a different mirror, there will be a different URL there.
What will happen if I change the download server?
Mirrors should have the same or almost the same repository contents as the main server. Things probably won't change much, unless one or other of the servers is having problems. You might see
- Changes in the output of
apt update and the speed at which it runs
- Changes in the download speed.
- Small changes in the package versions available
What should I do then?
I would recommend clicking on "select the best server". If this doesn't solve your problem, try switching between "Main server", your country's server, and servers in nearby countries.
