This morning I upgraded the mail servers at work. They were down for maybe two seconds each. I doubt anyone even noticed.
If you’re used to installing software on Windows, you probably think I’m kidding. Didn’t I have to shut the service down before installing the upgrade? Didn’t I have to reboot when it was done? Nope!
The reason you have to go through all that trouble when you install something on Windows is that it won’t let you change files that are in use. So you can’t install the new Exchange without closing down the old one. And some files aren’t completely closed until you shut down Windows.
On Linux (and possibly all Unix-like systems), if you delete or replace an open file, the system hangs onto it until the program using it is done. So if you need to upgrade Sendmail, you can go through the entire process while the old Sendmail is still running, then tell it to restart the service once it’s ready.
It’s not quite zero, but it’s close!