While using testdrive gui, how to configure it to use zsync?
There seems to be no option to change that. Or is there a config file somewhere which can be edited?
TestDrive will use rsync, and then zsync, and then wget by default.
If you'd like to force zsync, I think that's a reasonable configuration option we should support. You're welcome to file a bug against the TestDrive project in Launchpad, http://launchpad.net/testdrive.
After some googling 1 it seems testdrive will use zsync by default as long it's installed.
Update: I have checked testdrive's source, it will use zsync if it can find the zsync command and if the image url starts with either http or ftp .