Hyper-V fun fact

Have you ever needed to back up a VHD with a script?  If you have, then you've probably wanted to make sure it copied successfully.

One simple test is to check the size of both the source file and the destination file; this sounds great until you bang your head against the wall in frustration when, two hours later, you can't figure out why the file sizes still don't match.

Allow me to explain.

When you back up a Hyper-V machine, the first thing you do is to stop the VM for the duration.  Essentially, this just tells the virtual machine manager to stop writing to the VHD, giving you a static target... but in the process, it marks the file which changes the size.  Ergo, you have a 4KB difference in your VHD size between "started" and "stopped" states.

The solution?  Make sure your VM is off for another second while you get that all-important file size.  Then start it up again, and off you go.