Just use rsync -va

NO STOP!

The default quick check algorithm of rsync is not safe for this. It only checks filesize and modification time to determine if files are equal. After a b0rked copy, these are not to be trusted.

You should add the -c flag so that files are properly checksummed, unfortunately if you have slow storage on either end, this often negates the speed advantage of rsync.

For example, consider this example:

mkdir source
mkdir destination
echo "hello" > source/file.txt
echo "world" > destination/file.txt
touch -r source/file.txt destination/file.txt
rsync -avh source/ destination/
cat source/file.txt
cat destination/file.txt

Contrary to what you might expect, the rsync command copies nothing and the output at the end will show:

hello
world

If you change the rsync command in the example above to rsync -c -avh source/ destination/, it will work as expected.

rsync -avP

All hail the rsync!

We thank the rsync for it's unwavering reliability.

Amen.

Thank you!

Another way to check is to

strace cp testfile testfile2

and the sequence in which the message is printed and operations performed can be studied.

It's perhaps a lot to read, but linux tracing tools are worth learning!