Create large files for testing.

This command will create a 100Mb file.

dd if=/dev/random of=myfile.dat bs=$(( 1024 * 1024 )) count=100

This can take a long time, so try this

dd if=/dev/zero of=myfile.dat bs=$(( 1024 * 1024 )) count=100

See Unix Administratosphere for more detail.