Editing Rsync (section)

=== Determining which files to send ===
By default, rsync determines which files differ between the sending and receiving systems by checking the modification time and size of each file. If time or size is different between the systems, it transfers the file from the sending to the receiving system. As this only requires reading file directory information, it is quick, but it will miss unusual modifications which change neither.<ref name="man page" />

Rsync performs a slower but comprehensive check if invoked with <code>--checksum</code>. This forces a full checksum comparison on every file present on both systems. Barring rare [[hash collision|checksum collisions]], this avoids the risk of missing changed files at the cost of reading every file present on both systems.