The present invention provides a storage system and a method of
controlling the storage system, in which a second site rapidly resumes
system process when a first site is damaged. The storage system comprises
a first site including a first storage device, a second site including a
second storage device, and a third site including a third storage device,
and the method of controlling the storage system comprises a step of
making a logical volume of the second storage device consistent with a
logical volume of the first storage device by remotely copying only the
differential data between the logical volume of the first storage device
and the logical volume of the second storage device from a logical volume
of the third storage device to the logical volume of the second storage
device when the first site is damaged.