Patent ID: 7698518

Claim:
A disk-array apparatus operatively coupled to a host computer, comprising: a primary volume for storing data; a pool volume for storing differential data, said data from said primary volume and said differential data being stored so as to define a virtual volume to be accessed by the host computer; a control processor for controlling data reading/writing into and from said primary volume and said pool volume, said control processor being configured to control a data storage capacity of said pool volume, wherein when an amount of differential data stored in said pool volume equals or exceeds a first predetermined threshold value, additional storage capacity is added to said pool volume, wherein said control processor is configured such that when the amount of differential data stored in said pool volume equals or exceeds a second predetermined threshold value, writing/deleting of differential data is selectively limited in said virtual volume, said second predetermined threshold value being higher than said first predetermined value, said control processor is configured such that when the amount of differential data stored in said pool volume equals or exceeds a third predetermined threshold value, deleting of differential data is selectively controlled in said virtual volume, said third predetermined threshold value being higher than said second predetermined value, and said control processor is configured such that when the amount of differential data stored in said pool volume equals or exceeds a fourth predetermined threshold value, deleting of data is selectively controlled in said primary volume, said fourth predetermined threshold value being higher than said third predetermined value.