A data storage control apparatus comprising: - a plurality of control modules (4-0, 4-1, 4-2, 4-3, 40-1, 40-2, 40) for accessing a plurality of memory devices (2-0,...,2-n) for storing data according to access instructions from a mainframe unit having a first protocol and an open system unit having a second protocol; - a first channel adaptor; and - a switch unit (6-0, 6-1) connected to the first channel adaptor and the plurality of control modules (4-0, 4-1, 4-2, 4-3, 40-1, 40-2, 40), and for selectively switching a connection between the first channel adaptor and the plurality of control modules (4-0, 4-1, 4-2, 4-3, 40-1, 40-2, 40), wherein each control module (4-0, 4-1, 4-2, 4-3, 40-1, 40-2, 40) comprises: - a cache memory for storing a portion of data stored in the memory devices (2-0,..., 2-n); - a second channel adaptor (41 for performing interface control to the open system unit via a connection to the open system unit; - a disk adaptor (42a, 42b) for performing interface control to the plurality of memory devices (2-0,..., 2-n); and - a control unit for controlling the cache memory in response to the access instructions, and for accessing the memory devices (2-0,..., 2-n) via the disk adaptor (42a, 42b) characterized in that - the first channel adaptor is adapted to connect to the mainframe unit from a start of access to an end of access according to the protocol of the mainframe unit, - and the second channel adaptor is adapted to disconnect the connection to the open system unit on receiving the access instructions from the open system unit, according to the protocol of the open system unit, - and said first channel adaptor is adapted to, in response to a write access from the mainframe unit, receive write data from the mainframe unit and write in parallel both -- by a first control module among the plurality of control modules into the first control module's cache memory the write data or data among said write data remaining to be written for having the write data written into said first control module's cache memory and -- by a second control module among said plurality of control modules into the second control module's cache memory the write data or data among said write data remaining to be written for having the write data written into said second control module's cache memory. - a plurality of control modules (4-0, 4-1, 4-2, 4-3, 40-1, 40-2, 40) for accessing a plurality of memory devices (2-0,...,2-n) for storing data according to access instructions from a mainframe unit having a first protocol and an open system unit having a second protocol; - a first channel adaptor; and - a switch unit (6-0, 6-1) connected to the first channel adaptor and the plurality of control modules (4-0, 4-1, 4-2, 4-3, 40-1, 40-2, 40), and for selectively switching a connection between the first channel adaptor and the plurality of control modules (4-0, 4-1, 4-2, 4-3, 40-1, 40-2, 40), - a cache memory for storing a portion of data stored in the memory devices (2-0,..., 2-n); - a second channel adaptor (41 for performing interface control to the open system unit via a connection to the open system unit; - a disk adaptor (42a, 42b) for performing interface control to the plurality of memory devices (2-0,..., 2-n); and - a control unit for controlling the cache memory in response to the access instructions, and for accessing the memory devices (2-0,..., 2-n) via the disk adaptor (42a, 42b) - the first channel adaptor is adapted to connect to the mainframe unit from a start of access to an end of access according to the protocol of the mainframe unit, - and the second channel adaptor is adapted to disconnect the connection to the open system unit on receiving the access instructions from the open system unit, according to the protocol of the open system unit, - and said first channel adaptor is adapted to, in response to a write access from the mainframe unit, receive write data from the mainframe unit and write in parallel both -- by a first control module among the plurality of control modules into the first control module's cache memory the write data or data among said write data remaining to be written for having the write data written into said first control module's cache memory and -- by a second control module among said plurality of control modules into the second control module's cache memory the write data or data among said write data remaining to be written for having the write data written into said second control module's cache memory. -- by a first control module among the plurality of control modules into the first control module's cache memory the write data or data among said write data remaining to be written for having the write data written into said first control module's cache memory and -- by a second control module among said plurality of control modules into the second control module's cache memory the write data or data among said write data remaining to be written for having the write data written into said second control module's cache memory.