A log structured array (LSA) controller apparatus controls the transfer of information
between a processor and a plurality of information storage devices configured as
an N+1 array in which the information is stored as stripes extending across the
devices of the array, each stripe comprising N information strips and one parity
strip, each information strip storing an integer number of logical tracks. The
controller defines an LSA directory that specifies the location of each logical
track in terms of the ID of the stripe to which the track belongs and the offset
of the track within the stripe; wherein on the addition of an information storage
device to the array, the additional strip provided for each stripe by the storage
device is logically appended to the end of each stripe in the LSA directory.