A system for providing a copy of data at a point in time, has: a data storage
device
including a master store arranged to store blocks of data, at least one subsidiary
store to store point in time copy data having blocks of data copied from said master
store at a particular point in time, and a bitmap store associated with each the
subsidiary store to store data indicating when a data block of the master store
differs from a corresponding data block stored in the associated subsidiary store.
Where more than one subsidiary stores and associated bitmap stores are provided,
a controller may make different point in time copies in different subsidiary stores.
A number of master stores may also be provided, each associated with a corresponding
subsidiary store and a bitmap store. In this case, the controller may ensure that
point in time copies of all the master stores are made at the same point in time.
Data from a master store may be stored in a subsidiary store only when it is to
be replaced in the master store. In this case, the subsidiary store may be smaller
in capacity than the master store and an overflow store may be provided for use
when the subsidiary store is full.
