Importance

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Abstract

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.

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First Claim