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Abstract

A computer data backup method where copying of data blocks from one medium to another is deferred until a data structure, e.g. a bit map, is generated defining those data blocks that in fact contain data. The data structure is then read to copy to the backup medium the data-containing blocks as defined by the data structure. Since the data structure is generated before the copying is effected, a range of contiguous data-containing blocks is determinable such that all the blocks of that contiguous range may be copied as a single continuous operation. The magnitude of required backup storage is reduced by eliminating the copying of empty blocks in general unless the number of empty blocks between two ranges of contiguous data-containing blocks is less than a predefined number.