| 摘要 |
Disclosed is to reduce an amount of memory required for a difference bit map corresponding to a storage extent of large city. A difference bit map 27 is made into multi-tiers. A difference is expressed by a bit 321 of an entry 320 of a second tier. An entry 310 of a first tier is provided with a pointer 311 to the entry of the second tier and a representative bit 313 , and the entry 320 of the second tier is made unnecessary when all values of the corresponding bits 321 are equal to a value of the representative bit 313.
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