Toms Kyte的书上讲了：当且仅当创建数据库时指定了一个非标准的块大小（不是2的幂）时，才会有6种不同的块大小 .

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A database can have as many as five data block sizes. The database always allocates and uses disk space in units of data blocks. The data block size is typically 8KB to 32KB and must be a multiple of the physical block size of the storage device.

Larger data block sizes are more common in data warehousing environments where the larger block size can yield shallower Btree indexes and thus better performance 。