A data block is the unit of physical I/O when Teradata Database writes data to disk. A data block contains one or more data rows from the same table. The system stores data blocks in segments, which are grouped in cylinders. You can set data block sizes for tables using SQL or set them globally using the DBS Control utility. If there is a conflict between the data block size value set for the system and for the table, the table value overrides the global value.