DATABLOCKSIZE sets the maximum data block size for blocks that contain multiple rows. The data block is the physical I/O unit for the file system.
Larger block sizes enhance full table scan operations by selecting more rows in a single I/O. Smaller block sizes are best for transaction-oriented tables to minimize overhead by retrieving only what is needed.
A row that is larger than the DATABLOCKSIZE setting is stored in its own data block. Vantage does not split rows across data blocks.
A sector is typically 4 KB. If the computed value for the DATABLOCKSIZE attribute does not fall within the allowable range, the system returns an error message to the requestor.
If you do not specify a value for DATABLOCKSIZE, the database uses the system-wide default data block size. The default value is typically 254 sectors and the maximum is 255 sectors for all systems.
The default DATABLOCKSIZE settings depend on the cylinder size that is set for your system.
Following are release-specific rules for minimum data block sizes.
The DATABLOCKSIZE value returned in response to a SHOW TABLE statement is either the value specified in the CREATE TABLE statement for the table or the value specified in the most recently entered ALTER TABLE statement.
Large-Cylinder Systems
Vantage systems use large cylinders. The following tables list the minimum and maximum data block sizes.
| Minimum Data Block Size | Description |
|---|---|
| 42 sectors (21504 bytes) | The minimum data block size accepted by the CREATE TABLE and ALTER TABLE SQL statements is 21248 bytes (41.5 sectors). Tables created or altered to use this minimum size have the value rounded up by the parser to 42 sectors, so these tables have an actual minimum data block size of 21504 bytes. |
| Maximum Data Block Size | Description |
|---|---|
| 2047 sectors (1048064 bytes) | The maximum data block size accepted by the CREATE TABLE and ALTER TABLE SQL statements is 1048319 bytes (2047.5 sectors). Tables created or altered to use this maximum size have the value rounded down by the parser to 2047 sectors, so these tables have an actual maximum data block size of 1048064 bytes. |