SQL/MX 3.2.1 Management Manual (H06.26+, J06.15+)

Matching Block Split Operation to Table Usage
In a table with key-sequenced organization, when an INSERT operation causes a data block to
overflow, the DP2 process must make room for the new row by splitting the block in the middle
and transferring some of its contents to a newly allocated block.
Because NonStop SQL/MX does not support the SEQUENTIAL BLOCKSPLIT option for the CONTROL
TABLE statement, you cannot split the block at the insertion point when rows are being inserted in
sequence.
308 Enhancing SQL/MX Database Performance