The BLOCKSIZE keyword specifies the physical I/O transfer size for the file. It takes the following form:
BLOCKSIZE = bks
For magnetic tape files, the value of bks specifies the physical record size in the range 18 to 32767 bytes. The default value is 2048 bytes.
For sequential disk files, the value of bks is rounded up to an integral number of 512-byte blocks and used to specify multiblock transfers. The number of blocks transferred can be 1 to 127; it is determined by RMS defaults.
For relative and indexed files, the value of bks is rounded up to an integral number of 512-byte blocks, and is used to specify the RMS bucket size in the range 1 to 63 blocks. The default is the smallest value capable of holding a single record.