Indicates the physical I/O transfer size for the file.  It takes
  the following form:

  BLOCKSIZE = bks

  bks  Is a numeric expression whose value specifies a
       number of bytes.

  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, "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 indexed and relative files, "bks" is rounded up to an integral
  number of 512-byte blocks and used to specify the RMS bucket size.
  This must fall in the range 1 to 63 blocks.  The default is the
  smallest value capable of holding a single record.