VMS Help INITIALIZE, Qualifiers, /CLUSTER_SIZE  *Conan The Librarian (sorry for the slow response - running on an old VAX)

       /CLUSTER_SIZE=number-of-blocks

    Defines, for disk volumes, the minimum allocation unit in blocks.
    The maximum size you can specify for a volume is 16382 blocks, or
    1/50th the volume size, whichever is smaller.

    For Files-11 On-Disk Structure Level 5 (ODS-5) disks, the default
    cluster size is 3. In this case the minimum value allowed by the
    following equation is applied:

    (disk size in number of blocks)/(65535 * 4096)

    Any fractional values must be rounded up to the nearest integer.

    For Files-11 On-Disk Structure Level 2 (ODS-2) disks, the default
    cluster size depends on the disk capacity; disks with less
    than 50,000 have a default of 1. Disks that are larger than
    50,000 have a default of either 3 or the result of the following
    formula, whichever is greater:

    (disk size in number of blocks)/(255 * 4096)

    Any fractional values must be rounded up to the nearest integer.

    For Files-11 On-Disk Structure Level 1 (ODS-1) disks, the cluster
    size must always be 1.

                                   NOTE

       For Version 7.2, you can specify a cluster size for ODS-2
       volumes smaller than allowed by the ODS-2 formula; however,
       if you try to mount this volume on a system running a
       version prior to 7.2, the mount fails with the following
       error:

         %MOUNT-F-FILESTRUCT, unsupported file structure level

       If you choose the default during the initialization of a
       Files-11 On-Disk Structure Level 2 (ODS-2) disk, your disk
       can be mounted on prior versions of OpenVMS.