/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 16380 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 16. 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
    and, by default, are rounded up to the next multiple of 16.

    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 16 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
    and, by default, are rounded up to the next multiple of 16.

                                   NOTE

       For Version 7.2 and later, 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 an
       ODS-2 disk, your disk can be mounted on prior versions of
       OpenVMS.

    For ODS-1 disks, the cluster size must always be 1.

                                   NOTE

       If you specify /LIMIT and do not specify a value for
       /CLUSTER_SIZE, a value of /CLUSTER_SIZE=16 is used.