# Volume Cluster Size

**Volume cluster size** is the minimum allocation unit for a volume. That is, if the volume cluster size is 16 blocks and you create a text file of 1 block, 16 blocks are allocated for this file on the volume and only one block is written.

# Limits

The maximum size you can specify for a volume is 16380 blocks, or 1/50th the volume size, whichever is smaller.

# Defaults

For 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 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.

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.