Every Oracle ASM disk is divided into allocation units (AU). An allocation unit is the fundamental unit of allocation within a disk group. A file extent consists of one or more allocation units. An Oracle ASM file consists of one or more file extents.
When you create a disk group, you can set the Oracle ASM allocation unit size with the AU_SIZE disk group attribute. The values can be 1, 2, 4, 8, 16, 32, or 64 MB, depending on the specific disk group compatibility level. Larger AU sizes typically provide performance advantages for data warehouse applications that use large sequential reads.
Refer to Example 4-1 for an example that shows how the AU_SIZE is specified with the CREATE DISKGROUP SQL statement. For information about allocation unit sizes and disk group compatibility attributes, see "Features Enabled By Disk Group Compatibility Attribute Settings".
Articles at My Oracle Support (https://support.oracle.com) for additional information about allocation units and Oracle ASM