Striping
ASM stripes its files across all the disks that belong to a disk group. It remains unclear if it follows a strict RAID 3 fashion of striping or a variant of RAID 3 that facilitates easy addition and removal of disks to and from the disk group. Oracle Corporation recommends that all the disks that belong to a disk group have the same size, in which case each disk gets the same number of extents. However, if a DBA configures disks of different sizes, each disk might get a different number of extents — based upon the size of the disk. An allocation unit typically has a size of 1MB.
ASM stripes help make data more reliably available and more secure than in other Oracle storage implementations.
Read more about this topic: Automatic Storage Management