Oracle Automatic Storage Management (Oracle ASM) always stripes across all devices presented to it as a disk group. A disk group is a logical storage pool in which you create data files. The default Oracle ASM stripe size is 1 MB, which is a good stripe size for a VLDB.
Use disks with the same performance characteristics in a disk group. All disks in a disk group should also be the same size for optimum data distribution and hence optimum performance and throughput. The disk group should span as many physical spindles as possible to get the best performance. The disk group configuration for a VLDB does not have to be different from the disk group configuration for a non-VLDB.
Oracle ASM can be used on top of previously striped storage devices. If you use such a configuration, then ensure that you do not introduce hot spots by defining disk groups that span logical devices which physically may be using the same resource (disk, controller, or channel to disk) rather than other available resources. Always ensure that Oracle ASM stripes are distributed equally across all physical devices.
Oracle Automatic Storage Management Administrator's Guide for more information about Oracle ASM striping