ASM is Oracle's recommended storage management solution that provides an alternative to conventional volume managers, filesystems, and raw devices.
ASM uses disk groups to store datafiles. An ASM disk group is a collection of disks that ASM manages as a unit. Within a disk group, ASM exposes a file system interface for Oracle database files. The content of files that are stored in a disk group are evenly distributed or striped, in order to eliminate hot spots and to provide uniform performance across the disks. The performance is comparable to the performance of raw devices. Oracle ASM has the following benefits:
ASM allows you to add or remove disks from a disk group while a database continues to access files from the disk group. When you add or remove disks from a disk group, ASM automatically redistributes the file contents and eliminates the need for downtime when redistributing the content.
The ASM volume manager functionality...