An ASM disk can be a disk device, a partition, or a network-attached file. When an ASM instance starts, it automatically discovers all available ASM disks. Discovery is the process of finding all disks that have been prepared for ASM by your system administrator, examining their disk headers, and determining which disks belong to disk groups and which are available for assignment to disk groups.
A disk group consists of a grouping of disks that are managed together as a unit. The disks in a disk group are referred to as ASM disks. On Windows operating systems, an ASM disk is always a partition. On all other platforms, an ASM disk can be:
A partition of a logical unit number (LUN)
A network-attached file
When an ASM instance starts, it automatically discovers all available ASM disks. Discovery is the process of determining every disk device to which the ASM instance has been given I/O permissions (by some operating system mechanism), and of examining the contents of the first block of such disks to see if they are recognized as belonging to a disk group. ASM discovers disks in the paths that are listed in an initialization parameter, or if the parameter is NULL, in an operating system-dependent default path.
Note: Although you can also present a volume for management by ASM, it is not recommended to run ASM on top of another host-based volume manager.