Functionality
ASM requires an ASM instance to manage diskgroups. DBAs can use SQL*Plus to connect to ASM and to do the usual database-level operations like startup and shutdown. However, one must set the ORACLE_SID variable to '+ASM' and not to the ORACLE_SID of a database. Views like V$ASM_DISK, V$ASM_DISKGROUP, V$ASM_FILE, V$ASM_ALIAS make it easier to perform queries regarding disk groups, disks in each disk group, their layout, extents etc.
Read more about this topic: Automatic Storage Management