Oracle ASM is a volume manager and a file system for Oracle database files that supports single-instance Oracle Database and Oracle Real Application Clusters (Oracle RAC) configurations. Oracle ASM is Oracle’s recommended storage management solution that provides an alternative to conventional volume managers, file systems, and raw devices.

Oracle ASM uses disk groups to store data files, an Oracle ASM disk group is a collection of disks that Oracle ASM manages as a unit.
Within a disk group, Oracle ASM exposes a file system interface for Oracle database files. The content of files that are stored in a disk group is evenly distributed to eliminate hot spots and to provide uniform performance across the disks.

Oracle ASM Instances

An Oracle ASM instance is built on the same technology as an Oracle Database instance. An Oracle ASM instance has a System Global Area (SGA) and background processes that are similar to those of Oracle Database.

However, because Oracle ASM performs fewer tasks than a database, an Oracle ASM SGA is much smaller than a database SGA. In addition, Oracle ASM has a minimal performance effect on a server. Oracle ASM instances mount disk groups to make Oracle ASM files available to database instances; Oracle ASM instances do not mount databases.

See the below structures:
Example - Oracle ASM Cluster with RAC

oracle-asm-rac

PS. Oracle ASM cluster in an Oracle RAC environment where Oracle ASM provides a clustered pool of storage. There is one Oracle ASM instance for each node serving multiple Oracle RAC or single-instance databases in the cluster. All of the databases are consolidated and share the same two Oracle ASM disk groups.

Example - Oracle ASM Cluster with single instance

oracle-asm-single

PS. A clustered storage pool can be shared by multiple single-instance Oracle Databases. In this case, multiple databases share common disk groups. A shared Oracle ASM storage pool is achieved by using Oracle Clusterware. However, in such environments an Oracle RAC license is not required.

Oracle ASM Disk Groups

A disk group consists of multiple disks and is the fundamental object that Oracle ASM manages. Each disk group contains the metadata that is required for the management of space in the disk group. Disk group components include disks, files, and allocation units.

Files are allocated from disk groups. Any Oracle ASM file is completely contained within a single disk group. However, a disk group might contain files belonging to several databases and a single database can use files from multiple disk groups. For most installations you need only a small number of disk groups, usually two, and rarely more than three.

Oracle ASM Disks

Oracle ASM disks are the storage devices that are provisioned to Oracle ASM disk groups. Examples of Oracle ASM disks include:

  • A disk or partition from a storage array
  • An entire disk or the partitions of a disk
  • Logical volumes
  • Network-attached files (NFS)

Oracle ASM spreads the files proportionally across all of the disks in the disk group. This allocation pattern maintains every disk at the same capacity level and ensures that all of the disks in a disk group have the same I/O load. Because Oracle ASM load balances among all of the disks in a disk group, different Oracle ASM disks should not share the same physical drive.

Oracle ASM Files

Files that are stored in Oracle ASM disk groups are called Oracle ASM files. Each Oracle ASM file is contained within a single Oracle ASM disk group. Oracle Database communicates with Oracle ASM in terms of files.
This is similar to the way Oracle Database uses files on any file system. You can store the various file types in Oracle ASM disk groups.

How to Use ASM

Start & Stop

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# start
$ export ORACLE_SID=+ASM
$ sqlplus "/ as sysdba"
SQL> startup
ASM instance started
Total System Global Area   83886080 bytes
Fixed Size                  1217836 bytes
Variable Size              57502420 bytes
ASM Cache                  25165824 bytes
ASM diskgroups mounted

# stop
$ export ORACLE_SID=+ASM
$ sqlplus "/ as sysdba"
SQL> shutdown immediate

Adding a diskgroup

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SQL> create diskgroup orag2 external redundancy disk 'ORCL:VOL5';
Diskgroup created.

SQL> select group_number,disk_number,mode_status,name from v$asm_disk;
GROUP_NUMBER DISK_NUMBER MODE_STATUS    NAME
------------ ----------- -------------- -------------------------------------
           0           5 ONLINE
           1           0 ONLINE         VOL1
           1           1 ONLINE         VOL2
           1           2 ONLINE         VOL3
           1           3 ONLINE         VOL4
           2           0 ONLINE         VOL5
6 rows selected.

Recreating a diskgroup

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dd if=/dev/zero of=/dev/rdsk/c1t4d0s4 bs=8192 count=12800
# When done, restart ASM and create the diskgroup from scratch.

Rebalancing

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# The rebalancing speed is controlled by the ASM_POWER_LIMIT initialization parameter.
# Setting it to 0 will disable disk rebalancing.
# To force rebalancing of a diskgroup:
ALTER DISKGROUP data REBALANCE POWER 11 WAIT;

Coverting to ASM

One can use Rman to convert a datafile, tablespace or entire database from/to ASM.
Here are the steps required to migrate an entire database to ASM

Convert a database to ASM

Ensure the database is using an SPFILE and not a PFILE (it’s about time after all!).
Set parameters on the target database.
For example, if we set both DB_CREATE_FILE_DEST and DB_RECOVERY_FILE_DEST we should get mirrored controlfiles and duplexed log files by default:

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SQL> alter system set DB_CREATE_FILE_DEST = '+DATA';
SQL> alter system set DB_RECOVERY_FILE_DEST_SIZE = 17G;
SQL> alter system set DB_RECOVERY_FILE_DEST = '+RECOVER';
SQL> alter system set CONTROL_FILES = '+DATA';

Start the database in NOMOUNT mode and restore the controlfile into the new location from the old location:

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RMAN> connect target /
RMAN> STARTUP NOMOUNT
RMAN> RESTORE CONTROLFILE FROM 'old_control_file_name';

Mount the database and copy the database into the ASM disk group:

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RMAN> ALTER DATABASE MOUNT;
RMAN> CONFIGURE DEVICE TYPE DISK PARALLELISM 8;
RMAN> BACKUP AS COPY DATABASE FORMAT '+DATA';

Switch all datafiles to the new ASM location and open the database:

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RMAN> SWITCH DATABASE TO COPY;
RMAN> ALTER DATABASE OPEN;

Add new tempfiles and drop the old tempfiles:

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SQL> alter tablespace temp add tempfile;
SQL> alter database tempfile '...' DROP;
SQL> select * from dba_temp_files;

Optionally, move SPFILE into ASM:

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SQL> CREATE SPFILE '+DATA' FROM PFILE;

Move redo log files into ASM - for each group:

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SQL> ALTER DATABASE DROP LOGFILE GROUP 1;
SQL> ALTER DATABASE ADD LOGFILE GROUP 1 SIZE 100M;
Convert a tablespace to ASM

Ensure the database in in archive log mode, and from rman:

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connect target;
sql "alter tablespace TSNAME offline";
backup as copy tablespace TSNAME format '+DATA';
switch tablespace TSNAME to copy;
sql "alter tablespace TSNAME online";
exit;
Convert a datafile to ASM

Ensure the database in in archive log mode, and from rman:

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connect target;
sql "alter database datafile '...' offline";
backup as copy datafile '...' format '+DATA';
switch datafile '..' to copy;
sql "alter database datafile '...' online";
exit;

Monitoring

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V$ASM_DISK -- ASM disks
V$ASM_DISK_STAT -- cached view of V$ASM_DISK for faster access (used by Enterprise Manager)
V$ASM_DISKGROUP -- ASM diskgroups
V$ASM_DISKGROUP_STAT -- cached view of V$ASM_DISKGROUP for faster access (used by Enterprise Manager)
V$ASM_OPERATION -- status of ongoing disk operations (like rebalancing)

Resources:
https://docs.oracle.com/cd/E11882_01/server.112/e18951/asmcon.htm#OSTMG94057
http://www.orafaq.com/wiki/Automatic_Storage_Management