Oracle ASM uses disk groups to store data files. An ASM disk group is a collection of disks managed 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 is evenly distributed to eliminate hot spots and to provide uniform performance across the disks. The performance is comparable to the performance of raw devices. [From Oracle® Automatic Storage Management Administrator's Guide 11g Release 2].
ASM Disk Groups
An ASM disk group consists of one or more disks and is the fundamental object that ASM manages. Each disk group is self contained and has its own ASM metadata. It is that ASM metadata that an ASM instance manages.
The idea with ASM is to have small number of disk groups. In ASM versions before 11.2, two disk groups should be sufficient – one for datafiles and one for backups/archive logs. In 11.2 you would want to create a separate disk group for ASM spfile, Oracle Cluster Registry (OCR) and voting disks – provided you opt to place those objects in an ASM disk group.
Disks to be used by ASM have to be set up and provisioned by OS/storage administrator before ASM installation/setup. Disks can be local physical devices (IDE, SATA, SCSI, etc), SAN based LUNs (iSCSI, FC, FCoE, etc) or NAS/NFS based disks. Disks to be used for ASM should be partitioned. Even if the whole disk is to be used by ASM, it should have a single partition.
The above is true for all environments except for Exadata – where ASM makes use of grid disks, created from cell disks and presented to ASM via LIBCELL interface.
An ASM disk group can have up to 10,000 disks. Maximum size for an individual ASM disk is 2 TB. Due to bug 6453944, it is possible to add disks over 2 TB to an ASM disk group. The fix for bug 6453944 is in 10.2.0.4, 184.108.40.206 and 11.2. MOS Doc ID 736891.1 has more on that.
ASM looks for disks in the OS location specified by ASM_DISKSTRING initialization parameter. All platforms have the default value, so this parameter does not have to be specified. In a cluster, ASM disks can have different OS names on different nodes. In fact, ASM does not care about the OS disk names, as those are not kept in ASM metadata.
Any ASM file is allocated from and completely contained within a single disk group. However, a disk group might contain files belonging to several databases and a single database can have files in multiple disk groups.
ASM can store all Oracle database file types – datafiles, control files, redo logs, backup sets, data pump files, etc – but not binaries or text files. In addition to that, ASM also stores its metadata files within the disk group. ASM has its own file numbering scheme – independent of database file numbering. ASM file numbers under 256 are reserved for ASM metadata files.
ASM Cluster File System (ACFS), introduced in 11.2, extends ASM support to database and application binaries, trace and log files, and in fact any files that can be stored on a traditional file systems. And most importantly, the ACFS is a cluster file system.