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ASM is a volume manager and a file system for Oracle database files that supports single-instance Oracle Database and Oracle Real Application Cluster (Oracle RAC) configuration. ASM is Oracle's recommended storage management solution that provides an alternative to conventional volume managers, file systems, 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, to eliminate hot spots and to provide uniform performance across the disks.

To ensure that a disk group always has sufficient space to store the critical organizational data, you will have to continuously track the space usage on the disk group. This will provide you with early pointers to potential space contentions on that group, and help you swiftly provide more space to the group by adding more disks. The ASM Disk Space test enables you to achieve this end. This test closely monitors how each disk in a disk group uses the space available to it, points you to the disks that are running out of space, and thus holds a mirror to space contentions on a disk group.

To enable the test, go to the ENABLE / DISABLE TESTS page using the menu sequence : Agents -> Tests -> Enable/Disable, pick Oracle Database as the Component type, Performance as the Test type, choose the test from the DISABLED TESTS list, and click on the << button to move the test to the ENABLED TESTS list. Finally, click the Update button.

Measurement	Description	Measurement Unit	Interpretation
usedSpace	Indicates the amount of space currently used in this disk.	MB	Ideally, the value of this measure should be low. A consistent increase in this value is a cause for concern.
freeMb	Indicates the amount of space in this disk that is currently free - i.e., available for use.	MB	Ideally, the value of this measure should be high. A consistent decrease in this value is a cause for concern.
freeSpacePct	Indicates the percentage of space in this disk that is currently unused.	Percent	A high value is typically desired for this measure. By comparing the value of this measure across disks and across disk groups, you can quickly isolate the disks/groups that are running short of space. If the free space is alarmingly low for all disks in a group, it indicates that the group requires more space. You can then consider making space by adding more disks to the group.
usedSpacePct	Indicates the percentage of space in this disk that is currently used.	Percent	A low value is typically desired for this measure. By comparing the value of this measure across disks and across disk groups, you can quickly isolate the disks/groups that are utilizing space excessively. If the used space is alarmingly high for all disks in a group, it indicates that the group is rapidly running out of space. You can then consider making space by adding more disks to the group.
usedSpaceGrowth	Indicates the growth in space usage of this disk since the last measurement period.	MB/Sec	If you observe the variations to this measure over time, you will be able to detect early whether the space in the disk is being steadily eroded or not. This way, you can initiate measures to conserve space much before the disk exhausts all the space available to it.